textproduct: Seattle/Tacoma

UPDATE

An upper-low offshore is tracking inland and steering rain showers into parts of western Washington this morning. Latest KATX/KLGX scans show the activity primarily along the coast and Olympics, spreading inland towards the Cascades later today. The NBM has A 15-25% chance of thunderstorms for the North Cascades this afternoon. High temperatures are forecast to top out in the mid 60s to mid 70s with the coldest temps expected along the coast. The previous discussion remains below along with an updated aviation section:

SYNOPSIS

Western Washington remaining in the upper level trough position through Saturday. A pair of weak systems embedded in the trough moving through today and tomorrow. Upper level low developing in the trough moving over the northern portion of the area Friday night into Saturday. Upper level low dissipating Saturday night with the trough moving east. Temporary upper level ridge Sunday. Dissipating front reaching the coast late Monday with a weak warm front brushing the area Tuesday. A much stronger front arriving Wednesday. This could be the strongest system to impact the area since spring.

SHORT TERM /TODAY THROUGH SATURDAY/

Satellite imagery shows upper level trough approaching Western Washington with the trough axis just offshore this morning. Middle level clouds out ahead of the trough over the area with low stratus along the coast into the Lower Chehalis Valley. Temperatures at 2 am/09z were in the mid 50s to mid 60s.

Upper level trough moving into Western Washington today with a chance of showers west of Puget Sound and in the Northern and Central Cascades. Slight chance of thunderstorms in the same areas beginning late morning and continuing through the afternoon. Some breaks in the cloud cover this afternoon. Highs below normal, in the mid 60s to mid 70s.

Upper level trough remaining over Western Washington tonight into Saturday. Weak disturbance embedded in the trough moving through Friday morning for another chance of showers in the northern portion of the area. Small upper level low developing in the trough moving over Western Washington Friday night into Saturday. Showers likely north and a chance of showers south part by late Saturday morning. Small pool of cool air aloft associated with the low destabilizing the air mass enough creating a slight chance of thunderstorms from about Seattle northward late Saturday morning into Saturday afternoon. Highs Friday in the mid 60s to mid 70s, Saturday in the 60s and lower 70s. Lows in the 50s.

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

Trough moving east Saturday night. Convergence zone over the Central Puget Sound extending into the Cascades will keep a chance of showers in the forecast. Temporary weak upper level ridge moving through Sunday providing a dry, but not very warm, day. Models in good agreement with a dissipating front reaching the coast later Monday. Weak warm front brushing the area Tuesday followed by the trailing cold front Wednesday. While there is agreement on this general pattern the ECMWF is much stronger with the cold front putting a 1000 mb surface low off the coast. If the ECMWF verifies break out the Pumpkin Spice. That has the look of a season ender. Not to say there will not be any warm days in September if the ECMWF verifies. In the last 20 years there has only been one September in Seattle, 2016, without at least one 80 degree plus day. Normal high temperatures cool off quickly in September. By this time next month the normal high in Seattle will only be 68 degrees. Felton

AVIATION

South to southwesterly flow aloft as an upper level trough with a weak front moves through the area today. This weak front will bring in periods of MVFR ceilings for most terminals this morning, along with some chance for some shower activity throughout the afternoon. Terminals along the coast will likely see lower ceilings for most of the day (KHQM). Mid level clouds will develop in the afternoon, allowing for VFR to return to most area terminals by 18z and continue through the evening. Shower activity will be sparse across the area and be located largely in the Olympics, although there is a 10-15% chance of thunderstorms in the afternoon mainly over the North Cascades. However, confidence is fairly low, so will not include it in TAFs at this time. S/SW winds this morning 5 to 10 knots, and will increase this afternoon with periodic gusts up to 20 kts across the interior.

KSEA...MVFR ceilings over the terminal expected to erode around 16z- 18z, with VFR returning by the afternoon. Winds will remain southwesterly and will increase this afternoon, 8 to 12 knots, with gusts up to 15 to 20 kts at times. Winds will start to decrease in strength into the evening.

Mazurkiewicz/15

MARINE

A weak frontal system will continue to move over the coastal waters today. South to southwesterly winds will increase over the area waters this afternoon, though winds are expected to stay just below Small Craft Strength this afternoon. The exception being Puget Sound and Hood Canal interior water zone, as latest guidance suggests a (15-20%) chance of brief gusts up to 21 kt this afternoon.

Weak systems will continue to move over the coastal waters through the end of the week, which will continue onshore flow and pushes down the Strait of Juan De Fuca. The push on Friday may be leaning towards Small Craft Advisory at this time. Otherwise, all other marine waters look benign.

Coastal seas remaining 4 to 6 feet through the weekend.

Mazurkiewicz

FIRE WEATHER

For the most part low level onshore flow, cooler temperatures and precipitation chances through most of the next week will help ease fire weather concerns over the area. The only worrisome feature is the slight chance ( 10-15 percent ) of thunderstorms mainly in the North Cascades this afternoon and again Saturday afternoon. If there is a thunderstorm it will not be accompanied by much rain and it could produce erratic and gusty outflow winds impacting existing fires. New lightning strikes will pose a hazard and could lead to new fire starts. Thunderstorm threat ending Saturday evening. Cooler weather pattern continues into next week. Possible system next Wednesday could bring wetting rains to the area. Felton

SEW WATCHES/WARNINGS/ADVISORIES

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