Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 232254 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 354 PM PDT Fri Mar 23 2018 .SYNOPSIS...A cold, showery upper low off the coast will weaken on Saturday with the remnant trough axis moving across Western Washington on Saturday night. Showers will taper off on Sunday. On Monday and Tuesday, a stalled warm front in northwest flow aloft will lead to light to moderate rain. Brief upper ridging next Wednesday and Thursday will lead to drier weather. && .SHORT TERM...A sprawling upper low off the coast contains very cold air aloft. The 12z UIL sounding showed a 500 mb temp of -37C, which is about as cold as it gets at that level around here. Such cold air aloft is leading to an unstable air mass and is bearing lots of convective showers. The cold air mass is bringing a mix of rain, snow, and small hail under many of the heavier shower cores. Cannot rule out a clap or two of thunder through sunset. In such a cold air mass, the number of lightning strikes is small. The impact is that you won`t hear thunder approaching before it is on top of you, so simply head indoors as a lightning precaution if you see dark clouds approach. Models show a wave spreading up from the south early on Saturday morning. As a result, a fairly solid area of precip will spread up across southwest Washington and northward to the east of Puget Sound. Considering a forecast freezing level around sunrise near 1000 feet, this will bring a rain snow mix down to just a couple hundred feet above sea level. This afternoon`s gridded forecast brings 1-2 inches of snow in the morning for some foothill communities such as North Bend and Enumclaw; a Winter Weather Advisory may eventually be needed. Minor accumulations are possible on higher hills closer to Puget Sound, but air temperatures will be well above freezing, and highs should reach well into the 40s tomorrow afternoon. Therefore, any accumulations will quickly vanish after precip ends and temps rise later Saturday morning. The sprawling upper low will move east and weaken into an open trough axis by the time it crosses Western Washington on Saturday night. Once it passes, there will be a lack of larger scale lift to organize showers on Sunday, so Sunday will be the drier day this weekend, with just a few shallow popcorn showers. A little more sunshine on Sunday will finally boost lowland temperatures back above 50F. By Monday, we will find ourselves in the dirty periphery of an offshore upper ridge in moderate to strong northwest flow aloft. With warm advection lift and fairly deep moisture, look for steady light to moderate rain on Monday. Haner .LONG TERM...Tuesday will be similar to Monday, with Western Washington under the dirty, drippy periphery of an offshore upper ridge in northwest flow aloft. On Wednesday and Thursday, the ridge axis will drift closer to the forecast area, and 500 mb heights will creep up close to 570 decameters. Cannot get rain out of the forecast completely, but Wed/Thu looks quite a bit drier than Mon/Tue. Models are in pretty good agreement with the northwest flow aloft sweeping a sharp cold front southeast down Vancouver Island next Friday. In the pre-frontal environment, warm advection and onshore flow will serve to increase precip chances next Friday. Haner && .AVIATION...A cold upper level low well off the Washington coast will slide SSW into the Oregon offshore waters late tonight through Saturday. Disturbances rotating around this upper low will give showers to the area tonight through Saturday morning, followed by diminishing showers from the north on Saturday. The airmass is moist and unstable, with the most unstable air offshore. The threat for isolated thunderstorms will be this evening over the coastal waters and western portions of the Olympic Peninsula. The air mass will stabilize somewhat late tonight through Saturday. There has been a break in shower activity in the interior this afternoon. Showers are expected to increase this evening and continue through Saturday morning. Conditions are mainly on the higher end of MVFR to lower end of VFR this afternoon. Conditions will likely deteriorate down into the lower end of MVFR later this evening through Saturday morning. Snow levels will be quite low again late tonight into Saturday morning, but precipitation rates with showers are expected to be lighter than this morning. Will include a TEMPO group for mixed rain and snow showers for the higher elevation terminals (above 300-400 ft MSL) for the 12-16Z period Saturday morning. CIGS and VSBYs in those conditions would likely fall into the IFR range at times. Albrecht KSEA...Another band of showers will lift northward through the terminal area early this evening and will be followed by another late tonight through Saturday morning. Snow levels with the second batch may be low enough to result in mixed precipitation at times, but temperatures will remain above freezing and no accumulation is expected. Ceilings and visibilities will remain on the low end of VFR or high end of MVFR early this evening, then fall into the lower end of MVFR or into the IFR category with the second shower band late tonight into Saturday morning. Winds South 9-13 kt late this afternoon will ease to 4-8 knots later tonight through Saturday morning. Albrecht && .MARINE...A surface and upper level low is centered west of the Washington offshore waters and will drop into the Oregon offshore waters late tonight into Saturday. Southerly winds will ease below Small Craft Advisory criteria late this afternoon as the low drops southward. Seas over southern portions of the coastal waters will remain above 10 feet through early this evening, so a small craft advisory for hazardous seas was issued for that area until 06Z. Weak high pressure will give light south winds to the waters Saturday through Sunday. A frontal system will move southeast through the waters Sunday night and may produce local small craft advisory conditions over some waters. Small craft advisory conditions are possible over much of the area later Monday through Tuesday as another system moves into the area from the northwest. 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-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands-Puget Sound and Hood Canal. Small Craft Advisory for hazardous seas until 10 PM PDT this evening for 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 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.