textproduct: Seattle/Tacoma

SYNOPSIS

Zonal flow aloft and onshore flow will lead to daytime high temperatures near seasonal normals today and again Tuesday. An upper trough passing across British Columbia will cool temperatures slightly and bring to a threat of showers in the North Cascades midweek. Warmer conditions return at the end of the week as upper ridging rebuilds over the region.

SHORT TERM /TODAY THROUGH WEDNESDAY/

Latest satellite imagery showing marine stratus more advanced than it was 24 hours ago, with cloudy conditions expected by sunrise stretching at least into the east Puget Sound lowlands...possibly even into some Cascade valleys. Current temps also reflecting this more successful marine push, running on average 2 to 4 degrees cooler than this time yesterday morning.

The forecast for today and Tuesday will remain consistent with the general pattern that has been in place for the past few days. Stratus likely to be burned off by late morning and enough daytime heating to get daytime highs akin to what they were yesterday, if maybe a degree or two shy. Latest models currently show that marine stratus push tonight might not be as successful as the current one. That will lead to clouds burning off quicker Tuesday and thusly a degree or two of warming.

Consensus remains intact so far with regard to an upper level trough passing just to the north of the Canadian border Wednesday. Any potential rainfall from that system still looks to be confined to the northern Cascades. Sufficient instability over that area will also allow for the prospect of isolated thunderstorms in the afternoon and evening. The more widespread impact from this passing system will be a dip in daytime highs with mainly mid to upper 70s in the interior lowlands and mid 60s along the coast.

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LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/

Models put some splotches of potential precip over some locations in W WA for Thursday...possibly due to a following shortwave...but it is a feature you would have to squint to find. As such, confidence remains low in this prospect, even though the NBM seems to have latched on to it in the extended. Instead, more likely is the start of a warming trend Thursday returning widespread lower 80s to the interior lowlands, with the coast warming up to the upper 60s to lower 70s. The heat remains on for Friday as highs for the interior top out in the mid to upper 80s. This would result in Moderate/Orange HeatRisk for the entire I-5 corridor stretching from Olympia to Bellingham...with perhaps a small gap around Mt. Vernon. For the weekend, nothing seems terribly impressive as models bring through a pretty low amplitude ridge followed quickly by an inverted counterpart of equally less impressive depth. These features are generally dry and should only result in a gradual cooling trend for the weekend. Even then, daytime highs for the interior are not expected to escape from the 80s.

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AVIATION

Westerly flow aloft will persist through the TAF period, along with onshore flow in the low levels. Marine stratus is currently making its way inland into the interior this morning, and will continue to filter in over the next few hours. This will bring a couple hours of IFR to localized LIFR conditions to the area terminals before stratus retreats back to the coast around mid to late morning. Mostly clear skies will prevail thereafter for the inland terminals, with some high cloud cover passing overhead. Winds are mostly light, persisting at 5kt or less, across the area this morning, except at KCLM where winds persist out of the west and are breezy at times with a westerly push down the Strait of Juan de Fuca. Winds will become more northerly across the interior and increase to 5-10 kt during the afternoon hours. VFR conditions will then prevail for most sites, except for KHQM on the coast, through the remainder of the day. Another round of stratus looks to push inland on Tuesday morning.

KSEA...VFR conditions with light winds at the terminal, primarily around 5 kt or less. Latest guidance indicates a 30 to 35 percent chance of stratus bringing a few hours of MVFR to IFR conditions to the terminal between 13-17Z this morning. Expect conditions to improve back to VFR by mid to late morning. Winds will shift back to the north this afternoon and will persist between 4-8 kt. Another round of stratus may push into the terminal on Tuesday morning, with guidance highlighting similar probabilities of 30-35 percent chance of MVFR conditions developing by early Tuesday.

MARINE

Broad surface high pressure will remain centered over the offshore waters with lower pressure east of the Cascades through much of 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 throughout the week may help 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 4-6 feet today, before subsiding towards 3-4 feet by Tuesday. Seas then look to increase to 5-7 feet again near midweek and will persist within the 3-6 foot range through the second half of the week.

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 early this morning for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory from 3 PM this afternoon to 5 AM PDT Tuesday for Central U.S. Waters Strait Of Juan De Fuca- East Entrance U.S. Waters Strait Of Juan De Fuca.