Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 020345 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 745 PM PST Mon Mar 1 2021 .SYNOPSIS...Mostly dry and warmer conditions across the region through Thursday as quasi-zonal flow gives way to high pressure aloft. A pattern change late Thursday will see a return of wet weather to Western Washington Friday into the weekend. && .SHORT TERM /TONIGHT THROUGH THURSDAY/...A passing weak upper level trough ushering in some mid to low level clouds over portions of W WA this evening and into tonight. Minimal precip chances accompany this system...and any showers that do develop will likely be confined to the northern half of the CWA. Current radar backs that up pretty well with a weak band of showers over northern Olympic Peninsula and a weakening convergence zone over the Cascade foothills of central Snohomish county. This system will eject Tuesday morning. Inherited forecast looks on track and see no reason for an evening update at this time. For additional forecast details, please refer to the From Previous Forecast section below. 18 From Previous Discussion...Quasi-zonal flow will remain in place Tuesday before a ridge of high pressure begins building over the Great Basin on Wednesday in response to the aforementioned upper low ejecting into the Cen Cal/SoCal coast. As this is occuring, a large upper level low will eject out of the Gulf of Alaska, flattening the ridge axis some Wednesday and Thursday and setting the stage for a return of wet weather for the weekend. In all, dry conditions expected through the period with warming conditions on Wednesday and Thursday. Some locales may threaten the 60 degree F mark for the first time this year. Aside from this, gusty winds will be possible along the coast with the approach of the next system. .LONG TERM /FRIDAY THROUGH MONDAY/...From Previous Discussion...The aforementioned ridge will break down late Thursday night into Friday with rain gradually spreading from west to east across Western Washington Friday. This system looks to tap into a large fetch of moisture across the northeast Pacific and point it at the West Coast as the aforementioned large upper low ejects out of the Gulf of Alaska. While confidence is high in rain returning to the region, the details of this system remain murky this afternoon and trends in the coming days will be important in sussing those out. Ensemble guidance this afternoon is trending toward a deeper trough setting up off of the northwest coast, with some ensemble blends hinting at another split flow situation where the bulk of the upper level energy impacts Southern OR/NorCal. A deeper trough, and/or split flow solution would point the bulk of the AR at the Oregon Coast, with the Olympic Peninsula seeing lower rainfall amounts. The good news regardless of what scenario unfolds is that the pattern will remain progressive through the weekend, cutting the moisture fetch off after 24 to 36 hours. Nonetheless, this will be worth keeping an eye on, as the GEFS and EPS members are converging on at least a weak AR event for the West Coast Friday into Saturday. At this time, flooding is not a concern due to only modest (forecast) rises in snow level and mostly dry conditions this week. Active weather looks to persist through the weekend and into early next week as a mean trough will remain over the region, keeping the door open for additional systems to traverse Western Washington. Pullin && .AVIATION...Southwesterly flow aloft tonight becoming westerly Tuesday morning as a weak upper trough moves over W WA. Low-level flow will remain weakly onshore. Widespread VFR conditions in place over much of the area, although locations along the coast and the Strait dipping down into MVFR as upper trough passes through. Cigs area-wide generally degrading overnight into Tuesday AM although most locations should remain low end VFR to maybe dipping down into high-end MVFR as onshore flow behind a dissipating front spreads lower clouds into the region. As usual, those locations more prone to lower cigs /OLM, PWT/ may see conditions dip into IFR conditions during this time frame. Cigs expected to recover with widespread VFR conditions expected to return for most locations Tuesday afternoon. KSEA...VFR conditions expected through this evening with MVFR conditions developing by around 09Z tonight. Some showers may be possible, but chances are not significant enough to merit any mention in TAF as most of the action will remain north of the terminal. Surface winds light S-SW. 27/18 && .MARINE...A weak front will dissipate as it moves onshore tonight. Low level onshore flow will increase as it does so. This will lead to borderline small craft advisory westerlies in the central and eastern Strait of Juan de Fuca tonight. Current obs bear this out as both Sheringham and Race Rocks on the Canadian side of the Strait are right at the lower end of SCA criteria. In addition, seas will also rebuild to around 10 ft with this system tonight into Tuesday. As such, inherited headlines look good and see no need for any alteration for the evening forecast update. A more significant frontal system will affect the waters Thursday into Friday will possible gales for the coastal waters and portions of the inland waters. Coastal seas will build to 15-18 feet. 27/18 && .HYDROLOGY...From Previous Discussion...Light rainfall amounts over the next several days will preclude the threat for river flooding. Continuing to keep an eye on Friday through the weekend as an atmospheric river will approach the west coast. Rainfall totals remain contingent on the evolution of this system and flooding is not expected at this time. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 4 PM PST Tuesday for 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. Small Craft Advisory until 4 PM PST Tuesday for Grays Harbor Bar. Small Craft Advisory until 1 AM PST Tuesday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. && $$ www.weather.gov/seattle

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