textproduct: Seattle/Tacoma

SYNOPSIS

Weak upper level ridge with light onshore surface flow through Thursday. An upper level low approaching from the southwest on Friday will move into the coastal waters Saturday. Shower chances increasing Friday night into Saturday with thunderstorms possible. Low exits to the north Saturday night. Upper level ridging Sunday and Monday will move east Tuesday as an upper level trough develops off the coast.

SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/

Dry and mild weather is expected across western WA through Friday with high pressure over the area. The low level flow remains onshore for morning clouds and afternoon sunbreaks. Temperatures will track close to average. An upper level low will spin offshore Friday night then start to shift inland. The air mass will be slightly unstable under moist, diffluent flow. This may result in high- based, light showers over the southern portion of the CWA Friday night. There's a low-end (10-15%) chance of thunderstorms in the southern Cascades. 33

LONG TERM /SATURDAY THROUGH WEDNESDAY/

An upper level trough will push into western WA on Saturday bringing showers, a chance of thunderstorms and cooler conditions. Rainfall amounts are generally under 0.10" with the highest chances of wetting rains over the northern Cascades near the Canadian border. Temperatures will be cooler and in the 60s to lower 70s. The low will shift NE into B.C. on Sunday with a few lingering showers over western WA. Moving forward, we're back to mainly dry and mild conditions early next week as weak high pressure returns and low pressure remains offshore. 33

AVIATION

Southwest flow aloft will continue with weak low level onshore flow through Thursday. Only a few high cirrus level clouds remain in the interior with VFR conditions. Winds are beginning to shift to the northwest, but most locations will see them only range 4-6 kt through the evening (with a few potentially reaching 8 kt). Winds become calm or light and variable for most locations tonight. Another marine push is likely for Thursday morning for the same areas as the previous night. The light winds may result in some mist and fog mixing in with the low stratus, but in general IFR/LIFR chances remain highest from the Kitsap Peninsula westward. Formation time will be from roughly 06Z along the coast, to 10-12Z in the interior, with clearing time ranging from 16-18Z in the interior, to 20-22Z along the coast. Winds Thursday will pick up out of the northwest during the day at 4- 6 kt.

KSEA...VFR through roughly 12Z, then stratus likely reaching the terminal at around 12Z and lasting through 16-18Z before clearing. IFR/LIFR probabilities continue to remain elevated enough (along with forecast/pattern consistency) for the forecast of low ceilings in the morning, although the likelihood for fog/mist remains less than 5%. Northwest winds shifting to the north 4-6 kt through the evening, then most likely becoming calm or light/variable overnight, before picking back up from the northwest later Thursday.

HPR/Mazurkiewicz

MARINE

High pressure over the waters will weaken going into Friday. A weak surface low and trough will move over the waters Saturday with a weak frontal system. There is a slight chance of showers and isolated thunderstorms for the interior waters going into Saturday afternoon. A few weaker pushes through the Strait of Juan de Fuca appear likely going into the weekend as a result of this front, although confidence remains low in any of the winds meeting advisory thresholds for small craft. Additionally, some portions of the Puget Sound waters may also get a touch breezy Saturday afternoon as the front pushes through. Otherwise, benign conditions are expected going into next week.

Seas will hold at roughly 3 to 6 ft during the forecast period.

HPR

FIRE WEATHER

Mild conditions and onshore flow promoting high overnight relative humidity recoveries will keep fire weather concerns at a minimum through Thursday.

An upper level trough will move into the Pacific Northwest later on Friday and into the weekend, bringing elevated fire weather concerns. Thunderstorms are forecast to develop over the southwestern Cascades late Friday and expand northward throughout the day Saturday. The highest chances for convection are over the Olympics and Cascades (25-35% chance), though the lowlands could also see some thunderstorm activity on Saturday. Rainfall amounts have increased for this system, and localized wetting rains are possible with any storms that develop alongside gusty and erratic outflow winds and abundant lightning. However, the air mass moving into western Washington is much cooler and more moist, so any lightning strikes may not have long to smolder with minimum relative humidity values bottoming out in the 60-70% range Saturday.

Conditions will calm on Sunday as the storm system moves out of the region and onshore flow resumes, bringing back continued high relative humidity recoveries. This will also be accompanied by morning low stratus and afternoon sun, near normal temperatures, and generally calm winds into the middle of next week.

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SEW WATCHES/WARNINGS/ADVISORIES

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