textproduct: Seattle/Tacoma

SYNOPSIS

Weak upper level low off the Central 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/

Satellite imagery shows low clouds have spread inland overnight and are moving east of Puget Sound at 3 am/10z. High clouds over the area above the stratus. Mild early morning with the cloud cover. Temperatures a couple of degrees either side of 60.

Some small differences in the pattern will make a big difference in the forecast this morning. Upper level low off the Oregon coast further back to the southwest than the models are indicating. Shortwave spinning out of the low that would be moving over Western Washington if the low was where the models predicted is instead going by offshore leaving Western Washington with just a marine push in progress. Low slowly moving northeast later today. Instead of another shortwave spinning out of the low, convective activity over the area will be mostly the result of daytime heating helping to destabilize the air mass more than the current levels. This pushes the chances for convection into the late morning and afternoon hours and mostly over the northern portion of the area where the stratus will dissipate first. Still a chance for thunderstorms over the Central and Northern Cascades this afternoon. Cooler air mass will lessen the chances for thunderstorms over the Olympics. Will modify the Red Flag warning changing the start time in the Cascades portion to later this morning and drop the Olympics from the warning. Black Hills visible on the satellite imagery putting the tops of the marine layer below 2500 feet. Stratus retreating to the coast this afternoon but with the southerly flow aloft still middle and high level clouds over the area. Highs in the mid 60s to mid 70s.

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. 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

Southerly flow aloft becoming variable later this afternoon and tonight. Low level onshore flow into Sunday morning. IFR to LIFR ceilings lifting to MVFR by late morning. Ceilings continuing to improve over the interior with VFR ceilings this afternoon. MVFR ceilings continuing along the coast. Low end MVFR/IFR ceilings returning to the interior after 12z Sunday.

KSEA...LIFR ceilings lifting to IFR mid morning, MVFR late morning and VFR by mid afternoon. IFR ceilings possible again Sunday morning after 12z. Southwest wind 6 to 12 knots easing to 4 to 8 knots tonight. Felton

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

Red Flag warning will continue for the Northern and Central Cascades today for the possibility of lightning and gusty erratic outflow winds. Chances for thunderstorms have gone down this morning in the Olympics and the warning for there will be dropped. Elevated fire weather concerns will continue until the upper level trough moves east of the area Sunday. 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

SEW WATCHES/WARNINGS/ADVISORIES

WA...Red Flag Warning from 11 AM this morning to 8 PM PDT this evening for West Slopes of the North Cascades Generally above 1500 Feet.

Red Flag Warning from 9 AM this morning to 5 PM PDT this afternoon for West Slopes of the Central Cascades Generally above 1500 Feet.

PZ...None.