by Edward R. LaChapelle and
M. M. Atwater
Avalanche Hazard Forecasters

May 1961


Anticipation of delayed-action climax avalanches is the most difficult problem facing the avalanche forecaster. The more common direct action slides fall as the result of circumstances developed over a few hours or at the most over a few days of prolonged storm. For the latter often brings an increase in stability which is more or less amenable to test in the field. Snow which is likely to form direct-action slab avalanches often gives warning of its dangerous character before it achieves critical instability on a given slope. Such is not the case with delayed action slides. Here the basic weaknesses may be established weeks or even months in advance of actual release, the decline in mechanical strength or bonding of a snow layer may proceed unnoticed for long periods, and a dangerous avalanche path may fail to show any signs of approaching danger be fore the final overload of otherwise innocent snow is added which brings it to the critical point. The delayed-action slab may achieve this critical point by the cumulative effect of a number of superimposed snow layers any one of which taken by itself is harmless. It can truly be the case of the straw which breaks the camel's back.

Doubtlessly many avalanche slopes reach the critical instability for a delayed-action avalanche release, but never fall for lack of an external trigger to initiate the actual rupture. Such slopes gradually return to a safe condition as warmer weather late in winter ameliorates the temperature gradients and permits destructive metamorphism to exercise its stabilizing effect. In an area of heavy use, such as a busy ski resort, it is not possible to wait for this to take place, for an accidental trigger may be supplied at any time; naturally by unexpected snow or weather developments, or artificially by a wandering skier. It is the true test of an avalanche hazard forecaster's skill that he be able to perceive the onset of delayed-action slab formation and be prepared to deal with it.

The warning signs of climax avalanche danger are not mysterious, but they are often subtle and hidden deep in the snow cover. They may be perceivable months in advance of the event; in fact this may be the only time when they stand out for the eye to see, before a smooth ice layer or unstable depth hoar becomes buried beneath subsequent snow falls. The simplest way to demonstrate these effects is to illustrate them with actual case histories from the Forest Service files at Alta, Utah, within the Wasatch National Forest, and Squaw Valley, California, within the Tahoe National Forest. The reader is therefore invited to join the USFS snow rangers as they follow the development of three actual hazard situations from storm to storm. The following storm reports are in the reporting ranger's own words, just as they are found in the Alta and Squaw Valley files. Additional commentary is added where appropriate to illuminate a particular point.

The record at Alta begins in January of 1959. In this case the warning sign of trouble to come was an unseasonal rainstorm which brought liquid precipitation on the shallow winter snow cover up to altitudes of 10,000 ft. or higher. This rain was followed by a drop in temperature which formed a very hard, smooth rain crust on the snow surface. Two sub sequent storms which built up a layer of powder snow over the crust as yet had shown no tendency to slide. The snow rangers were conscious of impending trouble, but it arrived even quicker than expected, riding the tail of an innocuous little storm.

Observer's Analysis

This storm was of little consequence in itself, but provided sufficient load to trigger climax slide conditions, and provided the first clear indication of slide activity to come as more snow was added by later storms. The highly unstable nature of the base, with at least two good falls of powder snow, in themselves stable, resting on a smooth rain crust, had indicated that extensive avalanching was to be expected. This 6" storm was the first one to reach the unstable point.

The first path to run was Superior, which came down at approximately 0720 on the 25th. This slide was witnessed by the Highway Dept. cat driver, who was driving his truck up to Alta and just rounded Windy Point as it came down.

Artillery fire and test skiing in the ski area failed to reveal any slide danger there, and most of the area was opened for use.

Late in the afternoon two skiers traversed the steep slopes above Wildcat Bowl and climbed high under the cornices above Westward Ho. Their line took them about 50 ft. higher up the slope than the usual traverse, which had been test-skied by the snow rangers in the morning. At the steepest part of the slope just under the cornice they dislodged a deep climax slab which carried them both a way down the hill, where they suffered minor bruises and equipment damage. Only the snow above the regularly used traverse slid, that below having been thoroughly stabilized by skiing. Both were thoroughly experienced skiers, and one a trainee in the avalanche school.

Shortly afterwards an Alta Lodge employee and a guest skied through the Eaglet's Nest in the general area known as "High Nowhere", and dislodged another climax slab under the steep cliffs of North Rustler. The employee was carried clear to the bottom, but escaped without injury.

Ample indication by this time was at hand that things were going to get worse before they got better. No closure actions were taken on the hill, because by the time of the above accidents the lift was closing down for the day. The forerunning squall of Storm # 6 had already arrived, and the next day looked promising for slide activity.

This is a case where the onset of possible danger had been recognized, but even after the warning sign of the Superior slide, the rangers tended to put too much faith in the previous stabilizing effect of heavy use on the ski hill. This faith was well-founded as far as the main runs and powder slopes were concerned, but those few patches which had escaped test skiing and use were unerringly ferreted out by the powder hounds, and the rapidly developing hazard stood revealed. Forewarned by these events, the rangers regarded the next storm with more caution and undertook more thorough control measures when it was over.

