Monday, January 28, 2008

Snow in Seattle tonight?

Lots of talk this morning concerning the snow potential for the Seattle area tonight. Below I've done my best to pull together what the forecast models, the National Weather Service, word on the atmos street, along with a little of my own intuition to say what I think is the most likely scenario for tonight.

Synopsis...A frontal system will be moving into the area tonight, bringing precipitation into an already cold air mass. However, the front will be bringing warmer air with it. It appears that the most likely scenario tonight is for rain in Seattle at elevations below 500 feet. The greatest chance of snow will be during the day today, ahead of the main system, where the airmass will be unstable. Expect scattered, locally heavy snow. The main system moves in tonight. Expect widespread precipitation to begin between 8 and 10 pm tonight. Precipitation may initially begin as snow, but should turn over to rain by midnight in all areas of the city.

The rest of today...Scattered snow showers with localized accumulations of 1-2" possible. Temperatures in the mid 30's.

Tonight...Rain developing, possibly mixing with snow before midnight, especially at elevations above 500 feet. Chance of precipitation near 100%. Low temperature of about 34, slowly rising overnight. Since there is uncertainty in the forecast, the following are approximate probabilities for (additional) accumulating snow in the city of Seattle at elevations below 500 feet:
0 = 60%
Tr-2" = 30%
2"+ = 10%

Tuesday....Rain, tapering off to scattered showers. Snow level rising to 1000 feet. Highs around 39.

Tuesday, January 15, 2008

Puget Sound Convergence Zone Strikes Capitol Hill

The so called "Puget Sound Convergence Zone" (PSCZ) hit parts of the Seattle area yesterday evening leaving an icy mess for the commuters this morning. Especially hard hit areas were Capitol Hill, downtown, the U-district, and other parts of North Seattle.

The PSCZ occurs after a front passes, when air moving in from the west or northwest can not move over the Olympic Mountains but instead moves around them and converges on the eastern side. This convergence of the winds sometimes results in an often narrow band of precipitation, as apparent on radar last night (first image below). If you look at the weather station observations, south of the precip band winds are southerly and north of the band winds are northerly.

Precipitation was in the form of rain turning to a mix of rain/snow/sleet or completely changing over to snow in some areas. After it stopped, skies cleared and nighttime temperatures dropped to below freezing, allowing everything to freeze.

Photos below:
First: Radar and surface weather observations in Puget Sound Region Jan 14 7pm. The interstates are shown in red and Capitol Hill is marked as a reference.
Second: Harrison St looking west Jan 15 8:30am.
Third: Harrison St from Summit looking east toward Broadway. Despite the icy conditions, schools were not delayed and school buses were still running normally.
Fourth: Buses were rerouted along Broadway Avenue, and delays occurred. Bus stops were unusually crowded, such as the one at Broadway and E John.
Fifth: This was our first winter weather event without you Taco Bell, we'll miss you.

All pictures of Capitol Hill streets were taken at approximately 8:30am on Tuesday January 15, 2008.