textproduct: Seattle/Tacoma

SYNOPSIS

Weak upper level low off the northern Oregon coast this morning will continue to weaken and move northeast today. The low will be off the Washington coast tonight and move inland as an upper level trough Sunday morning. Upper level ridge building in behind trough later Sunday will move east Monday. Western Washington on the backside of the ridge Tuesday. Another upper level trough arriving Wednesday or Thursday hanging around into Friday.

SHORT TERM /TODAY THROUGH MONDAY/

The upper low is slowly approaching the western Washington coast this morning. Convection developed early this morning, and continues across the Cascades, and primarily north of King County. Widespread thunderstorm activity has been observed over the last few hours over the Cascades, with minimal rainfall due to fast storm motions. There may be a brief break in the convection around midday before surface heating leads to destabilization and another round of afternoon and evening convection, primarily focused in the North Cascades. Will also need to monitor the low, but non-zero, threat for severe wind gusts in any stronger t-storm this afternoon into the Northern Cascades. Lightning may lead to new fire starts as well.

The low weakens to an upper level trough tonight. Trough moving inland near the Canadian border after midnight keeping at least a slight chance of showers over most of the area. Marine layer moving inland again approaching the Puget Sound by 12z Sunday. Lows in the 50s.

Trough moving off to the east with an upper level ridge building into Western Washington Sunday afternoon. Shower activity confined to the North Cascades Sunday afternoon. Low level onshore flow with the marine layer returning to the coast in the afternoon. Highs once again in the mid 60s to mid 70s.

Upper level ridge continuing to build Sunday night before moving east Monday. Onshore flow weakening bringing up the possibility that the marine layer does not get east of Puget Sound Monday morning. Even if it does it will be shallow and be gone by mid to late morning. More sunshine and warmer temperatures aloft adding up to a warmer day Monday. Highs in the 70s to lower 80s. Lows Monday morning in the 50s. JD/Felton

LONG TERM /MONDAY NIGHT THROUGH FRIDAY/

Extended models in good agreement this morning. Western Washington on the backside of the upper level ridge Monday night into Tuesday for one more warm day. Highs in the 70s to mid 80s.

Upper level trough moving into the area Wednesday hanging around into Friday. Best chance for showers later Wednesday into Thursday. Much cooler with highs in the mid 60s to mid 70s all three days.

Through the 21st Seattle is only 0.5 degrees above normal for the month of August. This cooler end to the month will make it a close call if August ends up being above normal for temperatures. July was slightly below normal. The last time July and August had below normal temperatures in Seattle was 2011. Felton

AVIATION

Fast moving scattered showers and thunderstorms have developed this morning. Expecting this activity to continue into the afternoon and then gradually subside this evening into tonight. Any thunderstorm that impacts the terminals has the potential for frequent lightning, gusty/erratic winds, heavy downpours, and reduced visibility. MVFR to IFR cigs this morning will continue to improve by this afternoon to VFR, with the exception of the coast. Another round of low cigs Sunday morning.

KSEA...IFR cigs will gradually improve this morning, with VFR expected by the afternoon. Scattered showers and thunderstorms may impact the terminal through this afternoon. However, confidence is not high enough at the moment to include TSRA in the TAF. Thunderstorms will have the potential for frequent lightning, gusty/erratic winds, heavy downpours, and reduced visibility. Outside of thunderstorms, southwesterly winds 6-8 kt.

29

MARINE

A weak surface low will move into the coastal waters today. The low will dissipate tonight. High pressure will rebuild Sunday and remain over the area through Tuesday. A weak frontal system will move across the waters midweek.

Winds picking up later this morning but staying 20 knots or less into the afternoon hours. Diurnal westerlies in the Strait of Juan de Fuca each evening for the next few days but at this point peak winds look to be around 20 knots.

Seas 3 to 5 feet for the next few days. Felton

FIRE WEATHER

Scattered showers and thunderstorms continue across western Washington this morning. As a result, a Red Flag Warning remains in place until the early evening. A brief lull is possible towards midday but RAP analysis is picking up on 1000-1500 j/kg of surface based CAPE in the central to northern Cascades this afternoon. As an upper-low offshore continues to track inland, diffluent southerly flow aloft will help to fire off an additional round of showers and thunderstorms - mainly over the North Cascades. Upper level ridge building in later Sunday. Low level onshore flow pattern later Sunday through Tuesday will result in good relative humidity recoveries in the overnight hours easing fire weather concerns. Another upper level trough arriving in the middle of next week increasing the chances for convection. Felton

41/Felton

SEW WATCHES/WARNINGS/ADVISORIES

WA...Red Flag Warning until 8 PM PDT this evening for West Slopes of the North Cascades Generally above 1500 Feet.

Red Flag Warning until 5 PM PDT this afternoon for West Slopes of the Central Cascades Generally above 1500 Feet.

PZ...None.