textproduct: Seattle/Tacoma

SYNOPSIS

One more warm day today with Western Washington on the back side of an upper level ridge. Upper level low approaching from the southwest will move into the coastal waters Saturday and into Southern British Columbia early Sunday morning. This system will bring cooler weather with a chance of showers and thunderstorms Saturday. Upper level ridge rebuilding Sunday will move east Monday and Tuesday. An upper level trough will arrive Wednesday.

SHORT TERM /TODAY THROUGH SUNDAY/

Satellite imagery shows stratus confined to the coast and Lower Chehalis Valley early this morning. Clear skies over the remainder of the area. Temperatures at 3 am/10Z were in the mid 50s to mid 60s.

Upper level low centered off the Northern California coast early this morning will continue to move slowly northeast today. Resulting upper level ridge out ahead of the low giving Western Washington southerly flow aloft. High clouds embedded in the flow aloft moving over the area later today. Light surface gradients this morning with the stratus deck not getting much further east. By midday the stratus deck will be just off the coast. High clouds this afternoon could knock a couple of degrees off the highs versus yesterday with highs in the mid 70s to mid 80s inland and mid 60s to lower 70s along the coast.

Upper level low off the Northern Oregon coast by 12z Saturday. Upper level disturbance spinning out of the low moving into the southwest portion of the area in the early morning hours for a chance of showers. Lows in the 50s and lower 60s.

Upper level disturbance making it's way through the remainder of Western Washington Saturday morning into the early afternoon hours. Chance of showers and thunderstorms with this feature. Fire weather watch for the Olympics and North Cascades will continue with this forecast package. By late afternoon the feature will be far enough to the north for the chances of convection to be north of Seattle. Cloudy skies will keep temperatures well below normal. Highs in the 60s and lower 70s.

Low moves inland into Southern British Columbia early Sunday morning. Chance of showers and thunderstorms continuing north of Seattle with the thunderstorms threat coming to an end in the overnight hours. Low level onshore flow will keep skies mostly cloudy behind the feature. Lows in the 50s.

Chance of showers lingers near the Canadian border Sunday morning as the low weakens and moves northeast. Low level onshore flow continuing keeping cloud cover over the area at least through the morning hours. Highs a little warmer than Saturday but still below normal, mid 60s to mid 70s. Felton

LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/

Upper level ridge building over the area Sunday night moving east Monday. Western Washington remaining on the backside of the ridge Tuesday. Models playing push it back with next upper level trough now timed into the area Wednesday instead of Tuesday on previous runs. Lots of variability with this feature. A few of the ensembles having wetting rain over the northern portions Wednesday. A vast majority of the ensemble solutions going for a chance of showers scenario. In the lower levels varying degrees of onshore flow. At this point Tuesday looks to be the warmest days with highs in the 70s and lower 80s for the interior. Highs mostly in the 70s on the other days. Highs on the coast in the mid to upper 60s.

There has been no measurable rain at Seattle-Tacoma airport this month. just a trace on the 1st. Having a dry first three weeks of August in Seattle is not that common. This year will be the 9th time in the 82 year history of weather records at Seattle- Tacoma airport. The last time it happened was 2012. This year will only be the 3rd time in the last 40 years. Felton

AVIATION

Southerly flow aloft with light flow in the lower levels. VFR conditions into early Saturday morning for the interior. LIFR along the coast this morning improving to VFR this afternoon. LIFR ceilings returning to the coast this evening. IFR ceilings spreading inland after 06z tonight getting east of Puget Sound around 12z Saturday.

KSEA...Increasing high clouds this afternoon and evening. IFR ceilings developing around 12z Saturday morning. Northerly winds 6 knots or less becoming southwesterly around 03z. Felton

MARINE

Broad high pressure over the waters will weaken today. A weak surface low will move into the coastal waters Saturday with the associated front moving through the inland waters Saturday morning. The low will dissipate Saturday night. High pressure will rebuild Sunday and remain over the area through Tuesday.

Low end small craft advisory westerlies in the Central and Eastern Strait of Juan de Fuca tonight. Increasing southerly winds over the inland waters Saturday morning currently forecast to peak around 20 knots.

Seas remaining in the 3 to 5 foot range. Felton

FIRE WEATHER

Dry and warm conditions continuing today. Upper level low offshore moving northeast ending up off the Washington coast early Saturday morning. Convection developing with this feature early Saturday morning over the southwest portion of the area. Convection spreading over the entire area by mid to late Saturday morning. Probabilities for convection are highest over the Olympics and North Cascades, peaking between 20 to 30 percent. Due to the potential of scattered thunderstorms for these areas and the very dry conditions at the surface, a Fire Weather Watch will continue for zones 652, 658, and 661. Convection may exist elsewhere however uncertainty exists in the coverage. Lightning may lead to new fire starts, as well as result in gusty and erratic winds.

Low level onshore flow increases Saturday night into Sunday with varying degrees of onshore flow through the middle of next week. Good relative humidity recoveries each night will keep fire weather concerns at a minimum. Chance of showers in the middle of the week with an upper level trough moving into the area. Felton/JD

SEW WATCHES/WARNINGS/ADVISORIES

WA...Fire Weather Watch from late tonight through Saturday evening for West Slopes of the North Cascades Generally above 1500 Feet.

Fire Weather Watch from late tonight through Saturday evening for East Portion of the Olympic Mountains-West Portion of the Olympic Mountains.

PZ...Small Craft Advisory from 6 PM this evening to 6 AM PDT Saturday for Central U.S. Waters Strait Of Juan De Fuca- East Entrance U.S. Waters Strait Of Juan De Fuca.