I’ve been posting relatively brief “Snupdates” on WeatherTogether’s Facebook Page tonight, but I’m seeing some interesting new developments on radar and thought I’d write a more in depth blog post about what I’m seeing and it’s consequences for snow to ring in the New Year.
Let’s start by looking at the current radar image.
This is actually a pretty incredible shot. It appears to be a convergence zone, but something is different. Convergence zones only exist in the Puget Sound region, while this feature is a band of snow and graupel (“snow pellets”) extending all the way from Port Angeles ESE towards Index in the Cascade foothills. Why are we seeing this unique feature?
The answer is that there is actually a very localized frontal feature over the Strait of Juan de Fuca. You can see how the wind barbs are converging, and this is what caused this line of precipitation to blossom up as the night went on.
Credit: University of Washington Atmospheric Sciences
Take a look at the sequence of radar images to get an idea of how this line of precipitation formed.
Though this front was weak, the precipitation with it is actually quite intense. The convergence at the surface leads to uplift, and because we have an unstable atmosphere (large decrease in temperature with height), this air can continue to rise and form clouds and precipitation. I had a report of THUNDERSNOW of all things from Brie Hawkins of Little Bear Creek Weather in Woodinville along with heavy graupel (snow pellets) later transitioning to moderate snow.
This reminds me of a similar event on November 27, 2006 during a Monday Night Football game between the Seahawks and the Packers. A narrow but intense front came through, dropping graupel and snow in many places. Behind the front, temperatures plummeted and one of the worst traffic jams in Seattle’s history ensued. Heck, some people might still be trying to get home.
Temperatures are nowhere near as cold behind this front as they were during that one and we have less moisture to work with, but we will still be below freezing in many locations Monday morning. Though most people have Monday off, those that will be on the roads should prepare for ice.
The latest “HRRR” (High Resolution Rapid Refresh) model gives the Seattle area very little snow, as it shows this front quickly coming through without much precipitation.
Credit: Weather Bell
Tonight’s WRF underestimated the formation of this front, but brings some more moisture through the area around 5 AM that would fall as snow.
In both models, Southwestern Washington actually gets the most snow. This is because the low dropping down from the north is directing more moisture into that region. Temperatures are still generally above freezing for most places, but they will continue to cool as this low drops south and the night goes on. Moreover, heavy showers there could bring snow levels down to the surface. Olympia is currently reporting snow as of 12:30 AM.
My latest forecast calls for 1-2 inches in NW Washington due to steady, light snow associated with occluded front of the low pressure system dropping southward and a trace to 2 inches everywhere else along the I-5 corridor due to showers, with highest amounts between Olympia and Chehalis where there will be the moist moisture. Amounts will be less here in the Puget Sound Region due to lack of moisture, but the highest amounts will likely occur in the Central Sound where there is more convergence. Keep an eye on the radar… that is your greatest tool at this point.
http://www.atmos.washington.edu/weather/radar.shtml
Happy New Year!
Charlie