Increasing Probability of Lowland Snow Late Thursday Afternoon Through Early Friday

Update Wed AM.   The latest runs are shifting the precipitation several hours later, with a start on the early evening on Thursday.  So the Thursday PM Seattle commute may be OK.  Each model run is shifting the low farther offshore and delaying the precipitation onset.  All runs suggest a large east-west snow variation, from little along the western slopes of the Cascades to serious amounts (roughly a half foot) over the area west and southwest of the Olympics (e.g., Hood Canal area).
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It is becoming increasingly likely that we will see some significant snowfall over the lowlands late Thursday afternoon into early Friday morning, with amounts of up to 2 to 5 inches before the precipitation turns to rain.  But there will be a large variation of snow across the lowlands, with less near the western Cascade foothills.

My work tonight is made more difficult by major systems failures at the National Weather Service, slowing of Comcast internet (surprise), and the lack of availability of the UW WRF runs (probably due to the NWS problems).

Thursday afternoon, cold and dry air will be in position over western Washington and a strong warm front will be moving up the coast (see graphic for 4 PM showing low-level temperature, sea level pressure, and winds).  Where you see blue colors the air will be cold enough for snow to reach the surface.

Moisture associated with the warm front will be able to fall through the cold air as snow.   By the next  (Friday) morning, as the warm front and associated low moves northward, rain will spread into western Washington (the situation at 4 AM Friday is shown).

The latest run of the UW WRF forecast system shows snow over the region of various amounts.  For example, the 24-h snowfall ending 4 AM Friday indicates 2-6 inches over over the western Kitsap Peninsula and  more over the high terrain, and several inches inches over NW King County.  There is less over the eastern Seattle suburbs due to easterly flow down the western Cascade slopes (downslope flow causes drying).

The European Center snow depth forecasts for 4 AM Friday (below) shows 2-3 inches around Seattle, with less right next to the Sound and over the eastern suburbs.

Talking over easterly flow, there should be strong winds in the western Cascade foothills (see graphic of max gusts at 10 am on Thursday).   45 knots in some locations SE of Seattle.   Very strong winds over the eastern Strait and offshore water.

This situation is more threatening than Monday, since we will have cooler and drier air in place and the amount of precipitation is greater.  Furthermore, with low pressure approaching cold air will be pulled into the region through the Fraser River Valley.

The timing right now suggests that snow will reach the Seattle Metro area between 4 and 6 PM Thursday.

The snow forecasts are critically dependent on the amount of precipitation and the exact temperature structures over us... and the models have been shifting in their solutions somewhat.  Thus, there is still considerable uncertainty in the forecasts, which I will quantify using ensemble predictions tomorrow.   But the nearly all of the ensembles I have seen so far start this event off as snow.  All warm up the area by Friday morning.  The question now is really about the amount of snow and the exact timing.