Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 170950 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 250 AM PDT Sun Sep 17 2017 .SYNOPSIS...
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A series of low pressure systems moving across the area will result in wet and cooler conditions through midweek. Temperatures will be below normal. There will be a slight chance of thunderstorms over the coast and southwest interior on Monday. Some of the heavier showers on Monday will contain small hail.
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&& .SHORT TERM...
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Wildfire smoke continued to drift across the interior at this time. Per the ASOS network, the smoke was not having much of an impact on the surface visibility. The lowest reported visibility was seven statute miles. The denser smoke was likely confined to the Cascade valleys. Expect the smoke to thin this morning due to strengthening southerly flow in the 900-700 mb layer. The smoke will be shoved east of the area this afternoon. Meanwhile, look for rain to move onto the coast this morning, overspreading the rest of the CWA by midafternoon. The cold front will sweep across the area later this afternoon for breezy conditions. Showery conditions will prevail tonight in the wake of the cold front. Expect the snow level to lower into roughly the 4500-5000 foot range late tonight. However, the precip will diminish during the night, so anticipating little, if any, snow accumulations above the 5000 foot level. Rainfall amounts during the 24-hr period ending 5 AM PDT, Monday, are forecast to be in the quarter to one inch range on the coast and in the mountains. Elsewhere, anticipate amounts to be generally about a quarter of an inch or less. A chilly upper level trof will prevail over the Pacific Northwest on Monday. Disturbances moving through the trof will result in showers across the CWA, especially during the afternoon and evening. The air mass will also be unstable enough for a risk of thunderstorms over the coast and southwest interior. Expect small hail to accompany the heavier showers. This will be especially the case with any thunderstorms. It will also be breezy at a number of places during the day Monday. There will be a relative lull in the activity late Monday night into Tuesday morning before the steady precipitation arrives Tuesday afternoon. The models were coming into better agreement in regards to the Tuesday system. Expect a cold upper level low to move over the CWA Tuesday night. It looks like this system will be quite wet, especially for the southern half of the CWA. Paradise, on Mount Rainier, could receive ten inches (or more) of snow from this system Tuesday night. The GFS solution has backed off on the strength of the winds Tuesday night due to the surface low moving across the southern part of the forecast area. This would tend to keep the strongest winds south of the CWA. The GFS was now more in line with the ECMWF solution. .LONG TERM... A vigorous upper short wave will swing inland Wednesday. This will bring showers Wednesday. An associated surface low is expected to track across the south part of the area. Drier northerly flow aloft will develop over Western Washington Thursday. It looks like most of the area will be completely dry by Friday and Saturday. Schneider
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&& .AVIATION...
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Clouds associated with the approaching front will start to filter in during the early morning today. Current obs show mainly high clouds over much of the area...but coastal locations such as HQM are showing some lower level stratus. Some of those stratus may push inland somewhat...but by the time they do it will be hard to tell the marine clouds from the frontal ones. Cigs still progged to be mainly VFR through the morning hours before dropping down to MVFR this afternoon. Once the front passes through...cigs may rebound a little...ranging from low-end VFR to high-end MVFR...but a second wave of showers expected Monday will likely bring cigs down again. An upper level trough will move into the area today and remain in place for Monday. Flow aloft mainly southwesterly today...turning more westerly Monday. SMR KSEA...SCT high clouds this morning with the chance for some low level marine stratus to work its way into the area after 12Z...just in time to mix with increasing clouds from approaching cold front. FROPA expected in the early afternoon with -RA...increased winds and MVFR cigs. Cigs rebound slightly tonight. SMR
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&& .MARINE...
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Southerly winds expected to increase over the coastal waters this morning--with 20-30kt south winds developing by mid to late morning. Inherited SCA looks good and remain in place. The winds over the coastal waters will shift to northwest with the frontal passage in the afternoon. Over the inland waters southerly winds up to around 20kts will be followed by a typical Puget Sound Convergence Zone wind pattern with onshore flow through tonight behind the front. There will be additional weather systems moving through the area Monday through Wednesday for more onshore flow. MM/SMR
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory from 6 AM this morning to 10 PM PDT this evening 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 3 AM PDT early this morning for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.
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&& $$ www.weather.gov/seattle An illustrated version of this forecast discussion can be seen at www.wrh.noaa.gov/sew/gafd/latest_webafd.html

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