textproduct: Seattle/Tacoma

SYNOPSIS

Zonal flow aloft and onshore flow will lead to daytime high temperatures near seasonal normals through the first half of the week. An upper trough passing across British Columbia will lead to a threat of showers in the North Cascades midweek before warmer conditions return at the end of the week as upper ridging rebuilds over the region.

SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/

More marine air has reached the interior of Western Washington this afternoon with temperatures running 3 to 5 degrees below this time yesterday. The synoptic pattern stays in place through Monday and Tuesday with zonal flow aloft and light low level onshore flow. This translates to Monday and Tuesday being virtual carbon copies of today with coastal stratus making a brief morning visit across the interior before burning off with high temperatures running within a couple degrees of seasonal averages. Models are reaching a better consensus with un upper level trough dipping southward across British Columbia on Wednesday. While conditions will remain mainly dry for most of the area, the threat of a few showers or thunderstorms remains in the cards for the North Cascades Wednesday afternoon and evening. This will remain a primary focus for fire weather concerns in the coming days.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

Heights recover toward the end of the week as upper ridging rebuilds over the region. Probabilistic heat guidance continues to paint the Seattle-Tacoma metro core in 50-70% probability for Moderate HeatRisk on Friday and Saturday with widespread high temperatures in the mid and upper 80s. Model trends suggest that this may be a relatively short warm spell. Ensembles are hinting at some anomalous troughing just offshore early next week which suggests increasing onshore flow and cooler temperatures, but nothing to speak of in the precipitation department at this time.

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AVIATION

Westerly flow aloft persists into Monday as low level flow remains onshore. Much like the previous couple of days, LIFR/IFR coastal stratus will spread partially inland late tonight into Monday morning before burning back to the coast midday. Otherwise mostly clear skies with some high clouds will continue to prevail. Northwesterly winds look to ease tonight becoming mostly light and calm overnight. Winds will increase again throughout the day, with a switch from south to north across the Puget Sound interior with a westerly push down the Strait of Juan de Fuca.

KSEA...VFR through the evening. Low clouds will likely return around the same time frame on Monday with a period of LIFR/IFR between 13Z and 17Z. Surface winds W/NW 7 knots or less this afternoon will back to S/SW tonight. Winds turn back to N after around 00Z Tuesday.

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MARINE

Broad surface high pressure will remain largely centered over the offshore waters with lower pressure east of the Cascades through the week ahead. This will maintain onshore flow over the next several days and bring diurnally driven small craft advisory strength westerlies to the central and east Strait of Juan de Fuca daily. A series of weak systems passing to the north of the area this upcoming week may strengthen onshore flow at times for additional risks of gales along the central and eastern Strait.

Seas across the coastal waters will primarily persist at 4 to 6 feet and may become steep at times under locally driven winds into Monday. Seas subside to 3 or 4 feet by Tuesday before increasing to 5 to 7 feet midweek.

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FIRE WEATHER

A dry, onshore flow pattern with seasonable temperatures and persistent good to excellent overnight RH recoveries through Tuesday. Ensembles continue to very confidently show an upper level shortwave clipping the area Wednesday, with some variability in the strength of the shortwave. This continues to reinforce the scenario of thunderstorm potential Wednesday afternoon and evening for the far northern Cascades. The NBM has a 20-30% probability over the Whatcom, Skagit and Snohomish Cascades through late Wednesday evening. Warmer and continued dry conditions remain on track through the second half of the week, likely resulting in a return of elevated fire weather concerns for some areas.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 5 AM PDT Monday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.