Observer's Analysis

Storm #6 started out with a brief and violent squall late afternoon on the 25th. This squall put down about an inch of damp pellet snow, which rendered the highway extremely slippery. Little snow then fell until about 2100, when steady and heavy precipitation began. This continued through most of the night, and ended at dawn. Wind was moderate at all times, but the snowfall intensity was high. The storm snow itself was a fairly stable form of powder, and did not show much slide activity, but it provided additional weight to trigger more climax slides.

The Superior and Little Superior slides ran the afternoon of the 26th as a result of sun action on the S. exposures. The atmospheric conditions this day seemed particularly favorable for a strong positive radiation balance at the snow surface, a condition not usually encountered in January.

Artillery fire early on the 26th released two-large climax slides in the ski area. The North Rustler slide was shot down with a shell directed at the deep drift above the N. Rustler cliffs. The fracture line extended west under Eagle's Nest and cleaned out all the steep slopes in that area. Had the Rustler lift still been there, it would have been once more thoroughly demolished.

The last shot of the morning was directed into the chutes on Peruvian Ridge just north of Peruvian Bowl. A large climax slide was released which included most of the Peruvian Ridge area north of the Bowl, and fell over the Wildcat Bowl and into the head of Rock Gulley. These chutes used to be subjected to regular control measures by skiing and blasting when the old Peruvian lift was in operation, but in recent years they have fallen into neglect by both skiers and snow rangers. The size of the slide which came out of them after this storm is ample evidence that they still remain a source of hazard.

During a test skiing exercise later in the morning, a climax slab was dislodged by one of the avalanche school instructors on the steep slope above Race Course and south of Sunspot. This was a slab about 4 ft. thick which ran back to the rain crust. It afforded an ideal opportunity to demonstrate to the school the characteristics of a slab avalanche, where all the contributing factors could be clearly seen.

Hand blasting operations on the 27th released a major climax slab from Greeley Bowl, which cleaned out the entire south end of the Bowl, including Gunsight, and ran clear to the creek in Albion Basin.

Hand blasting on the 26th, as a demonstration exercise for the avalanche school, released a very large slab avalanche from the East Greeley face. This also ran back to the rain crust.

Once more a rather minor sort of storm had sufficiently overloaded the snow cover to create a rather widespread and dangerous instability. Poor bond of the underlying layers to the smooth rain crust was still the primary cause of trouble. Examination of the fracture lines suggested that some weakening of the snow layer just above the crust had also taken place by constructive metamorphism. Danger in the ski area was countered by extensive blasting and artillery fire. The ready manner in which these slides broke loose suggested that an even more serious situation would develop when another storm added more weight to the slopes and brought to the critical point large slide paths which are not normally active. The skier-released slab above Sunspot indicated that instability had reached the point where even a light trigger was effective. It should be noted again that the character of Storm #6 was not particularly dangerous, and that the type of snow and the manner in which it was deposited was not that normally associated with slab formation. The general situation which now existed in this part of the Wasatch Mountains was a clear illustration of the general principle that the slab avalanche is a mechanical state involving many factors besides a given type of snow or its physical appearance.

Most of the steeper and more active avalanche paths in the ski area had now been cleaned out by artificial release. The full consequence of some additional weight on other large slide paths with a slightly shallower slope in the release zone did not remain long in doubt, for another storm followed hard on the heels of #6.


The major climax avalanche on Greeley Hill at Alta, Utah,
which fell as a result of an early-season rain-crust. Released
by a 75 mm howitzer shell on the morning of 29 January 1959.
(See Storm Report No. 7) Arrow indicates the target point.

Observer's Analysis

This storm in itself was not particularly dangerous. Snowfall intensity was only moderate and the wind well below critical levels. Ordinarily it would have brought some good deep powder skiing and little else. The 1.5 inches of water it added to the load on the steep slopes was enough to bring the pending climax situation to full development.

The only marked natural slide activity was on Superior, which was cleaned out by several small slides during the storm and did not present a threat to the highway. With expectation that some climax slides were due, thorough artillery control was initiated on the 29th with spectacular results. The clouds cleared off and the sun came out by mid-morning; the avalanche school trainees were treated to an instructive morning of shooting.

The firing began on Hellgate with the 75 mm howitzer. A climax slide was dislodged which ran back to rain crust, as did all of the large slides released this morning. It hit the road with full vigor, and the dust cloud ascended the slopes of Westward Ho for several hundred feet. The next target was Flagstaff Bowl, the whole western half of which came out right from the ridge, and also hit the road. Little snow reached the creek, but the dust cloud struck the east end of Peruvian Lodge and gave the guests a thrill to go with their morning coffee.

