Wednesday, February 23, 2011

Snow in Seattle (and a moderate risk!)

Quick post tonight as I have some other things I need to take care of.  But, I promised to talk about snow in Seattle today...

First, as many people have noted, the SPC has upgraded to a moderate risk for the severe weather potential tomorrow in the southern US:
Fig 1 -- SPC day 2 convective outlook as of 1730Z, Feb. 23, 2011.  From the SPC website.
So things are continuing to shape up there...

However today as promised I wanted to focus on the snow event in Seattle--and how locationally variant it has been.  Models have shuffled back and forth amazingly over just where the snow is going to fall.  Here is what the local 4km UW-WRF model was forecasting would be the 24 hour snowfall accumulations as of tomorrow morning--basically, how much snow (and where) the model predicted would fall with this event.  This first image is what the model was saying on Tuesday morning:
Fig 2 -- 48 hour forecast of the previous 24 hours of snowfall accumulations at 12Z, Thursday, Feb. 24, 2011.
Back then, note the heaviest snow was forecast to be along the northern slope of the Olympics, a lot along the Cascades, and basically heavy snow to the north of Seattle.  Compare that to what the Tuesday evening run of the model said for this same time of snow accumulation.
Fig 3 -- 36 hour forecast of the previous 24 hours of snowfall accumulations at 12Z, Thursday, Feb. 24, 2011.
Dramatically different. The snow accumulations moved much, much further south.  In this model run the heaviest snows seem to fall under a band that goes right over downtown Seattle.  The snowfall amounts are also forecast to be in the 12 inch+ range across the Seattle area.  That strong, thick band over the central Puget Sound region looks a lot like an enhanced Puget Sound convergence zone.  The formerly snowed-in north also seems oddly dry as compared to the previous model run.  Now for this morning's model run:
Fig 3 -- 24 hour forecast of the previous 24 hours of snowfall accumulations at 12Z, Thursday, Feb. 24, 2011.
Now the heaviest snows have moved back north again, with only two or three inches over Seattle.  This is one reason there has been a lot of confusion surrounding this event--the models just haven't agreed.

So far today, the northern snow solution has been favored--five to six inches of snow had fallen in areas north of Everett as of the middle of this afternoon.  But the snow is now beginning to move south--and is definitely linked to a convergence zone.  Here's a radar image from early this evening:
Fig 4 -- KATX 0.5 degree base reflectivity from 0115Z, Feb. 23, 2011.  
Surface observations are overlaid on the radar image above.  Note the line of enhanced reflectivities marching south through Seattle at that time.  This line was accompanied by a huge graupel downpour and has now given way to snow behind it.  That definitely looks like it's being enhanced by the convergence zone--note that the wind barbs to the north of the line are showing winds out of the north and to the south of the line we see winds out of the south.  Classic convergence zone.

But is that really a convergence zone?  That band of enhanced snow seemed to be moving pretty far south and east an hour and half later:
Fig 5 -  KATX 0.5 degree base reflectivity from 0245Z, Feb. 23, 2011.
It looks more like a front now.  In fact, most analyses on the local news call this an "arctic front" pushing south through the area.  The location of that front is going to have a lot to do with the location of the surface low off the coast.  So this front's movements will also factor into what locations are going to receive enhanced snow amounts.

So what are we expecting to happen?  Currently the surface low is sitting just off of Cape Flattery--the northwest tip of the Olympic peninsula.  As that low shifts south, the "arctic front" (and the cold air to the north of it) will shift south as well.  Note, however, in the radar image that the snow is rather diffuse--it's not one big huge sheet of reflectivities like we usually see.  This hints that there isn't as much lift as we'd be expecting for a widespread snow event.  This also leaves only three real sources of lift:

  1. Orographic lift as air is forced to rise up over the mountains (the Olympics and Cascades).
  2. Convergence due to the Puget Sound convergence zone or convergence along the Strait of Juan de Fuca. 
  3. Convergence along the cold front moving south through the sound.

This means if you're not in the mountains, the snow is probably going to be rather localized.  As the local convergence zone(s) fidgets around tonight, lift over the convergence zone will cause areas of snow to move around throughout the Puget Sound region, probably trending further south as the evening wears on.  I really can't see us getting that much accumulations--certainly not the foot that was hinted last night.  I'd guess only an inch or two at most in the Seattle area.  However, if the convergence zone decides to park over a certain area, the snow could really accumulate.

Of course, the mountains will get a ton of snow.  They have built in lift around them...

So, in short, this isn't really like the big snow event of last November.  Roads may get a bit slick, but things shouldn't be too bad...

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