Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 220358 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 900 PM PDT Thu Sep 21 2017 .SYNOPSIS...An offshore upper level ridge will bring more sunshine and a little warming Friday and Saturday morning. The weak ridge will remain over the region Sunday through Tuesday but will probably allow transient weak weather systems to bring more clouds and some spotty light rain to mainly the coast and north part. The ridge will build Tuesday or Wednesday for mostly sunny and warmer weather. && .SHORT TERM...An upper level trough is still close enough this evening to keep scattered showers going in the Cascades from about Stevens Pass south. Temperatures are a bit warmer than earlier this week; the snow level has risen to around 6000 feet. A big patch of mid level clouds is moving south from British Columbia and this might make Friday somewhat less sunny than forecast. The clouds will probably clear north to south during the day Friday. The upper level ridge will be over southern British Columbia on Saturday, but it will be weak enough to allow transient weather systems to pass through the ridge and brush mainly the northern part of Western Washington with some light rain. Mid level clouds from the first system will probably spread over the area from the northwest Saturday morning, but this system should be pretty much dry. The models have been inconsistent with the system on Sunday, especially with the coverage of light rain, ranging from zero to some spotty light rain over the north part of the area. What is more certain is that it will be cloudy with highs remaining a little below normal. Kam/Burke .LONG TERM...Previous discussion...The weak upper level ridge over southern B.C. will remain in place Sunday and Monday, still wide open to weak transient weather systems. The GFS brings another system Monday through Monday night with a bit more coverage of light rain. The ECMWF is in better agreement but still a little slower, bringing some light rain Monday afternoon through Tuesday morning. Despite the better model agreement, confidence is still low. Models are in better agreement that the upper level ridge will build enough by Wednesday for mostly sunny weather Wednesday and Thursday with temperatures near or slightly above normal. Kam && .AVIATION...There is an upper level trough over the West with northerly flow aloft over Western Washington. Isolated showers in the Cascades are diminishing this evening. Satellite and surface observations show a cloud deck pushing south into W WA with bases around 8,000 to 10,000 ft. Most areas will stay VFR overnight but may see patchy fog in the valleys. The trough will shift toward the Great Basin region on Friday with northerly flow continuing over WA. Models show isolated showers over the Cascades with dry weather elsewhere. 33 KSEA...VFR conditions expected this evening with light/variable wind at the surface. Patchy fog may form early in the morning. Dry weather on Friday with winds switching to light westerly. 33 && .MARINE...Pressure gradients are weak across Western Washington tonight for generally light winds over the waters. High pressure over the NE Pac will maintain onshore flow through the weekend. A warm front will clip the region on Monday. Offshore flow will develop by midweek as a thermal trough forms along the coast. 33 && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$ 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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