textproduct: Seattle/Tacoma

SYNOPSIS

Weak upper level ridge over the area through Thursday with varying degrees of low level onshore flow. Upper level low well offshore slowly moving northeast to off the coast Friday. Weak system spinning out of the low could produce some showers over the area Friday. Low moving through Saturday keeping a chance of showers in the forecast for some locations. Weak upper level ridge with low level onshore flow returns Sunday into the first part of next week.

SHORT TERM /TODAY THROUGH THURSDAY/

The current forecast remains on track with no planned updates this morning. 33

Previous discussion...Satellite imagery shows high clouds moving out of the area. Stratus moving inland as far east as the Southern Hood Canal to the south and just west of Whidbey Island to the north. Temperatures at 3 am/10z were in the mid 50s to mid 60s.

Stratus will continue to slowly move inland getting east of Puget Sound a little after sunrise. The base of the layer is near 500 feet so higher terrain like around Paine Field could get some visibility restrictions for a few hours this morning. The layer is shallow and will retreat back to the coastline by midday/early afternoon. Highs in the 70s and lower 80s inland, mid and upper 60s coast.

Little change in the pattern tonight through Thursday with weak upper level ridging aloft and low level onshore flow. Marine layer moving inland tonight into Wednesday morning getting east of Puget Sound after sunrise. The layer will be a little shallower Thursday morning with a good chance it doesn't make it east of the Sound. If it does it will not last long. Highs Wednesday similar to today, 70s and lower 80s inland and mid and upper 60s coast. Highs a couple of degrees warmer Thursday with mid 70s to mid 80s inland and mid 60s to lower 70s coast. Lows through the period in the 50s. Felton

LONG TERM /THURSDAY NIGHT THROUGH MONDAY/

Not a lot of confidence in the extended forecast for Friday and Saturday. There is good agreement that a weak upper level low will move into the offshore waters Friday then kick out to the northeast into Southern British Columbia and weaken Friday night into Saturday. There is not good agreement on how much shower activity will occur over the area while the low is in the vicinity. Flow aloft becoming southerly Friday into Friday night which brings up the possibility of convection over the Cascades and Olympics Friday afternoon and evening. Ensembles aren't much help with a wide variety of solutions for Friday night and Saturday for the lowlands. Current blend solution has the shower activity confined to the coast and the northern half of the area. Given the wide variety of ensemble solutions look for this forecast to change in future forecast packages.

Models in better agreement Sunday and Monday with a return to the pattern we are currently in with weak upper level ridging aloft and low level onshore flow giving the area normal latter half of August weather.

The record high for Seattle today is 88 degrees. This is the first record high below 90 degrees since June 21st. The chances for a 90 degree plus day in Seattle drop significantly beginning today. There have been 91 90 degree plus days in August in Seattle since records started at Seattle Tacoma airport. Only 15 have occurred August 18th or later. There have been 19 days in September with highs in the 90s. The total of 34 90 degree plus days August 18th or later is only 12% of 90 degree plus days on record ( 34 out of 290 ). Almost half ( 15 out of 34 ) of the late season 90 degree days have occurred in the last 10 years. Felton

AVIATION

IFR/LIFR cigs and reduced visibility continue across the coast and Puget Sound terminals. Slow improvement expected this morning, with VFR conditions expected for interior terminals by the afternoon. As for the coast, minimal improvement this afternoon to IFR. SW this morning will transition to W/SW this afternoon. An additional round of marine stratus is likely overnight Tuesday into Wednesday morning, with more LIFR to low-end MVFR coverage.

KSEA...LIFR cigs with reduced visibility this morning. Already seeing some improvement in visibility and ceilings, and will continue to trend this way into the afternoon. Ceilings will scatter out this afternoon and VFR conditions will prevail into tonight. IFR/LIFR cigs possible Wednesday morning with another round of marine stratus. SW winds today 4-6 kt.

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MARINE

High pressure will remain over the far offshore waters and slowly build into area waters through the middle of the week, which will maintain northwesterly winds over the coastal waters. A weak system will approach area waters today, with little to no impact to the waters. Northwesterly winds will increase into Tuesday over the outermost waters, which may develop steep seas around 6-7 ft with a dominant period of 7 seconds. The threat for steep seas may continue through mid- week. High pressure will weaken late in the week as a frontal system passes over the coastal waters. Weak westerly pushes down the Strait of Juan de Fuca will continue this week. The next round of westerlies that may exceed 21 kt is Friday, with the latest probabilities around 60-80%.

FIRE WEATHER

The current weather pattern with low level onshore flow creating good humidity recoveries overnight into the early morning hours will keep fire weather concerns at a minimum through Thursday.

Pattern change Friday will elevate fire weather concerns with the potential for convection in the Cascades and to a lesser extent the Olympics Friday and Saturday. While precipitation amounts may bring localized wetting rains, any storms that develop will also be capable of generating gusty and erratic outflow winds and abundant lightning.

Fire weather concerns easing Sunday into the first part of next week with the return of weak upper level ridging aloft and low level onshore flow. Felton

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...None.