It Will Snow over Central/Northern Puget Sound However It Will not Final Lengthy

Essentially the most highly effective high-resolution meteorological instruments at the moment are out there for the snow occasion tomorrow evening.   Elements of the lowlands will get snow–up to a number of inches in locations.

However it will not final.

And the scenario tomorrow isn’t a great one for accumulating lowland snow in our area.

The Golden Rule of Snow

Snowfall is NOT snow depth.  

That is going to be essential for the upcoming occasion.   And this subtlety is usually lacking from the analyses of some beginner climate web sites.

A key subject is the temperature of the bottom and significantly roadway surfaces.  Under are this morning’s values from the town of Seattle’s Snowwatch web site, with the temperatures within the packing containers exhibiting the roadway floor temperature.   Most are within the higher 30s to close 40F.

The air above us proper now has a freezing degree of about 1300 ft (see under from SnowWatch under for SeaTac Airport).  Snow melts under that degree,  taking about 1000 ft to achieve the snow degree, beneath which it’s all rain.  So the present snow degree is about 300 ft.

 

With showers transferring by way of this morning, among the larger hills within the area received some snow (see under).  And the Cascade passes have been hit laborious, with chains required at Stevens Cross proper now.

Round 1000 ft ASL in Bellevue.  Image courtesy of Dr. Peter Benda

Tomorrow’s Snow Occasion

    As famous in my earlier weblog, a entrance will method our area tomorrow afternoon and night, bringing precipitation.  Cool air will likely be in place…. simply on the sting for being cool sufficient to permit snow to achieve sea degree.

However there’s something else that can come into play.  Cooling by melting and evaporation.

As precipitation falls into drier air under, it will possibly cool by evaporation, serving to to drive the snow degree to the floor.   Snow falling into above-freezing air additionally melts…and that takes vitality.   The result’s much more cooling, with the quantity of cooling depending on precipitation depth.

On the opposite aspect, heat ocean air will likely be transferring in behind the entrance!

Now that you’re all skilled up….let me present you some graphics I hardly ever current on this weblog: forecast precipitation maps that present whether or not the precipitation reaching the floor is rain (black and grey shades) or snow (colours).

And to make it even higher, these forecasts are from the highest-resolution UW system (1.3 km grid spacing).

First, right here is the forecast for the 3-h whole precipitation ending at 4 PM Tuesday.  Snow across the Olympics Peninsula, however heavy rain offshore and lightweight rain over the inside.

For the three hr ending 10 PM Tuesday, the rain (and robust southerly winds have reached the coast), whereas snow intensifies over the Olympics and the Kitsap Peninsula, the place southeasterly upslope circulation intensifies the precipitation.  Some gentle snow from Seattle downtown northward.

The subsequent three hours (ending at 1 AM) are the middle of the snow enjoyable for northern Puget Sound (see under).  The upper elevations from Lynnwood to Everett might get a number of inches of snowfall (NOT SNOW DEPTH).

However notice the rain (black colours and grey) quickly transferring in from the south.

Depressingly,  the identical graphic ending 7 AM reveals rain sweeping in over the lowlands, however snow persevering with at larger elevations.

The 24-h snowfall whole ending 4 AM Wednesday reveals the snow north of Seattle and Kitsap.

The snow depths (in metric models…sorry) at 4 AM Wednesday are far much less spectacular, with solely restricted lowland areas attending to .1 meter (3.7 inches)….and that will be quickly melting on the time.

I instructed my class that I used to be fairly sure Wednesday wouldn’t be a snow day on the UW.  An excellent occasion for skiers although….