It was now clear that the high angle slopes were highly unstable, and some careful thought was given before firing the next shot. It was finally decided that Flagstaff Mountain would have to be cleaned out, for it almost certainly would run naturally, and the longer the wait, the bigger it would come. Accordingly, safety instructions were given, residents of the Deep Powder House, bunkhouse and Alta Lodge ordered to the basements, rescue equipment removed from the FS garage where it might be buried, and the 75 mm gun transported by Weasel to safe slopes along the road past Grizzly Gulch. By this time the sun was out full, and the avalanche school was assembled at the gun to watch. One shot was fired at the shoulder of Flagstaff Peak. The fracture line propagated slowly eastward, jumping from gully to gully along the whole ridge from Flagstaff to Emma Hill, ample evidence that this face would have come down by itself in such a highly unstable condition. Fortunately the heavy sliding was confined to the high angle slopes, and lost momentum in the transition. The regular Flagstaff slide path between Rustler and Alta lodges filled the parking lot in good form, but little spilled over into the valley floor, though the dust cloud filled the valley for a few minutes. Most of the rest of the slide snow was arrested on the benches before it could reach the parking lots and lodge area.

With the 75 mm howitzer conveniently located to fire on Greeley Hill, it was turned around and a shot fired at this target. The results were, if anything, even more impressive than on Flagstaff. The whole hill came out right back to the ground, carrying the walls of Snakepit with it and filling the pit to a depth of 40 or 50 ft. The fracture line was up to 10-12 feet high.

The north side of the canyon having been cleared, the 105 recoilless was now brought into action for clearing the ski hill. Some small slides in the new snow were dislodged in the regular target points, and another large climax slide was released from Baldy Shoulder, getting another potential danger to the ski area out of the way.

On the 30th, hand-placed charges released large slides from Gunsight and Greeley Bowl.

It is interesting to note that all of the climax slides released on the 29th exhibited a hangfire characteristic. For a few seconds after the shell burst, nothing would happen, and then slowly the whole hillside would come into motion.

It is also interesting to note that absolutely no evidence of climax development occurred on the dangerous slidepaths overhanging the ski area, such as High Rustler, Stonecrusher and Sunspot. These paths had been regularly cleaned out after every storm by artillery fire, and gave ample evidence of the effectiveness of control measures.

The 29th was spent in road clearing work by the cat, and the road was not opened to traffic until 1000 on the 30th.

With Storm #7, the climax avalanche situation had reached full development. A widespread and exceedingly dangerous instability demanded drastic control action before the highway, parking lots and ski area could safely be opened to public use. Any lingering doubts about the overloading effects of another snow layer on the unstable base were removed when the first shot was fired and the vigorous reaction of the Hellgate slide could be seen. At this point the slopes might have eventually reverted to a stable state if allowed to stand undisturbed for a long period of time, but the risk of the climax slabs being triggered by natural or artificial means was too great to be countenanced. The most likely natural trigger was the addition of still another snow layer to the slopes, breaking down the slab supports by sheer overweight. The next storm later in the week proved that this was exactly what would happen.

Observer's Analysis

This storm started out quietly the afternoon of the 3rd, and gradually picked up intensity and wind velocity as the evening wore on. The Weather Bureau 5-day forecast on the 2nd called for no precipitation in sight. It was a high- level storm throughout, being concentrated at the heads of the canyons, while almost no precipitation fell lower down. By midnight the storm was going full blast, and continued with heavy precipitation and winds above the critical levels for most of the next 24 hours. The wind was gusty, frequently reaching 35-40 mph at the guard station. The lifts were shut down, the road closed, and traffic between lodges restricted all day on the 4th (lodge restrictions after 12:30). Intensity of the storm gradually eased off the evening of the 4th, and some light, dry snow fell early AM on the 5th, followed by clearing conditions after sunrise.

The snow type was predominantly pellet, with some granular. New snow density reached a maximum of 0.20 on the 4th. Snowfall at the end was dry powder. The high winds effectively scoured all windward exposures, and piled deep drifts on the lee slopes.

The storm snow was highly unstable, and apparently many of the slide paths ran very early in the storm before much depth had accumulated. Largest natural slide observed on the ski hill was the Baldy Chute, which apparently ran early on the 4th. The unstable base developed early in winter continued to provide conditions for climax slides where previous snowfalls had not yet been cleaned out by earlier avalanching. The East Emma slide was a good example of another climax slide triggered by overload of snow, an indication that these may continue to fall after each storm in the back country where no control work has been done.

Hand blasting released the large Yellow Trail slide, which involved new snow only, the base having been stabilized by previous blasting. As this slide descended, the vibrations from it were enough to release the climax slide in Glory Hole, which slid back to the rain crust around the entire edge of the bowl.

Skiing was good after the storm on the lee slopes, and the area was opened except for a few restrictions on the morning of the 5th. The bulldozer cleared the Superior slide about 1430, and the road was opened at that time.

The 105 rifle was fired on Superior and East Hellgate the morning of the 6th with only minor sloughs.

The East Emma slide was representative in slope and exposure of the Hellgate, Flagstaff and Emma slides which were released by artillery fire following Storm #7, but it had not been subjected to artillery fire. Its natural fall as a climax slide following Storm 48 was indicative of the way these other slide paths would have behaved if left uncontrolled.

