Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 231640 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 940 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 sprawling upper low is currently centered west of Vancouver Island near 50N 132W. There is very cold air aloft over a large area around the low, with 500 mb temps of -37C to -40C. Such cold air aloft is leading to an unstable air mass, with Western Washington in the more unsettled east-southeast quadrant of the upper low. This is a classic setup for spring convection, with showers pivoting up from the south-southwest today. Models show the coastal side of the Olympic Peninsula being most unstable this afternoon, with CAPE values forecast to reach the 400-700 J/kg range. Forecast already has thunder in the forecast this afternoon, and that seems good. This will be the type of day when a shower produces a single clap or two of lightning, seemingly out of the blue and without lead-up. So if you see dark clouds approaching, even you don`t hear thunder, take your lightning precautions anyway by most ideally going indoors. With such cold air aloft, could easily get small hail in convective showers, similar to yesterday. As for this morning, have gotten several reports of snow falling on hills above about 300 feet along the I-5 corridor. Hard to be excited about anything more than brief accumulations given air temperatures in the 36F-39F range, well above freezing. 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. Haner .LONG TERM...From Previous Discussion: 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...A cold upper level low west of Vancouver Island will drop southeastward and onto the southwestern Oregon coast on Saturday. The air mass is moist and unstable with showers and some thunderstorms rotating around the upper level low. Thunderstorms will be most likely along the Washington coast, but will be possible in the interior during the afternoon and early evening hours as the air mass inland destabilizes with daytime heating. There are multiple cloud layers with showers rotating around the upper low. Heavier showers will bring ceilings down to around 015-020 through this afternoon, but for the most part, ceilings will be around layers at 3500 and 5000 feet. Small hail or graupel, along with brief changeover to snow or mixed rain and snow, is possible in heavier showers through early this afternoon. Showers will slowly decrease from the north this evening as the offshore upper low drops southward offshore and toward the southern Oregon coast. Areas of MVFR conditions are likely again late tonight into Saturday morning as low level moisture lingers and we get nocturnal cooling. Albrecht KSEA...Heavier showers are producing MVFR conditions with ceilings down to 015 this morning. Expect ceilings to improve to the higher end of MVFR or lower end of VFR this afternoon and evening as the air mass destabilizes and we get some daytime heating. Snow levels are low, so heavier showers may produce a brief mixture of rain and snow or give small hail or graupel. Thunderstorms cannot be ruled out through this evening, but the chances are low enough at the terminal to keep the mention out. Will amend the terminal forecast as needed if tstms are seen moving in that direction. Winds will be south 10 kt with gusts to 20 kt in the afternoon. Winds will ease to east to southeast at 5 kt or less this evening through the overnight hours. Albrecht && .MARINE...A surface and upper level low is centered near 50N 135W. This low will move slowly southward onto the southwest Oregon coast on Saturday as it slowly weakens. Small craft advisory conditions are expected to continue through this afternoon over all waters with gale conditions until about noon over the eastern entrance of the Strait. Winds will diminish as the upper low moves south of the area tonight into Saturday and a weak ridge of high pressure builds into the waters. A frontal system will move southeast through the waters Sunday night. Small craft advisory conditions are possible over some of the waters as this system moves across the area. Albrecht && .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-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. Small Craft Advisory until 5 PM PDT this afternoon for East Entrance U.S. Waters Strait Of Juan De Fuca. Gale Warning until noon PDT today for East Entrance U.S. Waters Strait Of Juan De Fuca. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.