Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 191727 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 900 AM PST Mon Feb 19 2018 .SYNOPSIS...Dry and cold northerly flow aloft will prevail over the region through tonight. A weak disturbance will bring some light precipitation to mainly the southwest part of the state on Tuesday. Another system will follow late Wednesday into Thursday. Temperatures will remain below normal. && .SHORT TERM...
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Snow showers have ended and residual clouds are clearing out this morning. Dry northerly flow will maintain a dry and cold air mass over the region through tonight. Surface gradients will gradually weaken with breezing north winds easing by tonight. If skies remain clear, radiational cooling should allow low temperatures to dip down to the low to mid 20s, and teens in colder valleys. The latest 12Z GFS Time-height does show some thin mid and high cloud arriving tonight as a system approaches from the north off the B.C. coast. Cloud cover could hinder cooling by a few degrees tonight and some slight adjustments to forecast lows may be needed. The new 00Z GFS is still the farthest west with the low pressure system driving south along 130 W on Tuesday. 500 mb heights indicate a split in the north flow with the bulk of energy well offshore. The portion of the split trough digging south through Wrn Wa has less moisture to work with but models do show a couple tenths liquid QPF from the coast to the southwest interior which could give localized accumulations of an inch or two. Further north across Puget Sound, not much more than flurries with no accumulation expected. Models show little change in the cold and dry northerly flow pattern through Wednesday. Another weak embedded system will brush the area Wednesday but moisture will be very limited. Mostly flurries or light snow showers possible with little or no accumulation. This system looks to dig southward quickly with dry weather late Wednesday and Thursday between systems. A chance of snow is currently forecast for Thursday and will need to evaluate all the new 12z model data before trimming or removing pops. Mercer .LONG TERM... Previous discussion...The Wednesday night upper level low will move southwest across the area on Thursday. Will need to keep an eye on this system because there is a chance that the precip amounts could end up higher than forecast, especially if the low`s trajectory ends up slightly farther west. Beyond Thursday, it looks like the region will remain under northerly flow aloft due to the ridge axis remaining more or less near 150W. This will keep the door open for systems in western Canada to drop in the CWA during this period. Will need to keep an eye on the end of the week system as it could bring the potential for significant snowfall to parts of the lowlands. Otherwise, the main message for this period is below normal temps and near or slightly below normal precipitation.
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&& .AVIATION...
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Northerly flow aloft and at the surface will continue today and tonight. The air mass is stable and dry except for some lingering moisture in the far south. VFR conditions will prevail through tonight. Northerly surface gradients will ease through the day bringing gusty northerly winds around the region to an end. KSEA...VFR with no ceiling. Northerly wind 10-12 kt will fall below 10 kt after 00Z and become light southerly late tonight. CHB
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&& .MARINE...
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Gradients have been easing all night and winds are now also easing. Winds will diminish today but will remain at small craft advisory levels over the inland waters until late this afternoon. On the coast, northeast winds will likely fall below advisory criteria by noon or so. A low center will move southeast well off the coast on Tuesday. For now the forecast calls for the outer coastal water winds to be southeast 15-25 kt while the inner coastal waters remain at 10-20 kt; will revisit this issue in the afternoon. A weaker system is forecast to move southeast through Western Washington on Wednesday night or so, but this is not forecast to bring any advisory level winds. A stronger system will arrive around Friday. CHB
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&& .HYDROLOGY...River flooding is not expected the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory until 4 PM PST this afternoon for Central U.S. Waters Strait Of Juan De Fuca-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-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands-Puget Sound and Hood Canal. Small Craft Advisory until noon PST today for Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm.
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&& $$ www.weather.gov/seattle

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