The inexorable build-up of a climax situation, involving first the smaller or more unstable slide paths, and then the large, infrequent and dangerous ones as storm follows storm, presents a challenging problem to an area administrator responsible for public snow safety. Once the basic weakness in the snow cover (in this case the smooth rain crust) has been revealed early in the winter, the consequences are almost inevitable. Seldom do they build to such a climax as in this sequence of storms, but when they do, sound judgment and positive action are required to avoid disaster in a crowded ski area. In this case the decision to undertake extended control measures was compounded in difficulty by the risk of property damage. This risk was taken, here in a calculated fashion as the alternative to closure of the lodge and highway area to all traffic and activity. The morning following Storm #7 turned clear by 10 AM, and there was a strong possibility that sun action on the large south exposures overhanging the ski area would have effected natural release of the climax slides by mid-afternoon. Thorough enforcement of closures on a sunny day in a resort full of eager skiers would probably have been impossible. In a situation like this, an accurate technical estimate of snow and avalanche conditions is only the first step; acting correctly on this estimate may be the larger problem.

The next sequence of snow storms leading to climax avalanche hazard occurred at Alta the following winter. The cause this time was different, a layer of weak and unstable depth hoar next to the ground on northerly exposures. Avalanche behavior also followed a different pattern. Instead of gradually leading to bigger and bigger slides, the hazard occurred more erratically, culminating finally in a single climax slide of extraordinary proportions.

Very little snow fell in November and December of 1959. The ski hill and northern exposures retained only a few inches of snow barely sufficient to provide a base for skiing. South exposures were completely bare of snow up until the first major storm on Christmas day. The long period of clear weather turned the shallow snow cover almost entirely to depth hoar on the north exposures except for the principal ski runs receiving heavy use. In anticipation of trouble from this weak snow layer, artificial compaction by ski and foot was undertaken in Peruvian Bowl and on the Stonecrusher and Lone Pine slide paths. The effectiveness of this treatment was put to a good test by Storm #1 which began very early on Christmas morning.

Observer's Analysis

Trouble had been expected on north exposures with the first storm to deposit much snow over the existing shallow depth hoar. The danger was further heightened by the existence of a crust layer in the depth hoar resulting from a November salt storm. The early part of Storm #1 brought damp, heavy snow which favored slab formation, and avalanche activity began early, when little more than 6-8" of snow had fallen.

Test skiing on the morning of the 25th revealed highly unstable conditions and the skiers were restricted to minimum hazard areas on the Collins lift for the rest of the day. A test release on one of the chutes of Peruvian Ridge resulted in release of a soft slab extending all the way from Peruvian Bowl to Westward Ho. Very extensive cracking of the snow was observed even early in this storm, and persisted for a day or two afterwards.

Artillery fire on the morning of the 26th brought down several large slides, the most extensive being those on Baldy Shoulder and Sunspot-Racecourse. Many small patches of slab existed throughout the ski area, and test skiing and blasting in the Peruvian Ridge-Wildcat areas proved these to be sufficiently dangerous that they were kept closed throughout the day. A small part of the ridge around the top of the Wildcat Lift was left open to permit use of this lift, but even here the snow was unstable, and two small slabs were dislodged by skiers, one of them burying another skier who had the misfortune to fall below. The latter was quickly dug out uninjured. A heavy influx of skiers out to take advantage of the first good skiing of the winter made the area control problem difficult, and there was considerable complaining about the restrictions until it became obvious to even the most careless skiers that a serious hazard situation existed.

It is interesting to note that the slopes of Stonecrusher and Lone Pine, which had been previously foot-packed, did not slide, though adjacent slopes slid vigorously following artillery release. Increasing evidence seems to show that foot packing is an effective countermeasure against the hazards of depth hoar . Peruvian Bowl also did not slide, for it had been thoroughly sidestepped with skis several days prior to the storm.

Blasting and artillery fire continued for a couple of days after the storm, and danger in outlying ski runs was eventually reduced. A serious condition of instability continues on north slopes, however, and more trouble may be expected from future storms. At the date of this writing (3 Jan) occasional patches of unstable slab have continued to be uncovered in the ski area, and a general hazard condition is presumed to exist in the back country where no control measures have been taken.

This was some of the most highly unstable snow the snow rangers could ever remember seeing. Fracture lines occurred naturally everywhere, even among heavy timber and on gentle slopes. The first half of the storm, which brought damp, rather heavy snow and occasionally high winds, resulted in widespread slab formation which was virtually unsupported by the northslope depth hoar. Test ski release of the Peruvian Ridge slide on the 25th, during early part of the storm with only 6" of new snow, gave ample warning of the hazard. When a turn was made into one of the usual small test skiing chutes on the Ridge, the fracture line jumped over the edges of the chute and propagated northward for 1500 ft. across varied terrain, releasing avalanches over most of this distance. Enforcement of restrictions the following day was difficult with the large crowd of skiers brought out by the first good skiing of winter. The new Wildcat chair lift was opened to the public for the first time on this day, but most of the slopes accessible from it had to be kept closed. One ski patrolman, who was checking the hazard in this area, entered a small slope too low and was buried to his shoulders until dug out by a companion. A more harmless sort of storm bringing powder snow and light winds might have kept the danger disguised until a heavier snow cover had been built up over the depth hoar, but in this case there was no mistaking the hazard right from the start.

