textproduct: Seattle/Tacoma

UPDATE

No changes made in this evening's update. The previous discussion remains below along with an updated aviation section.

SYNOPSIS

Strong upper level ridge offshore weakening Thursday. Low level flow become light beginning Monday allowing more morning fog to form each morning through Wednesday. Weak system moving over the top of the ridge will try and move into Western Washington late in the week into the weekend.

SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/

Satellite imagery shows clear skies over most of Western Washington this afternoon. Small patch of fog hanging tough around Olympia. Temperatures at 3 pm/23z were in the mid 40s to mid 50s except around Olympia where temperatures are still near 40.

High amplitude upper level ridge offshore remaining in place through Wednesday. Interior surface gradients have already gone light and cross Cascade gradient trending back towards zero. Inversion still in place ( temperature at Paradise on Mount Rainier and at 5000 feet on Mount Baker 53 degrees at 3 pm ) and will strengthen overnight. The cross Cascade gradient will continue to move towards zero through Wednesday. The lighter surface gradients will lead to more fog coverage each morning Monday and Tuesday. By Wednesday expect the fog and low clouds to be thick enough to last into the afternoon hours. High temperatures will drift lower each day starting out in the mid 40s to lower 50s Monday and ending up in just the 40s on Wednesday.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

Low confidence in the extended forecast. Previous models runs have had a weak system moving over the top of the weakening ridge offshore. The operational runs have gone dry with the system dissipating before reaching Western Washington with another, all be it weaker, upper level ridge rebuilding offshore. Ensemble wet solutions for Friday are decreasing and now down to less than a quarter of the members. Wet solutions over the weekend also decreasing in the ensembles with a majority of the wet solutions Saturday night and Sunday. Pushing back the possible precipitation has allowed the lower levels of the air mass to warm up with less than 10 percent of the solutions indicating light snow in the lowlands Saturday night and Sunday with no snow solutions before Saturday. With the continued uncertainty will continue with the broadbrush chance pop solution from most of the period. If the trends continue look for the forecast to dry out at least for Thursday and Friday. High temperatures in the 40s with lows in the mid and upper 30s through the period.

AVIATION

Mostly VFR across terminals this evening with weak flow under high pressure. Patchy areas of fog near KOLM & vicinity generating LIFR conditions. Decreasing pressure gradients again tonight under a temperature inversion will leave a moderate chance (40-60%) for fog redeveloping later this evening into Monday morning, with increasing coverage in the morning into portions of Puget Sound, with clearing by the late morning/afternoon Monday. Winds tonight/Monday morning are light NE or variable less than 5 kt.

KSEA...VFR through tonight, with light N/NE wind 4-8 kt decreasing to less than 5 kt overnight (may become variable at times). Fog likely to be in the vicinity of the terminal as early as ~06Z with a 30-50% of it drifting over the terminal between 06Z-18Z. VFR for the remainder of Monday.

HPR/15

MARINE

High pressure/riding will remain offshore through a majority of the week with a thermal trough along the coastline. Fog will be possible in the interior waters Monday morning, with a 50% chance of visibilities less than a mile. There remains no wind concern with the pattern this week (even as the ridge breaks down and a system skirts through towards the end of the week). Seas: 4-6 ft through next weekend.

HPR

HYDROLOGY

No river flooding in the next 7 days.

SEW WATCHES/WARNINGS/ADVISORIES

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