Almost cold enough...but not quite

There is another chance of snow for the lowlands (including the city of Seattle) for Saturday. A low pressure will be moving southeast out of the Pacific and likely moving south of us Saturday night (see top panel in Figure). This is generally a good setup for snow for the lowlands because it allows moisture to come in from the Pacific and cold air to filter in from the interior over western WA. HOWEVER, the temperatures are not looking quite cold enough for snow down to sea level, and most of the moisture may stay south of the area, leaving only a marginal chance of lowland snow at best.

The bottom panel shows the probabilites for freezing temperatures and the probability for precipitation Saturday night using a combination of the forecast models. Using that and other information, here is the forecast:

Saturday...Mostly cloudy with showers likely. High temperatures around 41. Chance of precipitation 70%. Snow levels around 500 feet. Chance of snow at sea level less than 20%.

Saturday night...Mostly cloudy with scattered showers. Lows around 34. Showers may mix with snow at times, especially on the higher hill tops. Chance of snow at sea level 20%.

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.

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.

Cold rains for Seattle this weekend

Synopsis...2 weather systems will be affecting the Seattle area this weekend. They are labelled "1" and "2" on the weather map to the right, which shows these systems circled on a satellite image with their forecast movement. System 1 will be bringing widespread rain into the area tonight, with scattered showers lingering during the day Saturday. System 2 will affect the area on Sunday. Rainfall amounts of up to an inch expected in Seattle this weekend. Snow levels will be around 2000 feet. Forecast models are in good agreement. On the scale very low, low, moderate, high, very high, confidence for this weekend's forecasts are high for system 1 and moderate for system 2, as the timing of the rains could be slightly off for Sunday.

Tonight...Rain developing, should become widespread by midnight. Cold with temperatures in the upper 30's. Chance of rain near 100 percent.

Tomorrow...Rain early, tapering off to scattered showers, possibly heavy at times. Chance of rain 70 percent. Highs around 45.

Saturday night and Sunday...Rain developing Saturday night and possibly lasting through much of the day on Sunday. Rain will taper off to showers behind the cold front, forecast to pass Sunday afternoon at this time. Low of about 38 and high of about 45. Chance of rain 90 percent.

Weekend forecast update

For Seattle.
Synopsis...A high pressure is currently settling in over the region providing cold and dry conditions. Temperatures will drop below freezing tonight and Saturday night. This pattern will persist through much of the weekend, however a weak upper level disturbance may bring a chance of precipitation on Sunday. With cold air in place, the precipitation could be in the form of snow. There is very little confidence in the forecast for Sunday as the forecast models are in great disagreement at this time.

Forecast confidence/certainty explanation:
Forecast certainty (FC) levels: very low, low, moderate, high, very high.
FC = very high means there is a lot of certainty in the forecast.
FC = very low means there is little certainty in the forecast.

The forecast:
Friday night...decreasing clouds and cold with lows in the mid 20's to around 30 in urban areas near water. FC = high.

Saturday....Partly cloudy with highs around 40. FC = high.

Saturday night...Partly cloudy with lows in the mid 20's to around 30 in urban areas near water. FC = low.

Sunday...Increasing clouds with a 30% chance of precipitation. Highs around 40. There is a 20% chance the precipitation will be in the form of snow. FC = very low. Monitor the forecasts throughout the weekend for updates.

Weekend outlook

An upper level trough combined with offshore flow at the surface will give us a cold but dry weather pattern for this weekend. Temperatures will be below average as highs may struggle to get out of the 30's Friday and Saturday. With a lack of cloud cover, expect temperatures to drop below freezing Friday and possibly Saturday morning. There has been some talk of snow on Sunday, but due to a lack of moisture and only marginally cold temperatures, the prospect of getting more snow is very unlikely.

Wettest day in Seattle history, 2nd wettest day in Sea-tac history

Yesterday, December 3, 2007 was the 1st wettest day on record (running 24-hour totals) in the city of Seattle and 2nd wettest on record at Sea-tac airport. The pattern that set this up was the so called "Pineapple Express" where the jet stream was pointed from near Hawaii straight into western Washington providing plenty of unobscured moisture. Further, there was a lot of melting snow from a system on Saturday that gave 1-4 inches of snow in the Seattle metro to several feet in the mountains.

Be careful when reading articles from the media, as they are mixing and confusing 24-hour totals (best measurement of "storm" events) with calendar day totals (inaccurate representation of a "storm" event).

Here are some peak 24-hour precip amounts ending Monday evening 3 Dec 2007:
Bremerton, 10.78 inches
Sand Point NWS: 5.07 inches (*** All time 24-hour rainfall record ***)
115th and Greenwood: 4.65 inches (2100 PST 3 Dec 2007)
Lake Forest Park, 4.44 inches (2100 PST 3 Dec 2007)
Pacific Science Center, 4.40 inches (2100 PST 3 Dec 2007)
Capitol Hill, 4.39 inches (2100 PST 3 Dec 2007)
UW, Atmospheric Sciences U of WA: 4.37 inches (2200 PST 3 Dec 2007)
Boeing Field, 4.33 inches (2200 PST 3 Dec 2007)
Seatac, 4.11 inches (***2nd wettest 24-hour period on record; record = 5.01 inches Oct 20, 2003***)
Sequim, 0.28 inches (The Olympic Mountain rain shadow really works!)

Here are some peak wind gusts:
Lincoln City, OR: 108 knots (125 mph)
Cape Blanco OR: 95 knots (109 mph)
Cape Arago OR: 78 knots (90 mph)
Clallam Bay: 78 knots (90 mph)
Newport OR: 71 knots (85 mph)
Destruction Island WA: 81 knots (93 mph)
Sekiu, WA: 81 knots (93 mph)
Tatoosh Island WA: 75 knots (86 mph)
Cannon Beach, OR: 69 knots (80 mph)
Bellingham, WA: 63 knots (73 mph)
Aberdeen, WA: 54 knots (62 mph)
Long Beach, WA: 52 knots (60 mph)