Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 171133 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 333 AM PST Fri Nov 17 2017 .SYNOPSIS...
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A chilly low pressure system over the area early this morning will move east by this afternoon, and will be followed by weak high pressure aloft. Weak high pressure aloft will then remain over the region through Saturday. Alas, a strong Pacific storm will impact the region on Sunday. This system promises to bring locally heavy rain and strong winds.
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&& .SHORT TERM...
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It looks like Paradise on Mount Rainier has picked up about eight or so inches of new snow since 4 PM PST, yesterday. Another seven inches is expected during the day today; therefore, the Winter Weather Advisory was extended until 4 PM this afternoon. Meanwhile, a chilly upper level trof over the region will move east of the area by this afternoon. A weak upper level ridge will prevail over the area in its wake. There will be sufficient instability and moisture during the day for a continued threat of showers; although, most of the showers will be confined to the mountains, where upslope flow will result in brief heavy showers. Weak high pressure aloft will remain over the region through Saturday. The ridge will be quite dirty; therefore, expect some light showers around tonight. Weak warm advection will result in a threat of light precipitation over the CWA, especially Saturday night. A strong low pressure system will impact the region on Sunday for a threat of significant rainfall and locally windy conditions. The heaviest rain is anticipated on the coast and Olympic range. Strong southerly flow at the low levels will likely impact those areas prone to strong southerly winds (e.g. central coast and Whidbey Island). .LONG TERM... The cold or occluded front is anticipated to sweep across the CWA Sunday night for falling snow levels. It`s possible that the south central Cascades (in Pierce and Lewis Counties) could receive significant snowfall before the precipitation tapers off behind the front. The medium range solutions were not in very good agreement as far as Monday was concerned; therefore, confidence in the forecast was low. It is also the reason the inherited forecast was left pretty much intact. Either there will be an upper level ridge over the CWA (GFS and Canadian) or a closed upper level low (ECMWF). Beyond Monday, the models were agreement that a strong low pressure will impact the region about the middle of next week for the potential for significant rainfall and strong winds. The differences were mainly in the details.
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&& .AVIATION...
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Moist northwesterly flow aloft will continue today, then become more westerly tonight as a weak upper ridge builds. The air mass will stabilize today. At the surface, southerly winds will ease today and tonight. Scattered showers will continue today, then will diminish tonight. Ceilings over the Puget Sound region continue to be generally MVFR 1-2k ft. Elsewhere conditions are better. Improvement to VFR is likely this afternoon as a weak surface ridge builds. Some fog is possible this morning in the most susceptible locations such as KOLM. KSEA...MVFR this morning, then VFR this afternoon. THe central Puget Sound region including KSEA is likely to be in a rain shadow later today with westerly flow aloft. Winds will be southerly 8-12 kt. CHB
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&& .MARINE...
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A weak ridge will build over the area today, and winds will ease. Small craft advisory strength westerlies will continue this morning in the strait. The ridge should be strong enough to keep most weather out of the area on Saturday. A strong front will slowly approach the area from the northwest late Saturday then move inland Sunday morning. A gale watch is in effect during that period for all waters except the central strait. Forecast KPDX-KBLI gradients are in the 10-12mb range. Another strong system will arrive around Tuesday. CHB
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&& .HYDROLOGY...
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The Sunday system is anticipated to be somewhat mild, with the snow level predicted to rise to near 6000 feet on the Olympic range. Forecast 24-hour rainfall amounts of about 2 to 4.5 inches on the Skokomish River Basin should be enough to drive the uniquely flood-prone Skokomish River above flood stage sometime Sunday. Beyond Sunday, there is a possibility of a milder and wetter system impacting the region about the middle of next week. This system has the potential to produce significant rainfall on the Olympic range and Cascades, possibly resulting in flooding on several rivers.
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Winter Weather Advisory until 4 PM PST this afternoon for Cascades of Pierce and Lewis Counties above 2500 feet. PZ...Gale Watch from Saturday afternoon through Sunday 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-West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until noon PST today 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 10 AM PST 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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