Slushmageddon

Will update at 10 AM Monday
________________________________

You heard about about Snowmageddon in previous blogs.

Well, tomorrow (Monday) it looks like we will transition to Slushmageddon during the evening.

As forecast (and right on time), light snow started falling around Puget Sound around 4 PM and since then 2-4 inches have fallen in most places. Many local roads are covered, so be careful out there...with temperatures near freezing, the roads are slippery.

The latest radar image (8:30 PM) shows most of the action is over from this disturbance for Puget Sound, with the snow band moving east, but a hint of some convergence zone activity in Snohomish County.

Local departments of transportation have overnight to clean this up, so I suspect the main roads will be passable in the morning.

The new model runs are in and confirm that snow will start falling tomorrow afternoon and a messy situation will occur later in the day.

The morning will be fine, but precipitation will reach central Puget Sound between 2 and 3 PM (earlier to the south).  With cold air in place, precipitation will start as snow.  But as the event proceeds, warm air will surge in aloft, and eventually precipitation will transition to cold rain.  And then Slushmagedon will begin.

At 4 PM tomorrow the low will be west of our north coast and a warm front will be pushing in (solid lines are pressure and the colors are temperature, with blue cold enough to snow).   With shallow cold air in place Puget Sound will get snow at that point.

 By 4 AM Tuesday the low will be close to our NW coast and the air will be warm enough for rain to extend over Puget Sound

 The low hardly moves during the next twelve hours, but the air will have warmed and rain will be falling over western WA.  BC is going to get hammered with snow though.

I have a great graphic to illustrate all this, with colors showing snowfall and gray shades indicating rain.  For the 3-h ending 4 PM, expect snow over central Puget Sound, but rain to the SW.

For the next three hours, snow barely holds in Seattle, and extends to the north.  Rain will just be reaching Sea Tac.

By 4 AM Tuesday, rain will extend over the lowlands.

The total snowfall over the region shows mega-snow over the mountains (skiers will rejoice) and perhaps 3-4 inches over Seattle northward.

So you will wake up tomorrow with 2-4 inches of fresh snow.  Midafternoon more snow will start and I expect that around 2-4 PM snow will start again over central Puget Sound.  In the south Sound, precipitation will quickly turn to rain, but from Seattle northward there will be another 2-5 inches before it turns to rain during the evening.  The evening rush hour Monday may be problematic with fresh snow.

And Tuesday morning you will wake up to a slushy mess.

This is the kind of forecast that can easily go wrong.  If the low ends up 100 km farther south, Slushmagedon becomes Snowmagedon.  The ensemble forecasts have a wide spread.  But I believe the above is the most probable evolution at this point.