The next storm brought a good fall of snow and winds high enough to create a general hazard situation. Many north slopes which had not actually slid during Storm #1 now became active under artillery and explosives control. Slab formation was general, and highly unstable where underlain by depth hoar.

Observer's Analysis

This storm on the 13th had winds above 15 mph all day. The fresh snow was getting deep on the protected slopes of the ski hill at noon on the 13th. With poor visibility, Germania Lift was shut down in order to prevent trespassers from going under Baldy. The storm plot did not show the depth that seemed to be accumulating on the lee slopes. The new Wildcat Lift continued to operate all day with skiing on the slopes from Collins Lift to Westward Ho. The skiers kept the snow cut up sufficiently to prevent slides. The snow rangers kept the snow sluffed off the steep ridges overhanging Wildcat Bowl.

The Yellow Trail avalanche of 14 January 1960 at Alta, Utah.
This slide was released by a 105 mm shell fired at a slide path
on the other side of the mountain, a striking example of extreme
instability resulting from depth hoar. (See Storm Report No. 2)

Skiing was excellent on the steep slopes, giving us one of the best days of skiing in a couple of years. The fresh snow on the 12th set up as it fell, also offering better than normal skiing.

On January 14, 18 rounds were fired with the 105 rifle and 13 rounds were fired with the 75 mm. Medium to large slides occurred on the main north and east facing slopes, excepting Stonecrusher and the ridge above Snakepit. Greeley Hill above Snakepit fractured but did not slide.

A shell into Eagles Nest slid it out, and also Snakepit, the center section of Greeley Bowl, High Nowhere, and a slide on the Yellow Trail. The slabs were very touchy with the depth hoar under them.

On 16 January, hand-placed explosives dislodged the slabs remaining in Greeley Bowl. The slabs slid back to the ground again. Greeley Bowl has slid out to the ground on two previous storms this season.

On the morning of the 13th, one round was fired blind into the top of High Rustler and another was fired blind into Stonecrusher. With heavy snow falling, no results or target points were observed. The upper end of High Rustler did not slide when the rest of High Rustler slid to the ground the next day under artillery fire. Stonecrusher, as stated, also did not slide.

The road was closed to traffic at 4:30 p.m. on January 13th. The storm ended at the time the road was closed. The road remained closed until 7:00 am on the 14th.

Serious instability persisted after this second major storm of the winter. The cause was the same: a very poor base on northern exposures. The critical nature of the hazard was clearly illustrated by reaction of the snow to control measures on the 14th. Especially indicative was the artillery shell fired into Eagle's Nest during the morning shoot. This single shell initiated a series of slides, the farthest one being nearly a mile away on the other side of the ridge. Later examination of these slopes showed extensive fracturing between the areas which actually slid. It appears that disruption caused either by the shell burst or fall of the Eagle's Nest slide triggered a release of creep tension which was propagated over an extremely wide area. Snow behavior such as this is rare, but it does occur from time to time, and the consequent hazards must be borne in mind when artificial control measures are undertaken. This is a sound reason for blasting or using artillery fire only when a large area has been cleared of everyone who might conceivably be exposed to danger; it is not sufficient merely to clear the slide path actually being controlled.

Part of the fractured area mentioned above was on a major avalanche path, Greeley Hill, which failed to slide. Apparently in this case, a northeast exposure, depth hoar formation had been insufficient to provide the necessary lubricating layer between slab and ground. As frequently happens when a heavy slab fractures but fails to avalanche, this slope was stabilized in place and did not slide the rest of the winter even under repeated artillery fire.

There could be no doubt about the serious hazard situation which now existed. It had been eliminated in the ski area by extensive control measures , but touring parties were warned to use great caution in the back country. Further hazard development, requiring more control work, was expected after the next heavy storm, and restrictions in the ski area were imposed early when this storm did arrive.

Observer's Analysis

This storm of light powder snow began to build up in the afternoon of February 2nd. Germania Lift was closed down due to poor visibility and the heavy snowfall. Skiing was open on the area from Collins Face to Westward Ho. Skiers kept the powder snow cut up and sluffed during the afternoon. The road was closed to traffic at 3:00 PM, and was reopened at 9:30 the following morning when the plows came up.

Good skiing was had during and after the storm.

The No. 4 chute on Rustler Mtn. had not slid yet this winter, and finally went with this snowfall and a little help from the 105 rifle. The areas on the ski hill with a good base were all stable. With only little settlement and the wind just under critical velocity, more slide action was anticipated than actually occurred. The low water content, sluffing, and a good base on the ski hill must have stabilized most of the area.

