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.|
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.|
|Fig 3 -- 36 hour forecast of the previous 24 hours of snowfall accumulations at 12Z, Thursday, Feb. 24, 2011.|
|Fig 3 -- 24 hour forecast of the previous 24 hours of snowfall accumulations at 12Z, Thursday, Feb. 24, 2011.|
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.|
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.|
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:
- Orographic lift as air is forced to rise up over the mountains (the Olympics and Cascades).
- Convergence due to the Puget Sound convergence zone or convergence along the Strait of Juan de Fuca.
- 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...