textproduct: Seattle/Tacoma

UPDATE

12z sounding from KUIL indicates a sharp inversion in the lower-levels but just above, mid-level lapse rates sit around 8.6 C/km. This instability has been tapped into to say the least as an upper-low offshore has ushered moist (PWATs 1.1-1.3 in.), diffluent southerly aloft across the area. These factors in concert with present instability has lead to scattered thunderstorms off the coast and over W WA. Main threats include lightning, small hail, erratic winds and brief heavy rainfall. The Red Flag Warning also remains in place through the day for the Olympic and Cascade mountains due to lightning activity.

Shower coverage is slated to decrease into the afternoon and the aforementioned upper-low begins to lift into BC. Cloud coverage will help to stifle high temperatures with forecast highs in the 60s to lower 70s. The previous discussion remains below along with an marine/aviation section.

SYNOPSIS

Widespread showers and the threat for thunderstorms today as a low pressure system passes through, exiting the region this weekend. High pressure builds over the area next week, bringing a return to warm and dry weather.

SHORT TERM /TODAY THROUGH SATURDAY/

Current satellite imagery showing a tale of two cloud types, the first, marine stratus along the coast and the second being mid to high level clouds over the Cascades...mainly slop-over from persistent showers and thunderstorms east of the crest. Latest radar confirms this, with echoes on the east side of the Cascades. Looking to the south though, one can already see shower development in western OR that will make its way up into W WA later on this morning.

While the forecast by and large remains consistent with messaging from previous days, the focus for precip and thunderstorms has shifted somewhat. Chances for both seem to favor the western half of the CWA. This does not mean that no activity is expected east of the Sound, but with lingering moisture from the first wave of activity last night, this may be enough to ease some of the instability and make any activity today a little sluggish to start. Of course, this inhibition would be most prevalent over the western slopes of the Cascades and the nearby foothills and valleys. For the East Puget Sound Lowlands, the I-5 Corridor and surrounding areas, the better chances start around noon today and, depending on how far north you are, will increase accordingly...the Seattle metro leaning more toward a prime time of early to mid afternoon while the best chances may not reach Bellingham until late afternoon or early evening.

Forecast remains consistent on precip exiting quickly with the event wrapping up in all but the northernmost portions of the CWA by midnight tonight. Some isolated showers may be possible Friday, but for the most part, dry conditions return promptly Friday morning and remain that way for the remainder of the short term period.

Cooler daytime highs areawide today with interior lowlands generally in the lower to mid 70s while coastal areas seeing temps about 10 degrees cooler in the lower to mid 60s. The warming trend kicks in quickly starting on Friday, although current forecast shying away from previous solutions of a steep increase in temperatures. Instead, Friday sees daytime highs in the mid to upper 70s /mid 60s along the coast/ and Saturday is only incrementally warmer with temps in the mid 70s to around 80 /mid to upper 60s along the coast/.

18

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

Although upper level troughing will remain in place over W WA for the remainder of the weekend, weather will be more determined by a pair of upper level ridges...one over the eastern Pacific and the other increasing in amplitude over the desert southwest/Four Corners region. This pair will keep any moisture out of the area and well to the north and provide a boost to temps on Sunday as highs climb into the lower to mid 80s in the lowlands and upper 60s to lower 70s along the coast. The eastern Pacific ridge shifts eastward, becoming the main influence over W WA for the start of next week. This will push temperatures up more as highs both days will be in the mid to upper 80s inland and lower to mid 70s along the coast. Although a weak upper trough moves into the area Wednesday, it appears to have zero impact on temps, as daytime highs remain static. Given these temps, HeatRisk will remain Minor/Yellow for Sunday, but as 80s become more widespread, so too will Moderate/Orange HeatRisk...with Moderate/Orange conditions being widespread for the remainder of the long term forecast period.

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AVIATION

A cutoff upper low tracking northward off the coast will yield southerly flow aloft over the airspace. Ahead of this system, low marine stratus cover along the Pacific coast will continue to support IFR/low-end MVFR cigs through much of the period. Light and variable winds early this morning will build out of the south to southwest to 8-12 kt with gusts around 15 kt possible, except along the Strait of Juan de Fuca including KCLM where west-northwest flow will continue. Radar continues to track two areas of showers and thunderstorms moving northward this morning - first group in the interior west of Mt. Rainier up into the east Seattle metro, and another area along the coast, although both groups of convection have been trending downwards as of 16Z. Main concerns is lightning, with perhaps a few cells producing small hail and gusty outflow winds. Tops of the storms have reached 30,000 ft so far, with ceilings down to MVFR. Residual convection may linger into the afternoon once the first wave moves through as the atmosphere remains unstable with the trough (20-30% chance). Ceiling probabilities for Friday morning favor MVFR in the interior, and IFR for the coast.

KSEA...Showers/vicinity thunderstorms to continue this morning though activity has been ramping down to the south and east. Can't rule out redevelopment this afternoon from 21Z-00Z, but this remains a 20-30% chance. There is a 40% chance of MVFR development Friday morning. Light west winds early this morning will back out of the southwest and build to 8-12 kt by 18z Thu, before easing below 5 kt again after 06-09z Fri.

Picard/HPR

MARINE

A weak surface low meandering over the coastal waters through Friday will see northwesterly winds early this morning back out of the south to southwest by this afternoon, then veering out of the northwest through the day on Friday as the low exits. Thunderstorms will move over the coastal waters this morning, with lightning, small hail and gusty winds all possible. Diurnal westerly flow will continue through the Strait of Juan de Fuca, peaking each afternoon and early evening. A Small Craft Advisory has been hoisted for the central Strait and eastern Strait entrance from 1-11 PM PDT today when winds will reach 15-25 kt, before easing tonight. Surface high pressure building offshore through this weekend will see a return to typical summertime northerly flow. Diurnal gusts may reach 20-25 kt in the afternoon, most likely beyond 30 NM.

Coastal seas of 5-8 ft will ease slightly to 4-6 ft by tonight, with a continued dominant west-northwest swell at 7-8 seconds. As northerly flow strengthens through the weekend, seas will again build to 5-8 ft, with the highest chances for seas reaching 8-10 ft across the outer waters beyond 30-40 NM.

Picard/HPR

FIRE WEATHER

A Red Flag Warning remains in effect for the mountains today. Showers and scattered thunderstorms will remain a concern through this evening with a highlight on the potential for dry lightning (storms resulting in less than 0.25 of an inch of rainfall) and strong outflows between 35-50 mph. Storms that develop on the west slopes of the Olympics are more likely to produce a wetting rain (0.25 inches or more), while storms along the Cascades will produce sub- wetting rain amounts of 0.10-0.20 inches. As mentioned above, latest forecast seems to be favoring more activity in and around the Olympics as opposed to the Cascades, however with fuels in the Cascades being drier than their Olympic counterparts, a significant risk remains even if chances there are a little lower. Fire weather concerns will linger through the remainder of the forecast period as a hotter and drier pattern returns quickly over Western Washington. This has the potential for increasing fire activity from any new lightning starts from today's convection.

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 Central Cascades Generally above 1500 Feet-West Slopes of the North Cascades Generally above 1500 Feet.

PZ...Small Craft Advisory from 1 PM this afternoon to 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.