The 75 mm mountain howitzer in action at Alta, Utah,
during the climax hazard period of January, 1960.

The character of this storm turned out to be rather harmless in itself in spite of more than two feet of snow failing within 24 hours. It nevertheless was treated with great caution, and routine artillery fire and blasting on the 3rd and 4th dislodged both direct-action and climax slabs where previous use or intensive control measures had not stabilized the base. Like some of the snowfalls in the climax sequence of winter 1958/59, this layer of powder also brought hidden hazards closer to actuality simply by adding more weight to the slopes and consequent greater sheer stress to the underlying weak layers. This evolving danger was clearly recognized, careful control measures were continued, and touring was still posted as dangerous. The next storm proved these precautions were wise.

Observer's Analysis

This storm arrived in sections. The first fall of snow came during the night of 7-8 February, totaling 5" of heavy snow, with moderate winds. There were high, gusty winds all day on the 8th, velocity exceeding 40 mph estimated on the ridges. Heavy snowfall, (damp, heavy snow) began that evening and continued through the night. Snowfall intensity decreased during the day of the 9th, and then picked up again by late afternoon. By evening the snowfall had begun to taper off again, and the storm ended the following morning. Wind was high evening of the 8th, and again late afternoon on the 9th.

The general picture of contributory factors pointed to little hazard development out of this storm, for the high snowfall intensities were of short duration and the temperature pattern favored bonding between old and new snow.

Routine artillery fire verified this expectation, for only a few small surface slides were released. The weight of this much water added to the slopes, however, was expected to produce possible climax development on those slopes which still retained the crust and depth hoar conditions from early winter and which had not recently been cleared of snow by avalanching. This proved true in the case of Baldy Shoulder, which discharged a large avalanche under artillery fire, involving both snow from this storm and the previous two storms which combined to form a hard climax slab.

Hand blasting released a major climax slide of unusually large proportions from the broad face of East Greeley Mtn. This slope had not heretofore slid this winter, although subjected to repeated control measures in the expectation that it would because of the poor base which existed from early winter. Apparently the addition of another 3 1/2 inches of water was the amount of weight required to bring the slope to an unstable state. This slide came out clear back to the ground, involving the entire winter snow cover deposited since late December. It crossed the creek, overran part of the Green Trail, and hit poles of the Albion Basin phone line.

No other slide activity was observed, and the snow of this storm settled rapidly to provide some excellent powder skiing.

The climax avalanche which fell from East Greeley was the largest slide observed on this path in the past twenty years. In places the fracture line was 15 ft. high; apparently the high wind on the 8th deposited large quantities of snow on this lee slope by drifting in addition to that which was precipitated. Both this slide and that from Baldy Shoulder illustrate a dangerous characteristic of the climax avalanche: there often is no sign of instability even in the face of repeated blasting until a certain critical overload is reached. Both of these slide paths had been repeatedly subjected to control measures following the previous storms. Part of Baldy Shoulder had slid after Storm No. 2, but the Feb. 10th release involved a much larger area of snow. East Greeley had shown absolutely no reaction to repeated blasts with tetrytol prior to this climax slide. In such a situation, the only clues to impending hazard will be found in snow stratigraphy on the avalanche path and the winter record of weather conditions and snow cover evolution. Furthermore, it should be noted that the stabilizing effects of explosives control measures cannot always be counted on to eliminate hazard under these snow conditions.

This very serious climax situation was probably less widespread on north exposures in the ski area for two reasons. First, the very unstable nature of the snow deposited by Storm #1 resulted in extensive avalanching which cleaned out many of the regular avalanche paths right from the start. A more harmless type of snow with little slab formation would actually have contributed to greater hazard a month later. Second, thorough control measures kept many of the regular paths stabilized. Included in these control measures was the foot-packing of three slide paths prior to Storm No. 1. None of these discharged anything but small surface slides throughout the entire winter.

The lesson of the 1959-60 storm sequence is written very plainly: early-season formation of depth hoar in a shallow snow cover is a danger sign which cannot be ignored.

One contrast in the character of the two climax avalanche sequences de scribed here should be mentioned. The situation created by depth hoar, though perhaps the more unstable of the two, was confined only to those shaded slopes with northerly exposures where active depth hoar formation had taken place during clear weather in December. The rainfall responsible for the danger in 1958-59 fell, on the other hand, on all slopes alike, creating an ubiquitous rain crust which was a source of hazard everywhere.

The winter of 1956-59 also found a smooth rain crust incorporated in the snow cover at Squaw Valley, California. Here, too, it led to a serious climax avalanche hazard and difficult administrative problems.

It should be noted that at Squaw Valley weather and snow conditions differ considerably between the mountain tops and the valley floor where many of the snow observations are taken. This introduces added complications to the hazard forecasting problem. On the other hand, the snow rangers enjoyed excellent mountain weather forecasts, with a Weather Bureau man on duty at the Valley by virtue of preparations for the Winter Olympics.

Observer's Analysis

The dangerous potentialities of this storm were obvious from the start. The Weather Bureau observer kept us well and accurately posted throughout.

