textproduct: Seattle/Tacoma

SYNOPSIS

Showers and thunderstorms will continue over the mountains over the next several days under the influence of an upper level trough. Southwest flow aloft will develop over the weekend, bringing some warming ahead of another weak trough at the beginning of next week that will likely bring slightly cooler and unsettled conditions.

SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/

An upper level trough will remain overhead over the next several days as onshore flow continues at the surface, resulting in fairly similar conditions between today and Friday. The marine stratus layer has eroded towards the coast for most inland locations, with enough instability this afternoon to generate showers and thunderstorms over the Olympics and western Cascades. While relative humidities in the mountains remain fairly high as a result of continued onshore flow, fuels are dry enough to cause concern with incoming lightning. The potential for abundant lightning and gusty and erratic outflow winds has resulted in the issuance of a Red Flag Warning through this evening. For more details, see the FIRE WEATHER section below.

The region will see another round of low stratus expanding inland tonight, though the stratus layer is not expected to linger too long into the morning Thursday for most inland areas. Continued instability under the influence of the upper level low will maintain chances for afternoon showers and thunderstorms over the Olympics and Cascades, and a Fire Weather Watch has been issued for these areas for Thursday, once again for the potential for abundant lightning and gusty outflow winds. More of the same can be expected on Friday, with morning stratus and chances for showers and thunderstorms over the mountains. Elsewhere across the lowlands, conditions will remain mild with light winds and mostly clear skies through the remainder of the week.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

Models remain in good agreement with southwest flow developing aloft over the weekend as a high pressure ridge builds over the southwestern US and a cutoff low drops southward offshore. While temperatures will remain mild, the southwest flow may open the door for moisture to advect into the region, with continued potential for afternoon thunderstorms to develop. By the start of next week, a trough axis is favored to cross western Washington, with slightly cooler temperatures and more widespread chances for light shower activity. However, model solutions start to show a larger spread of solutions beyond the weekend, so conditions into next week remain uncertain.

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AVIATION

Shortwave trough remains overhead the terminals today, with flow switching from southwest to northwest as it transits to the east. VFR conditions primarily for most stations this afternoon (except for the coast where stratus remains present and will remain so down to IFR). Few popcorn isolated cells will continue to form in the north Cascades and Olympics this afternoon with no impacts expected for nearby terminals. Winds have transitioned to the west/northwest around 5-10 kt, and will decrease to 5 kt out of the southwest tonight/Thursday morning. LAMP guidance maintains a 30-40% chance of MVFR in the interior Puget Sound terminals from 12Z-18Z, with a 10- 20% chance of IFR. Probabilities for IFR/LIFR increase significantly moving west into the Kitsap, and towards the coast. Additionally, smoke will increase aloft from fires in B.C., but impacts down towards the surface remain minimal.

KSEA...VFR through tonight, then a 30-40% chance of MVFR ceilings between 12Z and 18Z Thursday morning (10-20% chance for a drop into IFR). Northwest winds 5-10 kt will decrease to 5 kt this evening, becoming southwest overnight, and northwest again late Thursday morning/afternoon 5-10 kt.

HPR

MARINE

High pressure in the Pacific will begin to weaken by the end of the week and into the weekend as it gets pushed southward with an approaching trough offshore that will slowly move towards land as it splits early next week. Diurnal pushes through the Strait of Juan de Fuca remains the primary impact, although small craft impact potential remains low until Friday afternoon and this weekend. Winds in the coastal waters will decrease going into the weekend as pressure gradients decrease.

Seas at 2-4 ft with a brief uptick to 6 ft in the outer coastal waters tonight into Thursday, and early next week.

HPR

FIRE WEATHER

A Red Flag Warning has been issued for today for the north Cascades and Olympic Mountains. A Fire Weather Watch has been issued for the same area plus the southern Cascades for Thursday.

Fire weather concerns will remain elevated to critical at times as an upper level trough will remain over the area today through Friday, creating weak instability in the atmosphere. This is bringing onshore flow with higher humidity but also chances of thunderstorms. Area thunderstorm chances have increased for the period, as high as 50-70% over the mountains. Some heavier showers are locally possible with the thunderstorms that, if they last long enough in one place, would put down wetting rains or more. There is a low but non-zero threat of flash flooding and debris flows in burn scars. Fire weather concerns moderate over the weekend.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Red Flag Warning until 11 PM PDT this evening for East Portion of the Olympic Mountains-West Portion of the Olympic Mountains-West Slopes of the North Cascades Generally above 1500 Feet.

Fire Weather Watch from Thursday morning through Thursday evening for East Portion of the Olympic Mountains-West Portion of the Olympic Mountains-West Slopes of the Central Cascades Generally above 1500 Feet-West Slopes of the North Cascades Generally above 1500 Feet.

PZ...None.