Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS66 KSEW 160439 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 939 PM PDT Fri Jun 15 2018 SYNOPSIS...A weak upper level low will move south through the area tonight. The low will stall over northern California on Saturday with a chance of showers or thunderstorms over the Cascades. High pressure will build offshore Sunday and Monday with dry northerly flow over the region. Temperatures will trend 10 to 20 degrees above normal, especially over the interior, as the ridge moves inland. The ridge will weaken by Thursday. && SHORT TERM...Onshore flow and providing enough marine influence to keep the air mass generally stable over Western WA. IR imagery shows convection mainly east of the Crest and north over the Southern B.C. Rockies. Models indicate the upper low centered near Vancouver Island will drop south overnight into Saturday. This will allow onshore flow to weaken and shift to period of light offshore flow. There will be some increase in mid level debris cloud as the remnants of thunderstorm activity dissipates and moves south with the low over the WA tonight. Instability is shown by models to be confined to mainly the Cascades and possibly the foothills Saturday afternoon and evening as the low moves south. The WRF shows very light spotty rain amounts over the Cascades Saturday afternoon and evening as a few showers and possibly some thunderstorms develop closer to the crest. While the flow becomes favorable for this activity to drift west, the interior lowlands will remain mostly stable. The current forecast keeps the shower threat limited to the foothills and Cascades which looks reasonable given the lack of moisture and instability over the lowlands. No updates necessary to the current forecast. As the low shifts south, high pressure aloft gradually builds over southern B.C. Sunday through the middle of next week. 500 MB heights on the GFS and ECMWF rise but don`t surpass 5780M until Monday. Thermally induced low pressure develops over the interior of Western WA, not along the coast, which indicates the spike in high temperatures will most noticeable around Puget Sound and the southwest interior eastward. Temperatures will reach at least 10 degrees above average by Sunday and 10 to 20 above average early next week with highs in the upper 80s away from the water. Mercer LONG TERM...Previous discussion...Warm and dry conditions will continue into Tuesday and Wednesday with high pressure over the region. Although recent guidance is slightly cooler, we are still expecting to see widespread 80s across the area, with a few low 90s possible. The coast will see a sharp temperature gradient with onshore flow. The beaches will likely stay in the 60s to lower 70s with low 80s inland. The upper level ridge will weaken by Thursday and models are showing an onshore push for cooler conditions overall. A weak trough will cross the region on Friday with a few showers possible in the Cascades. 33 && .AVIATION...
-- Changed Discussion --
An upper low near Vancouver Island will drop south through Western WA tonight and Saturday. Light northwest flow aloft will become northeasterly by Saturday afternoon as the low moves south. Air mass somewhat moist in the lower to mid levels. Stable except weakly unstable near the Cascade crest, then increasing instability over the Cascades Saturday afternoon and evening. The threat of thunderstorms will remain near the Cascades. Most of the lowlands will remain VFR with few-sct clouds in the lower to mid levels. Patchy MVFR is possible in prone locations after midnight, mainly at KPAE. MVFR will be more widespread at times along the coast, but any low clouds will lift quickly mid to late Saturday morning with the change in flow and mixing. KSEA...VFR conditions are expected thru Saturday. Showers and isolated thunder should remain over the Cascades during the afternoon and evening. Radar will need to be watched as the flow becomes northeasterly allowing some moisture to shift west of the Cascades. However, all models show any showers or thunder dissipating before reaching the interior lowlands due to more stable conditions.
-- End Changed Discussion --
&& .MARINE...
-- Changed Discussion --
Onshore or northwest flow will persist through Saturday evening. Winds over the inner coastal waters appear to have subsided so the small craft was dropped, but continues over the outer waters through tonight. Westerly flow through the Strait will also reach small craft strength at times, mainly through around midnight. The flow will become weak offshore or northerly late Saturday night, continuing through Sunday morning, due to a 1020 mb high over mainland British Columbia. Expect a diurnally driven pattern of onshore or northwest flow during the afternoons and evenings Sunday through Tuesday. Not anticipating any major changes to the pattern until perhaps the latter part of next week.
-- End Changed Discussion --
&& .SEW WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
WA...None. PZ...Small Craft Advisory until 3 AM PDT Saturday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 6 AM PDT Saturday 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.
-- End Changed Discussion --
&& $$ www.weather.gov/seattle

USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.