Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 180329 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 829 PM PDT Sun Sep 17 2017 .SYNOPSIS...
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Onshore flow will continue tonight with showers mainly in the Cascades and north sound. An upper trough and additional weather systems will keep the weather cool and showery into the middle of the week. Dry weather is likely late in the week.
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&& .SHORT TERM...
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A big change in the weather today as a cold front brought measurable rain to the region. Totals so far ranged from 0.50-1.00" along the coast to 0.10-0.25" in the interior. We`re in post-frontal onshore flow now with showers mainly in the Cascades and north sound (where there`s low level convergence). Showers will taper down late tonight. The air mass will remain cool, moist and unstable through Monday as an upper level low moves over the Pac NW. The cooler air mass will keep temps in the 50s to lower 60s. Expect more showers with snow levels in the mountains dropping to around 5,000 feet. Models are showing an increase in lift and moisture with a trough axis Monday night. Places like Paradise at Mount Rainier may see up to 6 inches of snow. This snowfall total satisfies our early warning criteria and a Winter Storm Watch is in effect. The next system moves inland Tuesday night into Wednesday but mainly passing just to our south. However, we should still see some precip clipping the area. Temperatures will continue to trend below normal. 33 .LONG TERM...Previous discussion...Drier northerly flow aloft will develop over Western Washington Thursday. It looks like things will dry out entirely around Friday and remain dry into the weekend with northerly flow aloft continuing. Schneider
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&& .AVIATION...
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A cold front will continue to move eastward this evening. Current IR satellite imagery puts the front on the east side of the Cascades at the time of this writing while post frontal rains remain over the Cascade foothills and up into the Cascade crest. For W WA in the wake of the front...current radar shows scattered showers starting to pop up over the coastal waters and move inland and there looks to be a weak Puget Sound Convergence Zone trying to set up over Snohomish county. This activity will make cigs tricky tonight...but general trend looks like most locations will either be low-end VFR or high-end MVFR...however locations that see showers may dip down into IFR conditions occasionally. Southwesterly flow aloft will turn more westerly by Monday afternoon as the upper level low draws closer to W WA and passes just to the north. Low level onshore flow will gradually ease overnight tonight...but will remain in place for Monday. The air mass will remain moist and will gradually destabilize Monday morning and into the afternoon and early evening...allowing for scattered showers and the possibility for an isolated thunderstorm. SMR KSEA...Scattered showers will still be possible near the terminal tonight...although it looks like the PSCZ will remain to the north. Scattered showers remain on tap for Monday. Winds will generally remain southerly at speeds ranging 7-11 kts. SMR
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&& .MARINE...
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Onshore flow in the wake of the cold front will ease overnight...allowing for the majority of SCAs to expire with the evening forecast package. Inherited SCA for coastal waters and Central and Eastern Strait goes out til 3 AM Monday morning...but current obs actually support taking those down as well. More breezy weather is possible Monday through Wednesday at times--as disturbances move through the upper level trough over the area. Will evaluate incoming 00Z model data to see if additional headlines are warranted in the near term. It looks like the strongest gradients and more blustery weather will head into Oregon, but advisories will probably be up at times--mainly for the coastal waters. MM/SMR
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Winter Storm Watch from Monday afternoon through Tuesday morning for Cascades of Pierce and Lewis Counties. PZ...None.
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&& $$ www.weather.gov/seattle You can see an illustrated version of this discussion at www.wrh.noaa.gov/sew/gafd/latest_webafd.html

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