A Wild Tuesday Ahead: Snow Showers Followed by Heavy Rain, Warm Temperatures and Wind

Tuesday is going to be a day of great contrasts in the Northwest, and particularly west of the Cascade crest.

It will start off cold--in the lower 30s-- with overcast skies.

As the clouds thicken, light snow could fall during late morning or early afternoon over some of the western lowlands.  Nothing to worry about.

The temperatures will rapidly warm and the snow will turn to rain during the afternoon

And then the rain will get steadily harder, while the winds increase and turn blustery.

Cold, snow, rain, wind, followed by warmth.   All the basic food groups of Northwest winter weather.  What a wonderful place to live!

Tonight, a plume of clouds and moisture is approaching our region (see below).

In this blog I will show you a new graphic from the UW high-resolution modeling system.  One that shows precipitation, but separates wet stuff into rain (gray scale) and snow (colors).  Very educational.

At 10AM tomorrow, rain is just reaching Puget Sound, where marginally cold enough air (for snow) will be in place (this graphic is actually showing the 3-h precipitation ending at 10  AM).  Snow starting in higher elevations.

But there will be evaporation and melting of snow falling in from aloft.  As a result, the next three hours (through 1 PM) is forecast to bring light snow to Seattle.  Wet snow.

 But is won't last.  Three hours later, there is rain in Seattle, with some snowflakes over Snohomish County and the western slopes of the Cascades.

 For the 3-hour period ending 7 PM tomorrow, rain, in some places heavy, has extended over all the lowlands, with snow retreating to higher elevations as the air warms.

 And by 4 AM Wednesday, heavy rain is now widespread and snow is only falling in the high Cascades.

What about winds?   The forecast winds of Seattle predicted by Seattle WindWatch are shown below.

This is a sophisticated ensemble (many forecast) product, which suggests an acceleration of the winds to roughly 30 mph tomorrow night, with the highest resolution forecast going for over 40 mph.  Not a huge windstorm, but with heavy rain, you will notice the breeze.

And did I mention the expected river flooding from all the warm rain expected during the next two days? Loads of local rivers will be at bankful or flooding (see the forecast from the NWS River Forecast Center below)

So find a good book or video, and be ready to settle down in a cozy spot tomorrow evening while the rain is providing some pleasant white noise outside.

Cold Air is Surging Into the the Northwest

After a period of wet conditions with far above-normal temperatures, much cooler air is flooding into the region behind a Pacific cold front.

The latest visible image from the GOES-17 satellite clearly shows the story, with the long cloud band of the front  just starting to exit western Washington, while cold, unstable air (indicated by energetic convective activity--indicated by the popcorn looking clouds) is found offshore.

The current weather radar imagery below, illustrates the weakening remnant of the frontal precipitation, now over Oregon.  Behind the front, a Puget Sound Convergence zone has formed north of Seattle.  You can see some of the cold, instability showers moving in over the coast.

This cold, unstable air mass, with lots of convective showers, should drop the temperatures by 10-20 degrees during the next day....already temperatures around western Washington are in the 40sF, after climbing into the mid 50s yesterday.   The snow level will decline to roughly 500 ft, so the higher hills may get whitened and don't be shocked if a few wet snowflakes get mixed in later today and Sunday.

The mountains are now changing over from rain to snow and the predicted accumulated snow total over the next 48 h (through 4 AM Monday) projects as much as 6-18 inches in terrain.  Some lowland areas (e.g., around Bellingham and the coast) may get some very light snow. 

Importantly, the road surfaces are relatively warm after the recent mild weather, so snow should melt quickly if it falls.  This air is not as cold as the previous event and there is no active surge of arctic air through the Fraser River Valley.

Finally, this winter so far has been one of extremes.  We start dry, then go wet.  And we have alternated between warm and cool periods (see temperatures at Sea Tac for the last 12 weeks, below).  A bipolar winter, which perhaps might reflect the national mood about other things....