textproduct: Seattle/Tacoma

SYNOPSIS

Weak upper ridging over the Pacific Northwest will bring dry weather to begin the week. A stronger early season system Wednesday into Thursday results in more widespread rainfall and gusty winds. The deep upper low lingers over the Pacific Northwest into next weekend for cooler conditions.

SHORT TERM /TONIGHT THROUGH WEDNESDAY/

Mainly clear skies early this morning across Western Washington with weak upper ridging in place. Localized fog and low stratus may develop early this morning, otherwise, high clouds will be around at times today. Dry conditions continue through tonight. Highs today peak in the 70s for most areas.

Troughing begins to approach the Pacific Northwest offshore on Tuesday with increasing clouds. Although Tuesday is expected to be predominantly dry into the afternoon for most locations, a few showers begin to increase in the afternoon, with a 10% probability of thunderstorms in the Cascades late due to increasing south flow aloft.

A very deep, anomalous early season upper low moves southwards out of the NE Pacific into the offshore waters on Wednesday. This system will bring widespread rain into Western Washington Wednesday through Thursday. The heaviest rainfall is expected over the Olympic Peninsula given the vicinity of the upper low, however, measurable rainfall is expected for all locations. In addition to the rain, gusty winds are expected, especially for early September standards, as noted on the ECMWF EFI. NBM highlights the probability for wind gusts exceeding 40 MPH between 30 to 70% for the immediate coast, Whidbey Island, San Juan Islands, and western Whatcom County. Elsewhere, gusts of 25 to 35 MPH are forecast at this time. Given the vegetation on trees, early season wind impacts such as blown down tree limbs and isolated power outages can't be ruled out. Temperatures drop into the 60s on Wednesday due to the presence of the stronger system by midweek.

LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/

Unsettled conditions continue into Thursday as the upper low placement remains offshore of Washington and Oregon. Showers will remain a threat Thursday. Thunderstorm probabilities also increase towards 15 to 30% on Thursday, and will need to monitor the lightning threat in heavier showers. Although winds will be lighter on Thursday, continued south winds remain locally breezy. Ensembles are in good agreement of troughing over Oregon and Washington Friday into Saturday. The rain threat is expected to slowly diminish during this period, but wrap around moisture may still bring continued precipitation to some areas. Uncertainty begins to increase by late weekend, however, ensembles do begin to show troughing slowly moving eastwards heading into next week. JD

AVIATION

A brief upper level ridge will build over the terminals today with high pressure underneath. A trough will dig southward offshore on Tuesday, which will result in the flow aloft switching from west/northwest to southwest. Satellite still reflects mid and high cloud coverage across most terminals. ACARS soundings this morning shows a large dew point depression over the Puget Sound terminals this morning (around 8 degrees this morning). As a result, the fog formation this morning is going to be limited to areas that are able to cool off the most, as well as with light or calm winds, and near moisture sources. Highest probabilities for IFR/LIFR conditions remain the Pacific/Strait of Juan de Fuca coastlines at 30-40% chance this morning. This trickles down to 10-20% for south interior, as well as north Puget Sound and South Interior locations (which will be more isolated compared to the coast). Any fog that does form will clear around 18Z for VFR skies the remainder of Monday. Winds today will remain out of the north at 5 kt or less this morning, becoming 5-10 kt during the day. They will become light and variable tonight and switch to the south Tuesday.

KSEA...VFR conditions anticipated to continue through the TAF period. Fog likelihood is around 10% this morning, but the formation is likely to remain isolated around the terminal (if indeed it does form). Winds increase to 5-10 kt during the day, but become variable under 5 kt tonight as they switch to the south. There is a 20-40% chance of MVFR ceilings Tuesday morning.

HPR

MARINE

High pressure will remain offshore over the coastal waters and into Tuesday. Benign conditions with light winds will continue into Tuesday as a weak system bypasses to the north. A significant pattern change remains on track midweek with a large-scale low pressure system and front dropping down from the Gulf of Alaska Wednesday into Thursday. Confidence has remained persistent with the 60-80% likelihood for most of the coastal waters, the east Strait of Juan de Fuca and Admiralty Inlet waters over the past 3 days with the model data. A gale watch was issued for these areas for the day Wednesday as sustained winds of 25-35 kt remain likely, and some potential for a couple gusts to 40 kt. Remaining waters will likely see some impact from gusty winds out of the south and southeast with this system, though the threat of gale gusts is much less (for Puget Sound/Admiralty, and the central/west Strait of Juan de Fuca sections). The system will also bring several rounds of rain and a slight chance of thunderstorms for the coastal waters on Wednesday. The system will move out again on Thursday, and winds will decrease significantly going into the weekend.

Seas will remain at 3-6 ft through late Tuesday. The midweek system will bring seas around 8-11 ft Wednesday into Thursday, but will drop back to 4-6 ft Friday into next weekend.

HPR

FIRE WEATHER

A brief warming and drying trend will occur today under weak high pressure. Relative humidity minimums in the central and southern Washington Cascades will decrease to around 25 to 30 percent this afternoon. Finer, flashier fuels will be more apt to respond to the dip in RH. More clouds arrive ahead of the stronger system for only partly sunny skies on Tuesday. By late Tuesday night into Wednesday, the stronger system arrives for widespread rain, windy conditions, and slight chances for thunderstorms.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Gale Watch from Tuesday evening through Wednesday evening for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands.