Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 202228 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 330 PM PDT Wed Jun 20 2018 .SYNOPSIS...An upper ridge east of the Cascades will bring another day of near record high temperatures to Western Washington today, with partly to mostly sunny skies and a chance of thunderstorms late in the day. Onshore flow will bring cooler marine air inland Thursday and Friday. High pressure aloft will cross the region this weekend, followed by an upper trough early next week. && .SHORT TERM...
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Showers and thunderstorms are starting to pop up on the radar...mostly over the Cascades but there is an isolated cell out on the Olympic Peninsula. Will include this area under the thunderstorm threat for the evening hours with the upcoming forecast package. Aside from that...not seeing much movement off the mountains...but still plenty of time for that to occur...given that model progs not really expecting much of a lowland thunderstorm threat until after 00z late this afternoon. Looking over temperatures...most locations running warmer than this time yesterday which was expected. The exception to this being Hoquiam...where clouds have remained in place over the area keeping temps suppressed. Looking over the actual temperatures at the time of this writing it appears Olympia has already tied its record for the day of 89...Tacoma is reporting 90 and Sea-Tac is just shy of its record at 85. Aside from including the Olympics under the thunder threat for this evening...not much in the way of changes to the forecast as model solutions remain consistent for the near term. Showers and thunderstorms will diminish tonight and an upper level trough will start to move inland. Strong onshore flow is expected to accompany this trough triggering a good marine push...resulting in plenty of low clouds overnight and into the morning Thursday with perhaps even some drizzle along the coast. Most importantly...this will lower temperatures considerably with highs across the interior lowlands in the upper 60s and 70s. Another upper trough will move into Western Washington Thursday night and Friday. This feature will be better defined, but will only bring a chance of light showers to the area -- mainly the coast, mountains, and eventually in the Puget Sound convergence zone Friday evening. Friday will be a bit cooler yet, with highs mainly in the mid 60s to lower 70s. An upper level ridge will follow...starting to impact the area during the overnight hours Friday night and dry conditions and warmer temperatures expected for Saturday. Temperatures will increase only a few degrees with mid to upper 60s expected along the coast and low to mid 70s in the interior lowlands. SMR .LONG TERM...The aforementioned ridge does not last very long over the area as the ridge axis exiting Saturday night and the feature itself east of the area by late Sunday morning...setting up the next main weather factor over the area...an upper level trough with its associated low situated well north of the Canadian border. Models still not really syncing up with this feature when it comes to the details...and precip signatures with both GFS and ECMWF are pretty weak...so while POPs are present in the extended...they are confined to the northern third of the CWA and only get as high as low-end chance wording...while partly to mostly sunny skies are expected for the remainder of the area. This feature will keep temps over W WA right near seasonal normals...with lowland temps hovering right around 70 degrees. SMR
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&& .AVIATION...
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Light southwest flow aloft tonight will become more westerly Thursday. The air mass is moist and unstable in mid and upper levels with thunderstorms over the Cascades and Olympics and adjacent lowlands. Most convection should die out by sunset at 04Z. Thunderstorms may impact the terminals around Puget Sound before then and they are carried in several TAFs. Low level onshore flow will become strong this evening and will bring a marine layer inland across much of the interior of Western Washington for IFR or low MVFR ceilings Thursday morning. KSEA...Discussion above applies. TAF has a TEMPO group for thunder 23Z-03Z. Ceilings will be VFR until after 11Z when marine stratus will bring MVFR ceilings. Northwest wind 4-8 kt this afternoon becoming southerly 5-10 kt overnight. CHB .MARINE...Strong onshore flow will develop late this afternoon and tonight. A gale warning is in effect for the strait, with small craft advisories for adjacent waters. Moderate to strong onshore pressure gradients will continue each day through the weekend. Winds of at least small craft advisory strength are likely each day in the strait, with gale force winds possible each day. For now have kept winds 20-30 kt each afternoon and evening. CHB
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Gale Warning until 5 AM PDT Thursday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 3 AM PDT Thursday for Admiralty Inlet- Northern Inland Waters Including The San Juan Islands-West Entrance U.S. Waters Strait Of Juan De Fuca.
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