textproduct: Seattle/Tacoma

SYNOPSIS

Weak upper level trough over the area with low level onshore flow through Friday. Weak upper level ridge building in from the southeast with southerly flow aloft over the weekend. Another weak upper level trough will arrive Monday and Tuesday.

SHORT TERM /TODAY THROUGH FRIDAY/

The latest forecast remains on track this morning with no updates. Weak upper level trough over the area today with low level onshore flow. Stratus will move a little more inland in the next couple of hours getting east of the Puget Sound. The layer is shallow ( the Black Hills ares showing up on satellite imagery putting the tops below 3000 feet ). This combined with weakening onshore gradients later this morning through the afternoon hours will cause the stratus deck to retreat back to the coastline by early afternoon. Upper level trough axis moving through this afternoon providing just enough instability for a chance of showers and a slight chance of thunderstorms over the Olympics and North Cascades. High temperatures a couple of degrees below normal, 70s inland and mid to upper 60s coast.

Little change in the pattern tonight through Friday. Weak upper level trough remaining over the area with low level onshore flow. Onshore flow increasing in the night and early morning hours and weakening in the afternoon. Stratus along the coast moving inland overnight reaching the Puget Sound area before sunrise. Like today the marine layer will not be deep enough to hang around into the afternoon hours for the interior. Just enough instability in the afternoon to keep a slight chance of showers or thunderstorms in the Olympics and Cascades both afternoons. A touch warmer Thursday and Friday with 70s and lower 80s inland. Highs on the coast remaining in the mid to upper 60s.

A bit unlucky this morning with the area of cirrus moving through right at the peak time of the Perseid meteor shower. Better chance to see them inland late tonight/early Thursday morning. Unfortunately the marine layer will be intact along the coast keeping skies cloudy in the overnight hours. Felton

LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/

Extended models in good agreement over the weekend with an upper level ridge well to the southeast trying to back build into Western Washington. End result is just some weak ridging over the area with 500 mb heights only getting into the lower 580 dms. Will have to keep an eye out on how this pattern develops. We could end up with southerly flow aloft which would open the door to the possibility of late day convection over the Cascades. Model solutions start to diverge Monday with the GFS keeping the weak ridge in place while the ECMWF brings another upper level trough into the area. Ensemble solutions favoring the ECMWF so will have a slight chance of showers in the forecast for Monday and Tuesday with temperatures near or slight below normal.

For the second time in the last three years there has been no measurable rain in Seattle in the first 11 days of August. This is not as common as you might think. In the 82 years of records at Seattle-Tacoma airport the first 11 days of August have been dry 31 times ( 38% ). Out of the 31 years only 7 have had zero precipitation with the other 24 years reporting a trace.

Felton

AVIATION

Westerly flow aloft will become variable tonight as a shortwave trough moves directly overhead western Washington. Stratus has pushed into parts of the interior this morning (from the Strait of Juan de Fuca and SW Interior) with observations ranging from MVFR to LIFR. Light west winds have prevented the ceilings from reaching the surface, but in areas where winds are light, there have been instances of mist/fog being reported (primarily west of the Olympics). Ceilings in the interior will lift to MVFR and VFR from around 17Z and 19Z respectively, with the coast only seeing improvements to MVFR this afternoon. There is a 30% chance of thunder over the Olympics and north Cascades this afternoon (particularly along the crest) but activity at this time is not anticipated to move towards any of the terminals. Light onshore winds early this morning will strengthen this afternoon. Light northerly winds through the Puget Sound look to briefly switch to southwest from 15-20Z this morning before winds switch back to northerly in the afternoon. Another push of stratus is likely tonight/Thursday morning, with a 50-60% chance of MVFR, and a 30-40% chance of IFR in the interior. Higher chances for IFR/LIFR towards the coast.

KSEA...IFR currently present at the terminal this morning - likely to lift to MVFR at 17Z and VFR by around 19-20Z. LAMP guidance also gives similar probabilities for MVFR and IFR conditions Thursday morning as this morning. Brief southwest winds at 5 kt 15-19Z this morning before switching back to west/northwesterly 6-8 kt through the remainder of the day, turning back to the southwest under 5 kt tonight.

62/HPR

MARINE

Broad high pressure over the northeast Pacific will slowly weaken through the week and get pushed to the south through, allowing for winds over the coastal waters to weaken Friday into the weekend.

Westerly pushes down the Strait of Juan de Fuca will continue through the week and into this weekend. Guidance is indicating that this evening's push looks to be less strong with weaker gradients; the stronger winds will likely be confined to the Canadian side. Thus, have held off on issuing a Small Craft Advisory for the Strait this evening. Probabilities for SCA winds increases Friday through the weekend.

Seas 2 to 4 ft today may increase closer to 6 ft through the outer coastal waters tonight into Thursday, otherwise remain around 2 to 4 ft Thursday night through the weekend.

62

FIRE WEATHER

In general a low level onshore flow with temperatures near normal pattern would not be concerning for fire weather. The chance, 20 to 30 percent, for afternoon convection in the mountains the next few days first with the weak instablilty with the upper level trough through Friday then over the weekend the possible southerly flow aloft will keep fire weather concerns elevated for the next few days. Any convection that does develop does not look like it will be accompanied by wetting rains.

Felton

SEW WATCHES/WARNINGS/ADVISORIES

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