Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 231040 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 330 AM PDT Fri Mar 23 2018 .SYNOPSIS...A cold upper low will produce showers and scattered thunderstorms in the afternoon and evening with small hail possible. Showers decrease late tonight, with lingering showers over the weekend along with sunbreaks. The next front brings light rain to Western Washington Monday, then decreasing Tuesday. High pressure will build over the Pacific Northwest the middle of next week with mostly dry and somewhat warmer weather. && .SHORT TERM...A large closed 500 mb low and -40C cold pool is circulating off the Pacific Northwest coast near 130W. IR satellite imagery inidicates widespread showers with cold topped cumulus rotating near the 998 mb surface low becoming vertically aligned with the upper low. WA/Oregon are in diffluent northeast quadrant of low as it shifts southward today. This will maintain showery and cool conditions across Western Washington today with highs only in the 40s. Any breaks in cloud cover probably will not last long, limiting surface based instability. Regardless, general instability with steep lapse rates aloft give the cold air at 500 mb will put Western Washington in a favorable area for some scattered thunderstorms, mainly this afternoon and evening. Small hail will be likely with any storms given the low freezing levels. Some wet snow could even mix in briefly with heavier showers above 500 feet. Models show a wave developing at the base of the trough and lifting northeast into south/central Oregon to the northeast, south of our area. Rain and thunderstorms will probably be most active with this feature to our south but does not preclude thunderstorms just about anywhere this afternoon/early evening. CAPE values and LI`s only near -1C do not indicate more than brief heavy rain, and small hail possible. Showers decrease later tonight into Saturday. The main low and dynamics shift south with some lingering diurnal showers still possible over Western Washington. Highs will stay cool in the mid to upper 40s. Sunday will probably remain mostly dry across the area between systems. However, there are some inconsistencies in timing of the next front. The forecast is based off the ECMWF which shows light rain arriving as early as Sunday night. .LONG TERM...High pressure offshore Monday will gradually build through at least mid week. Fronts arriving in northwest flow will bring periods of light rain Monday and Tuesday. Eventually the ridge shifts eastward shifting fronts further inland. Dry weather should develop later Tuesday through about Thursday. Expect some partial sunshine and temperatures a bit warmer in the 50s. Mercer && .AVIATION...Cold upper level low offshore with southwesterly flow aloft weakening tonight. Air mass unstable throughout the day becoming more unstable during the afternoon and early evening hours. Multiple cloud layers into Saturday. In general scattered layer around 2500 feet scattered to broken layer around 4000 feet with another layer around 6000 feet. Ceilings lowering to 2000 to 3000 feet in the heavier showers with small hail or graupel possible this afternoon. KSEA...Mostly VFR ceilings through tonight. Brief MVFR ceilings in the 2000-2500 foot range possible with heavier showers mostly this afternoon. Small hail or graupel possible with the heavier showers. Southeast wind 4 to 8 knots becoming southerly 10 to 15 knots this afternoon into the early evening hours. Felton && .MARINE...Breezy cool unstable air with plenty of showers today. The breezy weather will ease tonight as low pressure offshore starts to drift south. Over the weekend onshore flow will persist, but then high pressure should build into the area early next week. It might not be strong enough at first to keep weather systems from brushing the area, but gradients don`t look all that strong--perhaps small craft advisory conditions for the coastal waters at times early next week. && .HYDROLOGY...River flooding is not expected in the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 5 PM PDT this afternoon for Admiralty Inlet-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-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands-Puget Sound and Hood Canal-West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory for rough bar until 11 PM PDT this evening for Grays Harbor Bar. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.