The 1 PM visible satellite image shows the approaching front/low pressure system with substantial convective clouds behind it. The leading edge of the front is now stretching from Forks, on the NW coast, to NW Washington.
The latest sounding above Sea-Tac airport (12:30 PM) shows a freezing level around 950 ft, which means the atmosphere is now close to cold enough for snow to reach sea level. This morning's model output suggests some warming aloft this afternoon.
However, the time-height cross section above Seattle does not suggest much cooling aloft, which gets me worried, as does the fact that temperatures are cooler than forecast.
Thus, I have some concerns that we might see a switch to snow earlier than forecast over Puget Sound (which was around midnight) and during a period when precipitation is heavier (after midnight there are only a few snow showers).
I am also concerned that the upper level pattern (see 500 hPa heights below for 1 AM Sunday) looks fairly close to the canonical snow pattern.
In such a situation difficult, rapidly changing situation, one turns to the HRRR...the constantly updated NWS High Resolution Rapid Refresh model. The 1-h snow forecast ending 4 PM shows snow reaching the surface over NW Washington.
By 7 PM, there is light snow reaching Everett and the eastern Seattle suburbs. The snow areas are where the precipitation rates are large enough to drive the snow level to the surface.
The 2 AM and 3 AM forecast show snow over Puget Sound (particularly Snohomish County and north King County in a convergence zone.
This is an extraordinarily difficult forecast because the temperatures are so close to the line for rain versus snow. The classic western WA dilemma. And above is only one solution. There are large uncertainties in the model solutions (see NWS Short Range Ensemble Forecasts of total snow) for Seattle from nearly no snow to several inches.
The bottom line: there is a good chance of snow showers later this afternoon and evening, particularly north of Everett. As the atmosphere cools down further later this evening and a convergence zone sets up, several inches could fall north and east of Seattle after midnight. Will have to watch this situation very carefully during the afternoon, as my colleagues at the NWS will certainly be doing.
The latest radar indicates showers along the coast and over NW Washington, all of which are reaching the surface as rain. A few snow flurries have been noted around the region.
The latest sounding above Sea-Tac airport (12:30 PM) shows a freezing level around 950 ft, which means the atmosphere is now close to cold enough for snow to reach sea level. This morning's model output suggests some warming aloft this afternoon.
I am also concerned that the upper level pattern (see 500 hPa heights below for 1 AM Sunday) looks fairly close to the canonical snow pattern.
In such a situation difficult, rapidly changing situation, one turns to the HRRR...the constantly updated NWS High Resolution Rapid Refresh model. The 1-h snow forecast ending 4 PM shows snow reaching the surface over NW Washington.
This is an extraordinarily difficult forecast because the temperatures are so close to the line for rain versus snow. The classic western WA dilemma. And above is only one solution. There are large uncertainties in the model solutions (see NWS Short Range Ensemble Forecasts of total snow) for Seattle from nearly no snow to several inches.
The bottom line: there is a good chance of snow showers later this afternoon and evening, particularly north of Everett. As the atmosphere cools down further later this evening and a convergence zone sets up, several inches could fall north and east of Seattle after midnight. Will have to watch this situation very carefully during the afternoon, as my colleagues at the NWS will certainly be doing.
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