textproduct: Seattle/Tacoma

SYNOPSIS

An upper-level ridge centered offshore with northwesterly flow aloft and light low level flow will allow temperatures to trend warmer through Friday. Cooler temperatures are expected for Saturday and Sunday before much warmer temperatures return for Monday and Tuesday. Elevated fire weather conditions are expected Monday and Tuesday.

SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/

An upper-level ridge is centered offshore the Pacific Northwest with broad troughing over central Canada this afternoon. Through Thursday, western Washington will be under subtle ridging with northwest flow aloft. Onshore flow will weaken tonight with less stratus coverage Thursday morning. Temperatures trend warmer Thursday with highs in the 70s along the coast and in the upper 70s and mid 80s across the interior. Ridging amplifies eastward on Friday as a low pressure system and associated cold front nears the Pacific Northwest coast. A return of slightly stronger onshore flow on Friday will result in cooler temperatures along the coast with warmer temperatures inland. Inland highs in the upper 70s to mid 80s brings widespread minor HeatRisk to the region with pockets of moderate HeatRisk in the Seattle metro area.

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LONG TERM /SATURDAY THROUGH WEDNESDAY/

Western Washington will be under the influence of broad troughing on Saturday with temperatures cooling by 5-10 degrees. This will be short-lived as ensembles are in good agreement for an amplified upper-level ridge offshore to begin shifting inland early next week. 25th-75th percentile maximum temperature spread is around 4-5 degrees across the inland areas, leading to high confidence for high temperatures in the upper 80s to low 90s for Monday and Tuesday. Confidence is lower across the coast where 25th-75th spread is between 8-11 degrees. This stems from uncertainty in the strength of the thermally induced surface pressure trough and its influence on the strength of the offshore flow. There is high confidence (90-100% chance) for widespread moderate HeatRisk for the inland areas with a low chance (20-30%) for major HeatRisk in the Seattle metro area.

Confidence in the synoptic pattern decreases on Wednesday with around 40 percent of ensembles suggesting a breakdown of the upper-level ridge bringing cooler temperatures. The other 60% holds onto some degree of ridging with warmer temperatures continuing into Wednesday and beyond. This is also reflected in the 25th-75th percentile max temperature spread increasing to 8-11 degrees across most of western Washington.

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AVIATION

Strong northwest flow aloft will continue just north of the terminals with higher surface pressure off the coast keeping the pattern onshore. Clouds will continue to scatter out this afternoon with conditions becoming VFR (few lingering areas of MVFR remain this afternoon west of Puget Sound but were scattering out). Winds will continue to shift this afternoon to the west and northwest as stronger winds aloft mix down. At the surface, 5-10 kt with an isolated gust to 20 kt being possible through the evening. For Thursday morning, the interior has a 10-20% chance of MVFR cigs in the morning, while the coast has a 30-40% chance of IFR ceilings overnight. Clearing time for any low cigs is by 18Z. Winds overnight become northeast at around 5 kt or less, then become northerly this afternoon 5-10 kt with a few gusts to 20 kt.

KSEA...VFR generally expected through the TAF period, with a 10-20% chance of an MVFR briefly from 15-18Z. Winds this afternoon and Thursday afternoon are out of the north 5-10 kt, with gusts to 20 kt more likely Thursday afternoon. Overnight, winds will turn northeast at 5 kt.

HPR

MARINE

High pressure at the surface will remain over the waters, with lower pressure inland keeping the pattern onshore through most of the weekend. Flow over the coastal waters will continue to remain strong over the coastal waters, with gusty northwesterly winds continuing through the outer coastal waters with gusts of up to 30 kt through Thursday. The winds will decrease on Thursday, before increasing again in the outer coastal waters late Thursday through Saturday, with winds up to 25 kt. Diurnal pushes through the Strait of Juan de Fuca will continue, but likelihood of them reaching 20 kt or greater remains low.

Seas will remain at 9 to 10 ft, and decrease to 6-8 ft in the inner coastal waters Thursday through Saturday. The outer coastal waters will see seas decrease to 6-8 ft Sunday through next week.

HPR

FIRE WEATHER

Onshore flow will continue to keep relative humidities moist through today and Thursday. Temperatures will warm into the mid 80s on Friday, with RH values dipping into the low 30s, and mid 20s in the Cascade Foothills. There is a cooldown going into Saturday, with improved RH values. The pattern warms significantly going into Monday and Tuesday, with highs into the 90s for portions of western WA. Widespread RH values into the mid and low 20s (potentially the teens) are expected for portions of the south Puget Sound/south interior, and portions of the Cascade Valleys. Winds only become breezy in the afternoons early next week for the south Cascades, with west winds 5-10 mph and potential for gusts to 20 mph. Fuels will continue to dry and the atmosphere may become conditionally unstable with a thermal trough moving along the coast Monday and Tuesday under a ridge.

HPR

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 11 PM PDT Thursday 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.