textproduct: Seattle/Tacoma

SYNOPSIS

A frontal system passing through western Washington will result in steady rainfall throughout the Thanksgiving holiday before tapering off overnight tonight. The weather pattern will trend drier and eventually cooler Friday into early next week.

SHORT TERM /TODAY THROUGH SATURDAY/

Latest radar imagery taking the shape of an upside-down L this morning as precip associated with the warm front continues to lift into the northern third of the CWA while precip associated with the cold front moves eastward through the western half of the area. While the pattern and rate of movement could hint at things starting to taper off earlier than forecast, upper level support from the main trough...expected to move through in the afternoon and evening hours...will likely keep rain in the forecast for the bulk of the day.

In fact, models are a little more sluggish in ejecting this upper level low, so while PoPs are expected to gradually taper off overnight, sufficient moisture may be present into Friday morning to keep at least a chance of showers in place for the eastern half of the area. From here, models remain consistent with bringing a pretty healthy upper level ridge onto the coast Friday, resulting in the bulk of the day being dry. Solution posed by the deterministic ECMWF, with a shortwave impulse making its way south along the leading edge of the ridge, bringing some rain back to the area Saturday seems to have caught on with both the deterministic GFS and a majority of ensemble members. That said, no solution is terribly impressed with the amount of moisture with this feature, bringing PoPs to low-end Chance /30 pct/ at best and any rainfall generally well lower than 0.10 of an inch.

Temps continue to sound like a broken record with daily high temps each day of the short term generally in the upper 40s to lower 50s. Overnight lows however will see a cooling trend, starting off in the lower to mid 40s tonight, dropping a few degrees Friday night to the upper 30s to lower 40s before widespread temps in the 30s set up shop for Saturday night.

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LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

The aforementioned impulse wastes no time getting out of the area by Saturday night allowing ridging to once again hold court over the eastern Pacific. This will at the very least keep the remainder of the weekend dry. The ridge will start to break down at the start of next week, however how quickly this occurs is a point of contention between the two deterministic models: GFS keeping Monday mostly dry while ECMWF has moisture entering the area as early as Monday afternoon. Ensembles do not rule out the latter, but at this time seem to be siding with the former. Current NBM is siding with the majority on this one, so will leave this as is for the forecast. Very strong high pressure remains over the eastern Pacific throughout this and once again asserts itself, returning dry conditions to the area by midweek.

Daytime highs continue the trend established in the short term, generally in the upper 40s to lower 50s. With the flip-flopping activity being just enough to bring clouds into the area from time to time, overnight lows will nudge upward, mainly in the upper 30s to low 40s, however the weak system expected for Tuesday will impact Monday's overnight lows, keeping them in the lower 40s.

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AVIATION

Southeast surface flow will persist for most terminals as the low pressure system nears the coast. Expect a transition to ENE flow this afternoon, with gusty conditions along the immediate coast and Chehalis Valley to 35 mph. Periods of rain through the afternoon before more widespread rain arrives late this afternoon into the overnight hours. Expect MVFR to IFR cigs in the heaviest rain showers throughout the day, with visibility reductions as well. Conditions will gradually improve early Friday morning with some sunshine possible by the afternoon.

KSEA...Rain showers today with VFR cigs presently. ESE flow will become ENE early this afternoon at 7 to 10 kts. Cigs are expected to drop into MVFR as the rain moves over the terminal, becoming predominantly MVFR into this afternoon and evening when the more widespread rainfall arrives. Rain will taper after 10Z.

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MARINE

A strong low pressure system will track near the coastal waters and onshore between Grays Harbor and the mouth of the Columbia River tonight before shifting inland and weakening. With the tracking of the low, expect backing winds over the waters from S/SE to N. While widespread gales are not expected, occasional gusts to 35 kt will likely develop ahead of and south of the low track. Updated the current advisory to break out the coastal waters into the three segments to better capture these differences. Seas will also build to around 11 to 15 ft. through the day today. In addition, expect strong east winds through the west and central Strait of Juan de Fuca. Wind and seas will ease early Friday. High pressure then rebuilds this weekend and into the start of next week, with the development of offshore flow over the waters. 33/12

HYDROLOGY

A passing frontal system will keep rain over the area for much of the day today and into tonight. Snow levels will remain above 4000 feet today into the evening, leaving the majority of the precipitation falling as rain. With expected rainfall amounts holding steady and topping out mostly in the 2 to 2.5 inch range with this system, confidence is increasing that we will not see any river flooding impacts. After tonight, there is no additional hydrologically significant precipitation expected through the next seven days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 4 AM PST Friday for Central U.S. Waters Strait Of Juan De Fuca-Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm- Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm-Grays Harbor Bar-West Entrance U.S. Waters Strait Of Juan De Fuca.