Team 4 Alyssa Halm and Dan Kurz

FORECAST VALID FROM 12Z WEDNESDAY THROUGH 12Z THURSDAY MARCH SNOWQUALMIE PASS WASHINGTON. A broad longwave trough that has been impacting the weather of the west coast will begin to prop to the E. In its wake the arrival of a strong zonal jet stream will unfold and its associated jet streak will remain offshore, but intensify the arriving disturbance. The Aleutian low continues to strengthen during this period and an associated trough extending along the west coast of AK/Canada dipping into WA coupled with its location in the left exit region of this strong jet streak creates essential lift and Pacific moisture to produce widespread precipitation as it moves onshore in the pac nw. The strong upper lvl div is going to be the largest factor in enhancing this system. There is also a llvl jet around the 850mb level that will also aide in the transport of moisture into the forecast region. The main forcing mechanism associated with this event will be the orographic lifting due to the Cascade mountain range. The western facing mountains are at the greatest risk of seeing enhanced precipitation due to the onshore flow that is apparent at all levels and will be almost perpendicular to western facing slopes. These mountains will help wring out excess moisture from this system. This entire event should remain an all snow event for the forecast area. The only chance of mixing will be during 22Z when the cold front is poised to pass through the area. Snow levels could rise to 3000 ft, according to the MM5, keeping our area all snow, but a slight mixing isn’t out of the question during this time period when sw winds will dominate and as WAA continues. Once the cold front passes, levels drop significantly and the snow/water ratio should increase from 1:10 to roughly 1:15. The freezing level is hard to follow throughout the forecast due to the approaching cold front and associated WAA due to the warm front. There will be a shallow layer of warm air at the sfc. ~34°F, as indicated from a cross section, but this layer will be shallow enough that precip. falling as snow will not have enough time to melt. This will enhance melting on the roads and likely make driving less hazardous before the cold front passes. Determining expected winds was done using MM5 12KM GFS model output. The model forecasted winds staying below the critical level of 25 mph through 03Z on Thursday. This is why we are forecasting the road to be closed to high profile vehicles at 04Z on Thursday since two hours of above 25mph wind will have occurred and gusts over 35 mph are likely as well. Snowfall amounts were also forecasted using the MM5 data. We used one-hour liquid precip. totals for the area and then tried to see how well the one-hour snowfall totals matched with these amounts. They both seemed a couple of hours off each other as far as determining the time that 4” will have fallen. This is likely due to the rain/snow ratio and how the model interprets it.

FORECAST FOR SNOWQUALMIE PASS VALID FROM 4 A.M. WEDNESDAY THROUGH 4 A.M. THRUSDAY. The road should be closed to high profile vehicles at 8 p.m. Wednesday. This will likely be the time when sustained winds of 25 mph will have occurred for 2 hours. Emergency personnel should be activated at 5 p.m. on Wednesday with 4” of snow accumulating by this time. Initial accumulation on the roads will occur at a slower pace due to elevated early afternoon temperatures and therefore it will push the time that the emergency personnel needs to be activated back a few hours. The chain law should go into effect at roughly 2 a.m. Thursday when 10” of snow will have likely accumulated. However, there won’t be enough snow to close the pass at anytime during the forecast period since 20” will not be reached at Snowqualmie Pass from this storm. Total storm totals should amount to roughly 12”.

Exiting longwave trough Jet streak Trough

Both of these soundings show moist layers throughout most of the atmosphere indicating upper level moisture support.

This map shows the low level jet at 850 mb. You can see the transport of moisture at the lower levels and winds just offshore of WA at knots.

The winds take a while to really impact locations away from the coast and therefore we think that high profile vehicles will be alright on the roads through 8 p.m. Wednesday

Critique of our Forecast Our forecast did not validate and we are currently applying for a position with the chain gang. We likely lost WDOT thousands of dollars with our mis-guided forecast. We said that high profile vehicles should be prohibited from driving at 8 p.m. on Wednesday and it gusted to just over 35 mph at 6 p.m. on Wednesday, a difference of 2 hours, fairly accurate. We forecasted emergency personnel to be activated at 5 p.m. on Wednesday since this is when we thought 4” of snow would likely have accumulated on the road. However, this occurred much earlier at around 1 p.m. This was forecasted incorrectly due to our lack of experience forecasting western systems and we also think that we might not have located Snoqualmie Pass accurately on each different model we used. This can have dramatic effects on precipitation amounts when orographic lifting is involved. Lastly, we forecasted the chain law to go into effect at roughly 2 a.m. on Thursday morning. However, this also occurred much earlier at around 8 p.m. on Wednesday when 10” of snow accumulated. This incorrect forecast can also be attributed to similar faults. All in all we did not forecast this event accurately and therefore hang our heads low in the weather community. The one thing that we did forecast accurately is the fact that the snow fall would never reach 20” and therefore the road would never have to be closed. Also, storm totals reached roughly 14.3”, and we forecasted 12”. This was a fairly accurate forecast, and amounts can often be the most difficult to accurately predict.