textproduct: Seattle/Tacoma

SYNOPSIS

Low pressure will drop out of the Gulf of Alaska tomorrow, remaining offshore but generating potential for rain and thunderstorms. The threat for thunderstorms expands on Saturday as the low moves overhead and provides supportive upper level dynamics. Cooler, cloudy, and at times showery weather will persist into early next week before conditions begin to warm up and dry out.

SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/

Mostly cloudy across western Washington this afternoon with highs so far in the mid 60s. Overnight into tomorrow, a deep area of low pressure will move offshore out of the Gulf of Alaska, sending a shortwave into the region. This will be the driver of another round of showers and a few thunderstorms. Tomorrow, the thunderstorm threat will be most likely within the Olympic mountains and in areas of the north interior including the North Cascades. Probabilities of thunderstorms range from 15% to 25% in these areas. The primary hazards will be lightning, gusty and erratic winds, and periods of heavy rain within the storms.

The aforementioned low pressure moves in directly overhead on Saturday, providing supportive mid and upper level dynamics for an expanded thunderstorm threat. Most unstable CAPE ranges from 300-400 J/kg, 0-6km lapse rates between 6-7 C/km, cold mid levels, and easily achievable convective temperatures. Most of the area, with the exception of the immediate coast and areas north of Skagit County have around a 20-30% chance of seeing a few thunderstorms throughout the day. The timing of thunderstorm development and activity will largely be between 11AM and 7PM local time. Please use caution when recreating and/or working outside and remember to head indoors when thunder roars.

LONG TERM /SUNDAY THROUGH THURSDAY/

Low pressure exits the region Saturday night, leaving Sunday in more of a transitional pattern. Transient and weak high pressure will fill in, only temporarily. Temperatures will be in the mid 60s, up a few degrees from Saturday. The bulk of the day should remain cloudy, with chances for light showers arriving late in the afternoon. Another round of low pressure arrives Monday for more scattered showers and a slight chance of coastal thunderstorms. This unsettled and cooler pattern lingers into Wednesday before there is any indication of things warming and drying. Temperatures warm slightly back into the low 70s by Thursday.

The latest 8-14 day outlook from the CPC shows increased confidence in above normal temperatures and below average precipitation beginning late next week and heading into the middle of the month.

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AVIATION

Westerly flow aloft will become southwesterly tonight as an upper trough situated west of Haida Gwaii pushes a frontal system into the region on Friday. Low level onshore flow is easing with VFR expected for most areas into this evening. Ceilings will lower to MVFR along the coast late tonight in increasing shower activity associated with an incoming trough. These conditions will spread to interior areas late Friday morning through the remainder of the day. The air mass will become weakly unstable by Friday afternoon for isolated thunderstorm activity.

KSEA...VFR conditions will continue into tonight. There's a less than 20% chance of MVFR ceilings early Friday with ceilings more likely to lower after 18Z-20Z as shower activity ramps up with the next frontal system. Surface winds will veer northwesterly 5 to 8 knots for a few hours late this afternoon and early evening before backing southerly again overnight and rising to 9 to 14 knots Friday AM.

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MARINE

Onshore flow will ease today as a weak surface ridge shifts inland and a broad surface low drops southward in the offshore and coastal waters. A weak front will cross the waters midday Friday into Friday afternoon. A vertically stacked trough trailing the front will move onshore during the day on Saturday with relatively little impact in terms of wind. A weak surface ridge will cross the waters Saturday night into Sunday before another frontal system reaches the area Sunday night and Monday with potential headlines for the coastal waters. Coastal seas look to remain below 10 feet through the period.

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FIRE WEATHER

Onshore flow has been beneficial to most areas of western Washington, yet afternoon RH values are still only in the 40% range for areas from Seattle south. The ERCs will respond to the increase in moisture over the next several days, especially in the fine fuels. Friday and Saturday will feature area wide chances for thunderstorms, but indications are looking more favorable for them containing adequate QPF. The pattern turns warmer and drier toward the end of next week, lasting beyond the middle of the month, so fire weather conditions will continue to be evaluated as that pattern evolves.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 11 PM PDT this evening for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.