textproduct: Seattle/Tacoma

SYNOPSIS

An upper-trough will swing a couple of disturbances into the region Thursday through Saturday. Brief ridging is in the offing to begin next week but the pattern is forecast to reopen by Tuesday as additional troughing takes aim.

SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/

Inherited forecast remains on track this evening with no need for forecast updates. For additional forecast details, please refer to the Previous Discussion section below. 18

From Previous Discussion...An upper-trough offshore is analyzed in the 500 mb levels. With it, marine stratus pushed well-inland today but under diurnal heating it has since burned off and retreated back to the coast this afternoon. Southerly flow aloft along with roughly 300-600 j/kg of CAPE has allowed for a slight chance (15%) of thunderstorms today, primarily over the North Cascades. The threat will greatly diminish this evening. For tonight, an additional round of marine stratus will attempt to march inland as the upper-trough shows progression. Overnight lows are forecast in the lower to upper 50s. Increasing rain showers along the coast and the Olympics into the early morning hours on Thursday.

The aforementioned upper-trough will be squarely overhead on Thursday. And with that, rain showers are forecast to become more widespread. The North Cascades and Olympics will see up to 0.25-0.50" of precipitation during the day, elsewhere less than 0.10". Again, can't rule out a slight chance (15%) of thunder for the North Cascades and Olympic mountains. Wind gusts up to 20-25 mph are also expected with the strongest gusts across Whidbey Island. We'll be in between disturbances on Friday but PoPs are favored to remain for the mountains. Convergence in the 850 mb levels is hinting at a developing PSCZ over King and Snohomish counties Friday night. High temperatures for both Thursday and Friday in the lower to mid 70s with overnight lows in the 50s. Coastal max temps will maintain in the 50s.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

Extended models in good agreement with upper level trough over the area Saturday shifting east Saturday night. Temporary upper level ridge moving through Sunday into the first part of Monday before additional troughing and associated upper-lows swing into the region Tuesday onward with cooler temps and more widespread PoPs.

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AVIATION

VFR conditions continue across most of western Washington this evening with a weak front just offshore of the coast. As this front moves across the area tonight, that will bring in periods of MVFR to IFR cigs across the area overnight into early Thursday morning. The area most likely to see more steady low ceilings are KHQM and KCLM, with the clouds being more transitory as a mid-level cloud deck develops behind the front that will bring low-end VFR conditions through much of the day on Thursday. A round of showers will also develop over the Olympic Peninsula Thursday morning and over the Cascades by the afternoon, though no impacts to area terminals are expected. Chances for thunderstorms Thursday afternoon remain fairly low (10-15%), so not confident enough to include at any terminals. South to southwesterly winds around 5-10 kt will continue through the evening tonight, decreasing overnight, and ramping up Thursday with S/SW winds 5-15 kt and gusts up to 20 kt.

KSEA...VFR through the evening with south to southwest winds generally 6 to 10 kt. MVFR ceilings are favored to develop once again early Thursday morning around 10-12z and erode by 16-18z, with a return to VFR with mid level clouds in the afternoon. Surface winds will lighten overnight, then increase to as high as 15 kt Thursday afternoon out of the southwest.

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MARINE

Southerly winds have picked up over the coastal waters today as a weak frontal system crosses over the waters. This frontal system will cause a westerly push through the Strait of Juan de Fuca tonight, though winds are expected to stay just below Small Craft strength.

Weak systems will continue to move over the coastal waters through the end of the week, which will continue to provide pushes down the Strait of Juan De Fuca, along with increasing southerlies over the coastal waters. At this time, winds mostly look to remain under small craft advisory, with increasing chances for Small Craft westerly gusts through the Strait of Juan de Fuca Friday evening. Chances for isolated thunderstorms over the waters also increase Thursday, which may locally produce gusty outflow winds, areas of low visibility, and lightning.

Coastal seas will remain around 4 to 6 feet through the weekend.

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FIRE WEATHER

Overall, the low level onshore flow producing good overnight relative humidity recoveries and cooler daytime temperatures will help ease fire weather concerns for the next few days. Higher elevation slopes and valleys will remain somewhat drier tonight. However, models indicate increased thunderstorm probabilities for the afternoon/evening for today and Thursday. There is a slight chance ( 15-25 percent ) of isolated thunderstorms mainly in the north Cascades but it can't be ruled out around Mount Rainier. If there is a thunderstorm it will not be accompanied by much rain and it could produce erratic and gusty outflow winds impacting existing fires. In addition, the lightning strikes pose a hazard and could lead to new fire starts. JBB/Felton

SEW WATCHES/WARNINGS/ADVISORIES

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