Avalanche Forecast

New snowfall received over the past week, and especially the most recent snowfall from Saturday night and Sunday, should continue to become unstable with strong sunshine and continued very warm temperatures Thursday.  The recent snowfall will continue to be very susceptible to strong spring solar effects and rapid warming Thursday.  This weather, following closely after such heavy recent spring snow storms, should maintain a significant avalanche cycle and a generally high avalanche danger in most areas, especially direct sun exposed terrain.

Natural or triggered avalanches should be likely to very likely Thursday and travel in avalanche terrain is not recommended.  Some of these slides may become large and potentially destructive in force.  Expect very dangerous avalanche conditions.

These expected very dangerous avalanche conditions are likely to persist through the week with the warmest weather of the season expected Thursday and possibly Friday. 

The last full week of April has felt more like winter than spring; the Forks, WA freezing level has averaged 4000 feet over the last week which is more than a 1000 feet lower than the average for the entire month of March. Upper NWAC stations at several sites have now reached their season maximum snowdepths, including: Green Valley, Crystal Mt, Paradise, MRNP, Pigtail Peak, White Pass, and Timberline, Mt. Hood.  Many other sites have neared their season maximums reached in late March.  

Water equivalents at NWAC stations near and west of the crest over the weekend were generally near the 1 inch range, with between  2.5-4.5 inches over the past week.  Snowfall received during this time near treeline zone is in the 1.5-3 foot range with more snow expected at higher elevations.  

Most west slope areas received about 1 foot of new snow Sunday.  This most recent snowfall, in addition to last week's snow, should be readily available to become involved in avalanches with the continued warm temperatures lasting into Friday.

In addition to the most recent new snow, WSDOT field observations on Chinook Pass, indicate that significant old wet snow is now becoming entrained in avalanches initiated in the most recent new snow.  This has led to some very large and dangerous avalanches, both last week and as recently as Tuesday, April 29th.  

Temperatures Wednesday reached into the 60's at most NWAC sites, the warmest day of the year thus far. The only thing helping to locally lower the danger is the moderate easterly winds along the crest.  These 15-30 mph easterly winds help to maintain a cooler and firmer snow surface on slopes exposed to the wind, thus limiting the snow melt and extent of wet snow avalanches.  

Avalanche observations (note that we receive far less observations than during winter):

Mt. Baker:  On Friday near Church Mtn, Brian Kennedy reported small wet loose slides on solar aspects becoming more reactive in the afternoon with sunbreaks and warming with a few point releases stepping down and entraining soft slabs about 20" (50 cm) down, but not propagating. 

Snoqualmie Pass: A triggered slab avalanche Sunday, caught, carried and severely injured a skier in a steep chute on Snoqualmie Mountain, requiring a long and arduous rescue.

Alpental: The Alpental ski patrol reported a widespread natural avalanche cycle Wednesday night, followed by easy ski triggering in the ATL and NTL zone Thursday, producing 8-12" storm slabs. Large wet loose avalanches were easily ski triggered below treeline Thursday.  With explosives Friday, Alpental patrol produced loose slides that entrained and ran further than the Wed night natural cycle. Ski cuts Friday produced isolated soft slabs. 

Tatoosh Range, MRNP: On Saturday, NWAC Observer Dallas Glass found shallow, but hard wind slab that was easily ski triggered on a N aspect above treeline (see below). Dallas also found widespread evidence of point releases from earlier in the week that had stepped down to lingering slab instabilities. Highlighting the power of late April insolation during this cool week, check out this picture from Dallas of a glide avalanche on a SW aspect at 5000'; the new snow from this week released on a lubricated smooth rock surface.  Dallas reported additional snowfall Sunday was producing more storm and wind slab concerns in all elevation bands.

Chinook Pass: On Tuesday the WSDOT crew reported a very large avalanche released from a large explosive that triggered an initial slab as much as 5 feet deep.  The slide extended and trigger additional slab releases, entraining impressive amounts of deep wet snow with the slide expanding to about 1000' wide!  Besides the recently fallen snow, there is a deep layer of weak and wet snow available to slide.  The old weakly bonded melt forms extended down to over 5 feet where tested. Wednesday saw equally impressive avalanches from more control work.  These photos from John Stimberis, WSDOT Avalanche Supervisor for the Chinook Pass operation, tell the tale of the current conditions just east of Chinook Pass over the past few days.  Note that, large slides were also triggered from small ski cuts, not just big explosives!