Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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258 FXUS66 KSEW 191619 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 920 AM PDT Tue Jun 19 2018 .SYNOPSIS...An upper ridge will move inland today and east of the Cascades tonight and Wednesday. Western Washington will be sunny with near record highs, but there`s a chance of thunderstorms over the Cascades. Onshore flow will bring cooler marine air inland Thursday and Friday. High pressure aloft will cross the region Saturday, followed by an upper trough early next week. && .SHORT TERM...
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Low temperatures this morning over W WA pretty much ranged throughout the 50s with locations along the Canadian border only cooling to around 60. Looking at current observations at the time of this writing...temps over much of the area appear to be hovering right around that 60 mark...whether it is the upper 50s along the coast to right at 60 in the SW interior and south Sound region to low 60s in the Seattle Metro. Switching over to satellite for a moment...we can see marine stratus set up over much of the Olympic peninsula...coinciding with the cooler temperatures. While current trends show no hints at their receding just yet...should see these clouds withdraw to the coastal waters by late this morning or noon today. Getting north of Everett...temps are already in the lower to mid 60s with obs around Bellingham and north reaching near 70. That all being said...current obs are about in line with what was observed 24 hours ago for the most part and as such...certainly a good indicator that W WA is on its way to another warm day. Current models appear to support that as upper level ridging makes its way inland today...with the resultant sunny skies and continued warm temps. However...high level moisture wrapping around the north side of the upper low exiting Idaho and entering into Montana, along with weak upper instability, could produce afternoon and evening showers over the mountains and possibly thunderstorms near the Cascade crest. The ridge axis exits the area during the overnight hours allowing for a shortwave feature to set up over the Cascades and help enhance the chances for showers/thunderstorms there Wed afternoon. For the remainder of W WA however...this may result in an upward nudge in RH values...adding a little mugginess to the continued heat expected Wed. This leads to the main forecast problem...as the continued heat with this little influx of moisture manages to pose some possible heat related health impacts and with that being the case...a heat advisory might be warranted...mainly for the Seattle Metro and the SW interior. Will take a look at 12Z model data as it comes in and will have a better feel for the situation in time for afternoon discussion. The good news is that more moderate conditions are on the way as stronger onshore flow will develop Wednesday evening, as the leading edge of an offshore upper trough moves into the Pac NW. A cool, moist marine layer should push inland Wed night and W WA will probably be more or less covered by marine stratus Thursday morning, which should give way to sun breaks in the afternoon. Highs across the interior lowlands should be back into the upper 60s and 70s, with mid 60s at the coast. SMR/McDonnal .LONG TERM...From Previous Discussion...Low level onshore flow will continue Friday as an upper shortwave moves across the area, which should produce a few showers and maybe a Puget Sound convergence zone. Then an upper ridge will build as it moves inland Saturday and east of the Cascades Sunday, for more sunshine and somewhat warmer weather. There is less agreement in the models for Monday, but most likely an upper trough will begin to pass by to our north. McDonnal
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&& .AVIATION...
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Wrn WA will be sandwiched btwn upper level ridges centered over British Columbia and off the nrn Baja CA coast, and upper level lows centered offshore and over MT. The flow aloft will be light and variable. Weak low level sly flow flow will become light onshore or wly this afternoon. Meanwhile, areas of mainly MVFR CIGs blanketed much of the lowlands from the Olympic peninsula wwd at this time. Expect the low clouds to scatter out or retreat to the coastline by 2100 UTC. KSEA...VFR. Light southerly winds will become light westerly this afternoon. Winds tonight will be light and variable, becoming light southerly or southwest Wednesday morning.
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&& .MARINE...
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Light southerly flow this morning will become weak onshore or westerly this afternoon. Expect onshore flow of varying strength tonight through the end of the week. Gale force winds are possible again over parts of the Strait of Juan de Fuca Wednesday and Thursday nights.
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Gale Watch from Wednesday evening through late Wednesday night for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 5 PM this afternoon to 1 AM PDT Wednesday for Central U.S. Waters Strait Of Juan De Fuca.
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&& $$ www.weather.gov/seattle

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