Mountain Snowfall Forecast Gaining More Credibility

Friday, October 7, 2011
11:15 A.M.

Looking at the WRF-GFS models from the UW this morning, I have some good news for winter sports and water supply enthusiasts everywhere! The forecast for significant snowfall in the mountains is gaining more credibility. It isn’t a slam dunk, but it definitely is looking more likely after the most recent model runs.

As I stated yesterday, the NWS from Seattle has access to the European (ECMWF or Euro) model, but I do not. The Euro shows heavy snowfall in the mountains above 4,500 feet. The GFS doesn’t quite show that yet. However, the most recent runs of the GFS are becoming more in line with the Euro.

First, let’s check out the jet stream. This graphic shows the winds at the 300mb level, and you can see that there are strong winds ramming right into Western Washington. Strong jet streams generally give stronger storms, and the orientation of the jet stream in the next couple days is expected to be perpendicular to the Cascades, resulting in favorable orographic enhancement for heavy precipitation!

Valid 05:00 am PDT Tue, 11 Oct 2011 – 96hr Fcst – UW 36km 12z WRF-GFS 300mb isotachs
You can clearly see a strong jet stream flowing right over the Pacific Northwest. The windiest parts of the jet stream could approach 160 knots! 
Now, let’s look at the 48 hour precipitation.
Valid 05:00 am PDT Wed, 12 Oct 2011 – 120hr Fcst – UW 12km 12z WRF-GFS 48-hour precipitation
Just to clarify, this graphic shows the precipitation forecast for the past 48 hours, not 48 hours in the future. You can see anywhere from 1 to over 3 inches of liquid precipitation throughout the Cascades, with more at higher elevations. I didn’t show the snow graphic because I believe that the GFS is too high with the snow level. Regardless, one inch of rain is generally equal to 10 inches of snow. This ratio will be lower where is is warmer (wet snow) and higher where it is colder (powdery snow), but it is a good benchmark. At face value, we could see snowfall totals of around 30 inches in isolated spots in the Cascades, and maybe even 40 inches at places on the volcanoes. 
Hopefully we will see some heavy duty snow! But even if we don’t in the next week, we should see gobs in the winter. La Nina baby!
Thanks for reading,
Charlie
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