Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 161103 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 403 AM PDT Sat Jun 16 2018 .SYNOPSIS...A weak upper level low along the coast will move south into Oregon this morning. The low will stall over southern Oregon/northern California later this afternoon into tonight with a chance of showers or thunderstorms over the Cascades. High pressure will build offshore Sunday and Monday with ridge shifting inland Tuesday night and Wednesday. High temperatures will warm over the weekend into the first part of next week peaking on Wednesday. Increasing onshore flow will result in cooler temperatures over the interior on Thursday and Friday. && .SHORT TERM...Satellite imagery shows weak upper level low along the Washington coast at 10z/3am this morning. Some light showers have been reported in the vicinity of the low. Mostly cloudy skies over the remainder of the area. Temperatures at 3 am were in the upper 40s and 50s. Upper level low will continue to move south into Oregon early this morning. Doppler radar has a few light showers on the northeast side of the low so have added a slight chance of showers for the early morning hours from about Port Angeles to Forks. Skies clearing over the lowlands later this morning into this afternoon with the east northeasterly flow aloft behind the low. Air mass over the Cascades becoming unstable this afternoon with lifted indexes still in the 0 to -3 range. CAPE values a little higher on the 00z run with a 1000 j/kg bullseye around Mount Rainier late in the afternoon. Convective temperatures off the model soundings for the mountains continue to be in the lower to mid 60s. Will keep showers in the forecast for the Cascades with a slight chance of thunderstorms in the afternoon and evening hours. Also a chance in the Cascade foothills with 700-500 mb winds east northeasterly 15 to 25 knots later this afternoon into the evening hours possibily pushing the storms off the Cascades. Warming trend continuing with highs today in the 70s. Flow aloft becoming more northeasterly tonight which will shut off the flow of wrap around mid level moisture into the Cascades bringing about an end to the shower/thunderstorm chances. Surface gradients going light with clear skies overnight. Lows will be in the upper 40s and 50s. Northeasterly flow aloft continuing on Sunday with the light surface gradients becoming northwesterly in the afternoon. Model guidance once again has some solutions that are very warm with 00z GFS going for 90 in Seattle. With the surface gradients not going offshore and the temperatures aloft, while warming up, still only in the plus 16 to 18C range by 00z Monday will stay will go more in line with the cooler ECMWF guidance which has the warmest locations in the mid to upper 80s Sunday ( going 84 for Seattle ) and the remainder of the area in the mid 70s to mid 80s. Little change in the pattern Sunday night into Monday with the upper level low to the south and east and northeasterly flow aloft over western Washington. Thermally induced surface trough along the coast in the morning shifts inland during the day. Temperatures aloft similar to a degree or two warmer than Sunday. Will add a few degrees to the high temperatures on Monday compared to Sunday`s readings with upper 70s to near 90 in the forecast. .LONG TERM...Extended models in good agreement Tuesday and Wednesday with the upper level ridge offshore shifting inland Tuesday night into Wednesday. Highs both days close to records, in the upper 70s to near 90 for the interior. Slightly cooler temperatures on the coast with an afternoon seabreeze. Surface onshore gradients increasing Wednesday night into Thursday bringing a push of cooler marine air into the interior. GFS is a little stronger with the push, 10 to 15 degrees of cooling, versus the ECMWF, 5 to 10 degrees of cooling. For now have used a model blend for Thursday with highs in the interior in the 70s for the most part. Temperatures continue to cool on Friday with low level onshore flow continuing pushing high temperatures back closer to normal, in the mid 60s to mid 70s. Felton && .AVIATION...Upper level low pressure driving south along the Washington coast will move into Oregon by afternoon. Light northwesterly flow aloft becoming light to moderate northeasterly flow this afternoon and tonight. High pressure will build into British Columbia on Sunday with light northeast flow over Washington. Air mass mostly stable, except weakly unstable aloft over the Cascades and Olympics. A few showers are possible over the Olympic peninsula today. Cumulus should build over the Cascades this afternoon with showers and few thunderstorms near the crest, diminishing after sunset. Predominately VFR conditions with SCT to BKN clouds around 6-8k feet. Isolated MVFR along the west side of Puget Sound affecting PWT and along the coast affecting HQM improving to VFR by late morning. KSEA...Northerly wind to 5 kt rising to 8 to 15 kt late this morning through early evening. Northeast wind to 5 kt after sunset. VFR with SCT-BKN clouds around 6-8k ft. Cumulus build ups expected to remain over the Cascades during the afternoon and evening. Mercer && .MARINE...A weak 1013 mb surface low will shift southward along the Washington coast this morning. Northwesterly winds over the coastal waters will shift to northerly this afternoon behind the low, reaching small craft strength at times. Northerly flow will prevail over the inland waters. Brief westerly small craft winds are possible in the Strait this evening before easing overnight. Northerly winds will increase over the waters Sunday and dominate into the middle of next week as surface high pressure builds into southern British Columbia. Diurnal onshore winds are expected at times over the coastal waters and in the Strait of Juan De Fuca during the afternoon and evening hours. Mercer && .CLIMATE...For the Seattle area the best chance for a record breaking high looks to be Wednesday. The record on Monday is 94 degrees, Tuesday 92 and Wednesday 88 degrees. Current forecast going for a record tying 88 on Wednesday. Felton && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 11 PM PDT this evening for 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. Small Craft Advisory from 11 AM this morning to 11 PM PDT this evening for Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.