9 February: Until noon, storm intensity was moderate but increasing. By noon, snowfall intensity was up to one inch per hour. No restrictions were necessary.

10 February: By 0600 we had 20" of snowfall. There was no question of operating KT-22 and Squaw Peak #1 for the public, regardless of hazard, due to severe wind and weather conditions. Papoose lift was checked out and ready for use by 1000. The storm was so severe and the new snow so deep and loose that few people ventured out. We fired from Papoose into KT-22 Bowl and from KT-22 on the Headwall, etc. Results could not be observed due to zero visibility. However, the Breakover on Papoose, under protective skiing, avalanched on a front two hundred yards wide, indicating unstable conditions. Storm intensity continued to increase.

When the Weather Bureau forecaster warned me at 1500 that a sudden increase in storm intensity was due, I took steps to get visitors out of the area and the work crews off Papoose. Snowfall intensity reached 3"/hr at 1600.

11 February: The storm ended at 0700 with 63" gross snowfall. We fired again on KT-22 Bowl and Squaw Peak #1 targets, and in addition worked over both areas and Papoose with explosives and protective skiing. The east face of KT-22 and Lady's Downhill avalanched simultaneously and ran halfway down KT-22 Gulley. The Headwall avalanched to a point level with Tower 20. The west face of KT-22 reacted violently to handblasting. Other major-slidepaths such as Tower 15 and the Slot had already avalanched.

With the exception of settlement, which was very high, all contributory factors were favorable to avalanches of large size.

This storm itself was an excellent example of direct-action soft slab development during a typical "Sierra blizzard" with extended periods of strong wind and high precipitation intensity. The result was a general cycle of slab avalanches throughout the area. By dumping large quantities of snow on the steep slopes, it also paved the way for later formation of a climax avalanche situation, for the avalanching during the storm removed only parts of the new- fallen snow. In this climate and with these snow types, the very high settlement and relatively warm temperatures rapidly lead to stabilization after the storm is over. Such deep, soft snow usually
ends up as a very stable layer in the snow cover, but such was not to be the case in this instance, for the weather persisted cold.

Note that none of this direct-action avalanching in five feet of new snowfall slid back to the rain crust, while under similar conditions at Alta a much lighter snow load had started sliding off such a crust. The key difference is probably temperature. The Squaw Valley ice layer apparently was covered with a warmer, stickier type of snow which found better adhesion than did the colder snows at Alta. When release did come, it was through the agency of a common and dangerous factor in coastal zone avalanches--heavy rain in the high mountains.

Observer's Analysis

Storm #5 was separated from #4 by four days of unsettled weather. An additional 8" snowfall occurred. High winds prevailed at upper elevations which transported large quantities of the light of Storm #4 to the heads of Squaw Valley slidepaths. As a result of cold temperatures, little consolidation took place in the 71" gross snowfall since 9 February. Outside the heavily used trails the powder was still so deep and loose as to be almost unskiable on anything but the steepest slopes.

15 February: Temperatures rose suddenly. Light snow began in the PM, changing to rain at 1600.

16 February: By 1400, 2.76" of rain had fallen on the poorly-consolidated base. The dry snow soaked it up like a blotter until it became saturated and top heavy. Water began to flow out of the gullies, under the snow, proving that the ice layer was lubricated. The meadow, including the compacted parking area, became a lake of slush.

Stillman and I considered that the snowpack was developing a total overload, not only from the lubricating effect of the rain but also the sudden addition of 14 lbs./sq. ft. A few wet avalanches took place. Our principal clue was a short but very deep avalanche on a steep pitch west of the tram. Penetration through the entire snowpack to the ice layer indicated climax conditions. KT-22 was closed. Squaw Peak #1 was open under restrictions. We called the course preparation crews off Papoose at noon. In the afternoon, at the request of the Race Committee, we field-checked Papoose. We could drive a ski pole handle deep into the snow anywhere, including the middle of the packed race course. On one slope, the snow cracked under Stillman in slab fashion but did not slide. Papoose remained closed. Rain changed to snow at 1400.

17 February: By 0700 5" of damp snow had fallen. Snowfall continued at moderate intensities, changing to powder. Under strong wind action it was slabbing on all lee slopes. After shooting and blasting the area, releasing numerous soft slab avalanches on the surface, we reopened Papoose, KT-22 and Squaw Peak #1 A second significant clue developed. At a spot near the start of the Ladies Downhill, an isolated section of hard and soft slab peeled off seven feet deep under explosives.

We realized that we were balancing on a very narrow margin of safety. Temperature was falling, a favorable development, but wind deposition of snow at the heads of all the slidepaths was heavy, rapidly adding weight to the already overloaded snowpack.

