Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 241241 AAA AFDSEW Area Forecast Discussion...UPDATED National Weather Service Seattle WA 541 AM PDT Sun Jun 24 2018 .UPDATE...
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The forecast was updated to expand the areal coverage of low clouds and to also include a mention of fog. The stratus bases and tops were quite low, so the clouds/fog should burn off quickly.
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&& .PREV DISCUSSION...
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/issued 333 AM PDT Sun Jun 24 2018/ SYNOPSIS...A low pressure system offshore will approach the area during the day today. Look for the cold front to move across the region tonight. Low pressure aloft will keep the area cool and unsettled on Monday. Weak high pressure aloft will prevail on Tuesday for near normal temperatures. SHORT TERM... An upper level trof and associated cold front offshore will continue moving east today, resulting in increasing clouds. A few showers should reach the north coast this afternoon. The air mass will also become weakly unstable over the north Cascades this afternoon for a slight risk of thunderstorms near the crest. Temperatures today should be warmer than yesterday. However, expect temperatures over the coast to peak early this afternoon before the marine push arrives. The cold front is expected to move across the CWA tonight. Expect scattered showers (more numerous over the northern part of the CWA) to break out over the area late today or this evening ahead of the front. There will also be a slight risk of thunderstorms near the north Cascade crest this evening. A Puget Sound Convergence Zone should develop in its typical location in the wake of the cold front late tonight. Look for the upper level trough to move over the region on Monday for below normal temperatures. There will be scattered showers over mainly the mountains, coast, and within the convergence zone during the day Monday. The upper level trough will move east of the area late Monday night, with weak high pressure aloft moving over the CWA in its wake on Tuesday. Temperatures on Tuesday are forecast to be near normal. Not too confident in the PoP forecast for Tuesday. There is a possibility that there could be a few light showers over the north Cascades Tuesday afternoon. LONG TERM... No major changes to the previous forecast. Overall, it looks like the Pacific Northwest will be mainly under the influence of upper level troughs during much of this period. This, coupled with continued low level onshore flow, will result in below normal temperatures. AVIATION...Increasing southwest flow aloft today as a frontal system offshore approaches Western Washington. Air mass is dry and stable except for low level marine moisture along the coast extending locally inland through the Strait of Juan de Fuca and the Chehalis Gap. IFR ceilings in stratus can keep expected along the coast and locally in the interior through 16Z-18Z. High and mid level moisture will increase ahead of the front this afternoon with some light rain reaching the coast around 03Z then spreading into the north half of the interior overnight. Widespread MVFR ceilings in scattered showers are expected across the region after 06Z tonight. KSEA...Stratus starting to form over the Hood Canal area and north Puget Sound area as of 930Z. Present terminal forecast does not include any IFR ceilings, but will need to keep an eye on things if stratus makes some southward progress through Puget Sound. Otherwise, expecting VFR through 06Z tonight. High and mid level clouds should abruptly increase later this afternoon ahead of the front with some shower activity possible by 09Z-12Z Monday with MVFR ceilings developing. Surface winds light and variable this morning becoming W-NW 4 to 8 knots in the afternoon. Winds back to southerly after sunset and increase to 10 to 15 knots. 27 MARINE...A 1021mb high over the area early this morning will give way to a warming air mass with falling pressures today. A push of marine air will begin later this afternoon and evening, and then a cold front will move ashore in the middle of the night. There wont be much if any of a wind shift with the cold front--just continuing onshore flow. Westerly gales are a pretty good bet for the Strait of Juan de Fuca this evening. The onshore flow will ease Monday through midday Tuesday, picks up again with the diurnal push Tuesday evening, and then another upper trough arrives so Wednesday evening could also have stronger onshore gradients as well.
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Gale Watch from 5 PM PDT this afternoon through late tonight for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 5 PM PDT this afternoon 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

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