Noon PST Friday 23 November 2001

A strong weather system moved across the Olympic, Washington 
Cascades and Mt Hood area on Thursday and Thursday night.  A 
weakening 986 mb surface low moved into the south Washington 
coast Thursday morning, a bit further south than originally 
indicated by computer models. Strong westerly flow following the 
low helped cause strong winds, heavy precipitation and lowering 
snow levels on Thursday and Thursday night. Westerly winds 
averaging 50 to 70 mph with gusts of 70 to 100 mph were measured 
by NWAC wind instruments at higher elevations. Several inches to 
18 inches of snow has been measured at sites above about 4-5000 
feet in the past 24 hours, with the heaviest amounts in the 
central Cascades, south Cascades, and Mt Hood area, with lighter 
amounts east of the crest.

This new snow follows the 6 to 11 inches of snow that fell 
Tuesday and Wednesday, and all of the recent snow above about 4-
5000 feet has accumulated on a crust that formed early in the 
week. This is likely to have caused some local slab layers to 
have formed mainly on steeper north to east aspects at higher 
elevations, but also possibly on other steep aspects, with 
initial lower density layers acting as weak layers, and other 
firmer storm cycle layers or the crust acting as a bed surface. A 
local considerable avalanche danger seems likely such slopes at 
higher elevations. Natural slab avalanche releases were seen 
Wednesday and Thursday at Crystal Mountain and Chinook Pass. On 
south and west aspects and on lower angle slopes the cooling 
trend should be more likely to help to bond new and previous snow 
layers.

Snow depths are still relatively shallow at lower elevations 
below about 4-5000 feet, with terrain and vegetation helping 
anchor the snowpack. But isolated avalanches might be possible at 
lower elevations as well where locally heavier snow deposits may 
lie on steep smooth surfaces such as the crust, smooth rock slabs 
or grassy slopes.

A weak ridge, and decreasing winds and decreasing moisture should 
cause snow showers to decrease Friday at lower snow levels. A 
large upper trough should move over the Northwest Saturday and 
Sunday, causing weak winds and periods of mostly light snow at 
continued low snow levels. A weather system moving into the 
Oregon California border area may help spread some heavier snow 
to mainly the Mt Hood area on Saturday. Overall this weather 
should help recent heavy snow accumulations at higher elevations 
to partly settle, further bond, and partly stabilize. Slab layers 
should be most likely to linger on steep north to east aspects, 
and the most caution should continue to be used on this type of 
slope.

Backcountry travelers should also use extra caution near Mt Hood 
area if additional significant snow falls in that area, which may 
add new loads to existing layers, or form new soft slab layers on 
steeper lee slopes. Also remember to actively monitor snow 
stability as you travel in the backcountry this weekend and this 
winter!