A Snow Event in Two Acts

Today's snow event will be in two acts, with the first one already begun.  Very light snow is now falling over Northwest Washington (locations such as Bellingham and Orcas Island)--something that is confirmed by the 8:30 AM weather radar (see below)

These snow showers are associated with an approaching upper level trough of lower pressure that is providing upward motion that is leading to a band of clouds and weak snow showers (the satellite picture at the same time is shown below).

But this is not the main act.  That will occur as the upper trough spawns a tight low pressure center in the lower atmosphere, as illustrated in the surface pressure forecast for 7 PM tonight.  An absolutely classical pressure pattern for a snow event over Puget Sound.   The low pressure draws in cold air form the north (note the large north-south pressure change) over Puget Sound and pushes moist air from off the Pacific above the low-level cool flow.

Close in to a major weather event, I like to look at the NOAA/NWS High Resolution Rapid Refresh Model--in which a new high resolution forecast is made each hour.  Below shows the accumulating snow in time.

There is  some light snow reaching Seattle around 11 AM....but this is stage 1 stuff.

The snow extends south by 4 PM at which time the amounts are just starting to increase.  Stage 2 is beginning as the low center develops along our coast.

By 10 PM tonight, there is a lot of snow as stage 2 revs up ...4-6 inches around Seattle and more to the east. There is going to be a region of heaviest snow where the moist flow off the Pacific converges with the cold flow coming out of the north.

And by 1 AM, there is a bit more around Puget Sound, but much more over northwest Oregon.

The essential point in all this, is that things should not be too bad before roughly 2-3 PM, but by rush hour, the situation should be declining rapidly.  The UW is closing around 12:30 PM, which is wise, since it allows folks to be home by mid-afternoon. 

Later today I will talk about the next big snow threats... late Sunday/Monday for SW Washington and Oregon and Tuesday for Washington.  Today is just a "warm up" as cold air remains in place for most of the week.   And no, I won't give a name to the upcoming snow events (but the picture below is a hint)