Archived for two avalanche incidents Apr 13 near Snoqualmie Pass Information for other areas and beyond that date are omitted Northwest Weather and Avalanche Center Seattle Washington 1800 PM PDT Fri Apr 12 2013 This forecast applies to back country avalanche terrain below 7000 feet and does not apply to developed ski areas or highways. Please note: The NWAC will end regularly scheduled mountain weather and avalanche forecasting on Sunday 14 April. Zone Avalanche Forecasts Forecast for: ... Snoqualmie Pass ... Summary Danger Forecast For Saturday Overall Danger Trend Saturday: Considerable avalanche danger above 4000 feet and moderate below increasing strongly in the afternoon. Avalanche danger decreasing late Saturday night. Concern: Storm slab avalanche Aspect: All Size: Initially shallow but increasing in depth Trend: Increasing strongly in the afternoon Concern: Wind Slab Avalanche Aspect: Lee N thru SE aspects of higher terrain Size: Isolated larger slabs possible higher lee slopes Trend: Increasing Slightly Snowpack Discussion A storm cycle returned last Thursday to Sunday with a series of frontal systems and a low pressure system crossing the Northwest. Snow levels started out at about 5000 feet in the north and 6000 feet in the south on Friday and fell roughly a thousand feet a day to about 3000 feet in the north and 4000 feet in the south by Sunday. Storm water equivalents ending Monday ranged from about 3-5.5 inches at NWAC stations near and west of the crest with storm snowfall totals at higher elevations of 2-3 feet. A transitory ridge caused mostly dry conditions with a moderate warming trend Monday and Tuesday. A front crossed the Northwest on Wednesday morning with rain below about 6000 feet in the north and 7000 feet in the south. Crystal mountain plus the WSDOT avalanche control near Washington Pass reported size-able wet-loose slides with the warming and rain on Wednesday. Wet loose releases continued into Thursday for Alpental where a convergence zone cause light rain at lower elevations. Ski area and DOT reports from Friday morning indicated that clear skies and good cooling Thursday night formed a stout melt-freeze crust. This crust may briefly weaken Friday afternoon with rain initially at lower elevations, but dropping temperatures Friday night should reform the crust and help keep the upper snowpack locked down. The mid and lower snow pack are considered stable and fairly consolidated. Detailed Forecasts Saturday Another winter-like weekend is in store for the PNW... After a strong front passes through Friday night, light to moderate snow showers Saturday morning will likely become moderate to occasionally heavy Saturday afternoon for the west slopes as an upper level shortwave enhances precipitation over the PNW. Pronounced cooling early Saturday should help new snow bond well to Friday night's snowfall. However, periods of high snowfall rates and strong crest level westerlies and NW flow aloft should lead to quickly escalating avalanche danger Saturday afternoon and evening. Showers and windy conditions should eventually ease Saturday night and lead to a slight decrease in the avalanche danger. Concerns: Storm slab avalanches should become more widespread and easier to trigger on all aspects in the afternoon due instabilities within the new storm snow Saturday, but avalanches should be contained to the new storm snow. New wind slab should build on the usual N thru SE lee slopes of higher terrain. Due to winter-like temperatures, new snow should be easier to transport and thus build thicker wind slabs especially up high. The bottom line is don't let your avalanche guard down just because the calendar says it's mid-April. Use the same cautious route-finding and terrain selection techniques you would use mid-winter during a storm cycle. Look for the warning signs like shooting cracks indicating instability propagation of a slab and for obvious indicators like recent natural avalanche activity as a message that it's time to pull back.