Friday Snow: Do the Models Have It Right?

 11:15 AM Update:  Snowing at Orcas Is, Friday Harbor, and Whidbey Is. Naval Air Station

Tomorrow morning will be a great test of weather prediction technology.  The high-resolution models, the leading edge of numerical weather prediction, are going for a few inches of snow over much of the western Washington interior.  In contrast, the much older statistical guidance products (Model Output Statistics, MOS) that use past statistical relationships and coarser model output have no snow.  And the temperature are really not that cold over the region:  at or near freezing at the surface at most locations.  However, we all that said, I believe the high-resolution models are correct for a number of reasons, most of which I won't go into here. Snow is coming.

So what do the latest model runs indicate?  The high-resolution US WRF model has snow, but delayed a few hours from the previous predictions.  Let me show you a new product produced by David Ovens, the person you can thank for building and running the UW WRF model system.  In these new graphics, 3-h snowfall is in color and rain is shown by gray shades.  This way you can tell where it is raining and snowing on the same chart!  Very nice. 

In the new runs, the snow holds off in the central Sound until about 3-4 AM.  Here is the three-hour total rain and snow for the period ending 4 AM  Snow over NW Washington, but just reaching Seattle.

 For the three-hour period ending 7AM, the Puget Sound interior gets hammered with roughly 2 inches. Rain is spreading to the coast and the western Strait of Juan de Fuca.  The eastern slopes and crest of the Cascades are getting lots of snow.

 The three-hour period ending 10 AM has more snow over Seattle, but the warm air wrapping around the Olympics is turning precipitation to rain its wake over the Strait and south of Tacoma.  Snow continues over NW Washington and the western Cascade slopes.

 By 1 PM precipitation has decreased greatly over the Puget Sound lowlands, which are now in the rain shadow of the Olympics (the winds turn westerly as the front goes by).  The lowland precipitation is mainly rain during this period. 

I like these new graphics.

What about the 24h snow total ending 4 PM?   More than a foot in the mountains and 2-4 inches around Seattle. More over parts of NW Washington.

 Now I believe this forecast is a good one, although the amounts could be off by a few inches.  Why do I believe this?  First, the WRF model has stuck with this solution for many prediction cycles--there is stability to the solution.  Meteorologists have less confidence in model solutions that jump around a lot.  Second, several different models are giving basically the same results, including the UW MM5 model initialized by a completely different NWS model, the NAM.   Third, one of the most advanced high-resolution forecasting systems, the NWS High Resolution Rapid Refresh (HRRR),which does a sophisticated initialization each hour with many different local datasets, is also going for the snow.  Here is the 9, 12, and 15 hr forecasts of accumulated snowfall from this system, valid at 2 AM, 5 AM, and 8 AM.  Same basic idea.

 The implications of all this is clear:  our best technology (which does have weaknesses) is going for snow later tonight and extending to 8-9 AM.

This is a complicated situation with fronts, mountains, melting snow, sublimation and evaporation, and many other processes acting simultaneously.  And the front is coming off the ocean.   It will be fascinating to see if we get this right.

You see why forecasting is so much fun?  Combines science, technology, and gambling.   Meteorologists don't need casinos, we've got something much better.

I will be doing an update on KPLU FM Friday Morning at 9 AM.

Help The Seattle School District Pick Better Math Textbooks for K-5

Most Seattle elementary schools are using very poor math textbooks:  Everyday Math.  FINALLY, the district is replacing them and you can help ensure that decent books are selected. For the next few weeks the finalist books will be available at the Seattle Schools HQ and you can peruse them and provide comments.   Here is more information on this opportunity.  With information form concerned citizens and parents, a math committee will make recommendations to the district.