Special Avalanche Statement issued Noon PDT Wednesday 22 May 2013
…Potentially dangerous avalanche conditions remainder of this week and possibly into the weekend…
An upper level low pressure system and a very cool unstable air mass began to linger over the Northwest on Tuesday centered over Washington. A shortwave and showers rotating counter-clockwise within the low caused the heaviest snow showers over the central and south Cascades with unseasonably low snow levels. New snow amounts ending Wednesday mid-morning range from minor amounts in the north to a few inches at higher elevations near Stevens and Snoqualmie to 6-15 inches from about Crystal to Timberline with the most at Paradise on Mount Rainier.
The cold upper level low should linger over the Northwest Wednesday and Thursday, possibly centered over western Washington. The main shortwave and shower activity should continue to rotate counter-clockwise within the low and cause the heaviest snow accumulation in the central and south Cascades with continued unseasonably low snow levels through Thursday. The center of the low along with the primary area of showers may shift to Oregon by Friday morning.
Another 5-10 inches of snow seems likely by Thursday morning with an additional few inches possibly by Friday morning in the central and south Cascades. New snow amounts may be fairly equal west and east of the crest. Less new snow should be seen in the north Cascades.
Concerns: Potentially dangerous storm slab avalanche conditions should be expected in back country or uncontrolled areas especially in the central and south Cascades on varied aspects. Regardless of the low snow levels and cool temperatures heavy snow at this time of year is very susceptible to sun effects and daytime warming. Snow that falls during the daytime hours on Wednesday and Thursday will also be heavier due to warmer daytime temperatures which will cause denser surface snow and hence further instability. Natural and triggered avalanches should be possible or likely at higher elevations on Wednesday and Thursday especially in the central and south Cascades where snowfall is heaviest. Potential dangerous avalanche conditions should be expected. Careful snowpack evaluation, cautious route finding and conservative decision making should be essential.
Loose wet snow avalanche conditions should also become more likely by Friday and into the weekend. Poor or dangerous snow and avalanche conditions should also be expected at higher elevations on the volcanoes especially in the central and south Cascades.
This statement will be updated as warranted.
Ferber/USFS NW Weather and Avalanche Center