18 February: By 0600 15" of snow had fallen. Strong wind action and slabbing on the surface continued. Again we shot up the area. Since visibility was zero, results could not be observed. While the blasting crew worked down the ridge above the Men's Giant Slalom, Stillman and I conferred at the summit of KT-22. We agreed that the mountain was getting ahead of us and should not be opened. On the way down, we picked up the blasting crew and found that they had released a climax hard and soft slab avalanche seven feet deep on Katy's Nose. The slide had run across the Giant Slalom Course. West Chutes 43 and #5 on KT-22 had released sympathetically from vibration. Chutes 1, 2 and 4 had already avalanched. External evidence observed later--the amount of snow deposited on top of the debris-- indicated that the gigantic Squaw Peak Bowl avalanche occurred during this period, the morning hours of 18 February. It broke off on a front extending from the south shoulder all the way around the Bowl to the Watson Monument, a mile wide and up to 14 feet deep. Penetration was to the January ice layer. The mound type avalanche barriers protecting the Bowl loading station of Squaw Peak #2 Lift stopped the avalanche in their zone, but were completely buried, indicating a need to extend this system in depth. Observations later showed that the North Fork of Squaw Creek, Bear Creek and Ward Creek drainages avalanched in a similar manner. In fact, this climax avalanche condition and cycle extended all the way to Mammoth Mountain.

At Squaw Valley, the avalanche paths which did not run were significant: the Headwall, Slot, Third Bowl, and KT-22 Bowl. On 18 February there was no way Stillman and I could determine whether our efforts had forced these slidepaths to stabilize in place or whether they were simply hanging fire. It was two months later before the cracks were exposed which prove that we broke the slab prematurely and forced it to stay in place.

That evening, Stillman and I reported to the Race Committee that under existing conditions we could hold Papoose and probably hold the Men's Giant Slalom Course. The weather forecast at this time was for a short break on the 19th to be quickly followed by a new storm of major proportions.

19 February: By 0600, 28" of snow had fallen and the storm was over. In good visibility, we once more blasted and shot up the area, peeling off the top layer of soft slab avalanches. A radical drop in temperature and wind had tightened up the base and stopped the addition of weight. The climax condition appeared to have run its course. Papoose was released to the Race Committee immediately; the Men's Giant Slalom by 1030; the Women's Downhill at 1200. In the afternoon Stillman and I inspected the Squaw Peak Bowl and reported to the Race Committee that we could clean up the remaining hazard--doubtful slopes and cornices--in half a day, and that if they wished work could start immediately on safe portions of the Men's Downhill. The weather forecast improved steadily as the new storm elected to sidestep us to the south.

20 February: We treated the Headwall to a 100-lb. multiple blast along its fracture line. Cratering and cracking were the only result. So ended a 12-day period of continuous action for the snow safety organization, during which we expended 90 rounds of 75 and 105 mm ammunition and over a thousand pounds of explosives. During the two major storms, every recognized slide path in the area was active, most of them repeatedly, and several new ones appeared on the hazard map. The resident forecaster of the Weather Bureau, John Lanning, did an outstanding job of keeping us forewarned throughout. This service is indispensable.

This second heavy storm also brought enough wind and new snow to create a direct-action avalanche cycle in its own right. In addition, the rainfall provided lubrication of the deeply buried ice layer and brought the climax situation to full development. All three factors leading to this development--the ice layer, very deep, loose powder snow, and rain-are common to the Coastal Alpine zone, and any one of them often occurs alone without precipitating serious avalanching. This particular combination, arranged in this order in the snow cover, led to avalanches of great size and destructiveness. The impending climax hazard was clearly recognized by the snow rangers, and appropriate safety measures and avalanche control action taken. The administrative problems were compounded by the presence of many competitors in the area for the North American Championships being held as a prelude to the 1960 Winter Olympics. The quantity of ammunition and explosives used indicates the scope of countermeasures taken to insure public safety and keep the slopes open. Even such a severe hazard condition in an area of many avalanche paths can be successfully met, as was demonstrated here, but the control action must be decisive, vigorous and well-organized.

Severity of this storm sequence was indicated by the fall of avalanches where none had previously been observed. This included even the outrun of the newly-constructed Olympic jumping hill, an unexpected example of the dangers of artificially creating avalanches by altering the natural terrain or timber cover.

The 75 mm recoilless rifle played an important part in
reduction of the serious climax avalanche hazard at Squaw
Valley, California, in February, 1959.

The origin of climax avalanches is complicated and often obscure. Some of the basic types of development have been learned through hard experience in the field. By bringing the substance of such experience together in this collection of reports, the newcomer to avalanche forecasting is afforded a chance to benefit from this experience, a chance he might have only rarely if he were to depend on his own observations, for such outstanding examples of climax hazard do not occur every winter, and often many winters may pass before they do. The importance of keeping accurate, up-to-date records of snow, weather and avalanche conditions therefore can not be exaggerated. This study would not have been possible if the snow rangers in question had not developed the habit of writing careful, complete storm reports. Standard methods of observing and reporting such in formation are treated in detail in Chapter 6 of the "Snow Avalanches Handbook".

Anyone engaged in regular avalanche observation and control work is urged to follow the methods outlined in the Handbook and the examples here in, for their records some day may prove instructive for others.

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