Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 291556 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 856 AM PDT Mon May 29 2023 .UPDATE...
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Onshore flow this morning with extensive stratus across Western Washington, clearer skies at higher elevations of the Cascades and Olympics. Clouds will likely continue through the morning with some clearing this afternoon, however, some areas may stay generally cloudy through much of today. Temperatures will likely be a bit cooler for this reason with highs in the upper 50s to low 60s on the coast, and 60s for the interior.
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&& .SYNOPSIS...
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Onshore flow will continue through mid week for morning clouds and some afternoon sunshine. Troughing will develop along the coast Tuesday through Thursday for cooler temperatures and potential for drizzle. Warmer temperatures likely next weekend into the following week.
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&& .SHORT TERM /TODAY THROUGH WEDNESDAY/...
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Zonal pattern remains in the upper levels to start the short term forecast period. An upper-level low stays down in California and a trough to the east will progress giving way to a long-wave trough coming from the Pacific by Wednesday. At 300 mb the jet core will be over WA by Tuesday night/Wednesday which will support rising motion in the right entrance region. There`s good consistency with this in between the ECMWF GFS and NAM with the aggressiveness of this jet. Dynamically there is some weak vorticity advection at 500 mb and 700 mb Tuesday night/Wednesday to support some lift. However moisture aloft remains relatively dry in the model guidance with no support at the aforementioned levels. Down near the surface a weak surface trough sets up along the Cascade Ridge with onshore push continuing through the short term period. There is sufficient moisture near the surface with RH values approaching 80-90 percent with the marine stratiform pushes coming on the coast. Given the dynamical environment and the onshore pushes will keep temps cooler with the expected morning marine stratus to push inland during the next few mornings. As temperatures climb do expect most areas to clear out for the afternoon and evening for all mornings. The weak synoptic environment will only allow for drizzle in the morning especially near the coast with the moisture near the surface and the drier air higher up. Winds remain light out of the west of 5 to 10 mph with the exception of the Strait of Juan De Fuca with winds 20 to 30 kts. HPR/Kovacik .LONG TERM /THURSDAY THROUGH SUNDAY/...The upper level pattern at the start of the long term forecast period will feature broad troughing extending from the Aleutian Islands into the PNW. A shortwave trough embedded within this flow will skirt across W WA through the day on Thursday. Weak dynamics and meager moisture will prevent much in the way of sensible weather impacts across the area, however an isolated shower or thunderstorm cannot be completely ruled out over the North Cascades. Continued onshore flow in the low levels will make for similar temperatures as what was experienced for much of the short term period. By Friday, the large scale upper level pattern will amplify, with strengthening high pressure over much of Canada and the northern tier of the US and a deepening trough over the far NE Pacific. As the ridge strengthens over Canada, its influence will build westward towards the PNW. Current ensemble solutions favor more of a high pressure influence over W WA vs Pacific troughing, however, continued onshore flow will help keep temperatures in check, allowing them warm back only into the low to mid 70s. Through the weekend, ensemble solutions favor increasing high pressure influence aloft, making for a warming trend. Some uncertainty remains in the degree of warming, but confidence is currently high in a return to temperatures in the upper 70s to near 80. There still remains some concern for a period of above- normal temperatures at the very end of the forecast period and just beyond, along with the potential for the development of weak offshore flow. Kovacik/HPR
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&& .AVIATION...
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An upper level trough in the Gulf of Alaska will bring zonal flow aloft over W WA for much of the day before gradually turning more southwesterly as this feature starts to sag southward. Surface flow remains onshore with generally stable conditions present. Satellite shows marine stratus has made it inland and all the way to the Cascades this morning, with the majority of area terminals under stratus mainly seeing IFR to LIFR conditions. Expect to see slow, but gradual improvement back to VFR conditions for area terminals this afternoon - likely around 21Z. Winds are generally light across the area, persisting at speeds less than 5 kts over through the morning hours. Winds will then pick up slightly after 18Z, ranging generally between 4-8 kts. KSEA...LIFR conditions in stratus. Expect stratus to linger into the early afternoon, before approximately lifting and scattering near 21Z. VFR conditions will resume thereafter. Light S/SW winds persisting this morning at the terminal. A slight uptick in winds expected this afternoon, with winds expected to generally persist between 4-8 kts. 18/14
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&& .MARINE...
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High pressure will remain situated over the waters the next several days, helping to maintain breezy northwesterly winds over the outer coastal waters at times and diurnal westerly pushes down the Strait of Juan de Fuca. Guidance remains consistent with a strong westerly push expected along the Strait this afternoon and evening and as such have maintained the Gale Warning for the Central and East Strait. Additionally, conditions look right for SCA conditions to develop in the adjacent Northern Inland Waters and Admiralty Inlet waters. In the meantime, northwesterlies also look to meet SCA criteria over the coastal waters and Western Strait, meriting additional headlines there. Current seas ranging 3-5 ft will build during the day, getting to 5- 7 ft this afternoon and 7-9 ft tonight before peaking at 9-11 ft Tuesday morning in the outer coastal waters. With seas dominated by short period fresh swell, expect quite choppy conditions to continue over the next few days. 18/14
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&& .HYDROLOGY...The daily hydrology discussion has ended until the start of the next rainy season; it will only be updated as needed. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory from 2 PM this afternoon to 11 PM PDT this evening for West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 5 PM PDT this afternoon for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. Gale Warning from 5 PM this afternoon to 8 AM PDT Tuesday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 5 PM this afternoon to 5 AM PDT Tuesday for Admiralty Inlet-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-Northern Inland Waters Including The San Juan Islands. Small Craft Advisory until 8 AM PDT Tuesday 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. && $$

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