Early and mid-may featured our first major warm-ups for the Cascades and Olympics. Two warm spells during the May 2 - May 10 allowed for 15-20" of snow melt/consolidation during this time frame in the Washington Cascades and Olympics with slightly higher melt amounts around Mt. Hood. However, a return to late winter conditions May 11-17 resulted in significant snow deposition near and west of the Cascade crest and the Olympics. Reports indicate significant new snow of 29" and 42" at Paradise and Timberline during that time frame. The biggest "heat wave" of the spring was May 21-23rd with daytime temperatures into the 60's and 70's at most locations in the Washington Cascades and Olympics. A potent late season low pressure system brought rapid cooling and moderately strong winds to our mountains. Earlier this week, temperatures dropped into the 20's Tuesday night and never made it out of the 30's Wednesday at many of the mid-elevation mountain stations. A warming trend has begun Thursday with freezing levels rising to 8,000-9,000 feet this afternoon.
The following chart illustrates the early may warm-ups that are pulling us out of what has been a long, cool and wet first half of the spring.
Temperature and Precipitation near crest level in the Cascades during Spring 2017:
Recent snowpack observations
Last weekend, reports indicated a transition from recent new snow to a more seasonal spring snowpack. That transition involved numerous loose wet avalanches as the combination of sun and high freezing levels rapidly transitioned the new snowfall.
On Tuesday, March 23rd, Forest McBrian reported good early morning travel conditions just south of Washington Pass. On Wednesday, March 24th the Chinook WSDOT reported a well-drained and consolidated snowpack with much of the season's cornice build-up already calved off.
A trough currently centered over the Pacific Northwest interior spawned a weak low in Eastern Washington Thursday, which is helping to kick off light convection in the Cascades Thursday afternoon with the chance of a Thunderstorm. The trough will move east by Friday, ushering in another high amplitude ridge of high pressure, Friday. The ridge should stay in place into at least Monday. The ridge will be associated with generally clear weather and very low chances of precipitation through Sunday. Weather models differ on the chance of Thunderstorms Monday afternoon, so I will put a chance of isolated Thunderstorms in the forecast. Daily maximum freezing levels will rise gradually from 8,000-9000 ft today to 10,000-11,000 ft. Friday, 11,000-12,000 feet Saturday/Sunday, and 12,000-13,000 feet on Monday.
The cooling Tuesday night with continued cool temperatures on Wednesday should help firm up an already saturated snowpack in most locations below 8,000 feet. With anticipated warming and elevated freezing levels Friday-Monday, small loose wet avalanches are likely in specific areas during solar heating and isolated larger avalanches may be triggered. Evaluate snow and terrain carefully following the standard spring travel advice: Move during the night and early morning hours when in areas of avalanche concern.
During the night and early morning hours when the surface snow refreezes due to heat loss to the surrounding atmosphere, lowering the avalanche danger. During the day, sun effects and warm air temperatures can rapidly melt and weaken surface snow layers and produce an increasing avalanche danger during the late morning and afternoon. Loose wet avalanche activity generally starts on east and southeast facing slopes receiving morning sunshine and progresses to the west and southwest facing slopes during the afternoon. Therefore the safest time to cross potential avalanche terrain is during early morning hours before the surface snow begins to warm and weaken.
Cornices have partially shed or melted back in most parts of the Cascades but are still present in the Cascades and the remaining cornices will have reduced strength and are increasingly likely to collapse during the warm weather. Cornices may initiate large loose wet avalanches. Limit exposure to these features during daylight hours.
Many cornices are still large, so give them a wide berth when traveling along ridgelines. Cornices often catch people by surprise when they break farther back onto flatter areas than expected. See a blog post regarding cornices here.
Slopes beneath glide cracks should be avoided as the entire seasonal snow cover may release from melt water lubrication and weakening. Glide avalanches are difficult to predict as they are not necessarily tied to the warmest part of the day.
Remember that even small wet avalanches may be dangerous, particularly when they might funnel into a terrain trap or over a cliff.