Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 181005 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 305 AM PDT Mon Sep 18 2017 .SYNOPSIS...Chilly low pressure aloft will bring wet and unsettled conditions to the region through Tuesday. There will be a slight risk of thunderstorms over parts of the area today. Conditions will gradually improve on Wednesday as high pressure aloft begins moving in from the west. Temperatures will be below normal through midweek. && .SHORT TERM... There were scattered showers over mainly the southwest half of the CWA at this time. There have been a few lightning strikes with the cells over the offshore waters. Expect the thunderstorm threat to increase over the WA coastal waters this morning as the cold pool aloft (temps near -25 degree Celsius) moves over the area. The air mass will destabilize over much of the CWA during the day, but especially over the coast, southwest interior, and the southern Puget Sound Region where isolated thunderstorms are a possibility. The heavier showers will likely contain small hail, possibly enough to cover the ground. The thunderstorm threat should end after sunset except over the coastal waters. Meanwhile, a chilly upper level trof will prevail over the Pacific Northwest for cool, showery weather today. The showers will be most numerous during the afternoon but since the air mass will begin destabilizing as early this morning, thought it would be a good idea to not try to time them. There should be glimpses of sun between showers. A disturbance will move across the CWA tonight. The main impact will be a subtle increase in the shower activity the first half of tonight or ahead of the disturbance. There is a possibility of locally heavy snow in the south central Cascades above the 5000 foot level tonight into Tuesday morning. Paradise on Mount Rainier could receive nine inches of new snow tonight; therefore, a Winter Storm Watch for the Cascades in Pierce and Lewis Counties will remain in effect. The flow aloft will become zonal or westerly on Tuesday. The models were in disagreement concerning the details, so confidence in the forecast was not high. The GFS was drier than the ECMWF; therefore, chose to compromise between the two and broadbrush the forecast. Another upper level low will approach the region Tuesday night. The models have been taking this system farther south with each cycle. The GFS was the farthest south and was drier for Tuesday night. Chose to compromise between the farther northward and wetter ECMWF solution and the GFS. It did appear that the occluded front will move across the far southern part of the CWA, and this should keep the strongest winds south of the area. The models were in agreement that conditions will improve on Wednesday as the upper trof moves slowly east. .LONG TERM... Drier northerly flow aloft will develop over Western Washington Thursday. It looks like things will dry out entirely around Friday and remain dry into the weekend with northerly flow aloft continuing. Schneider && .AVIATION...An upper level trough moving through the area will allow for scattered showers over W WA this morning. Southwesterly flow aloft will turn more westerly by this afternoon as the upper level low draws closer and passes just to the north. Low level onshore south to southwesterly flow will will remain in place today. The air mass will remain moist and will gradually destabilize this morning and into the afternoon and early evening...allowing for scattered showers to continue as well as the possibility for an isolated thunderstorm. This activity will make cigs tricky today...but general trend looks like most locations will either be low-end VFR or MVFR...however locations that see showers may dip down into IFR conditions occasionally. SMR KSEA...Scattered showers will still be possible near the terminal today with best chances being this afternoon as the atmosphere becomes more unstable. Winds light and variable this early morning but otherwise will generally remain southerly at speeds ranging 7-11 kts. SMR && .MARINE...Onshore south to southeasterly flow will continue today as low pressure approaches the coast. Winds look to be strongest over the coastal waters where SCA speeds are expected to develop this afternoon. Speeds for the inner coastal waters are expected to drop off tonight while said speeds are expected to linger over the outer coastal waters into Tuesday. Thus...SCAs from evening forecast will remain in place. More breezy weather is possible Tuesday and Wednesday at times--as disturbances move through the upper level trough over the area. Models remain on track that the strongest gradients and more blustery weather will head into Oregon, but additional advisories may be needed--mainly for the coastal waters. MM/SMR && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Winter Storm Watch from this evening through Tuesday morning for the Cascades of Pierce and Lewis Counties above 5000 feet. PZ...Small Craft Advisory from 11 AM this morning to 11 AM PDT Tuesday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm. Small Craft Advisory from 11 AM this morning to 11 PM PDT this evening for Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. && $$ An illustrated version of this forecast discussion can be seen at is the U.S. government's official web portal to all federal, state and local government web resources and services.