textproduct: Seattle/Tacoma

SYNOPSIS

An upper ridge over Western Washington will slide to the east into Thursday. A weakening frontal system will move through late Thursday into Friday. An upper trough will deepen offshore on Friday. A compact low pressure moves into the area late Saturday through Sunday. Lingering precipitation continues into Monday.

SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/

Mainly clear skies continue for most of Western Washington this afternoon, with continued fog and low stratus from Seattle to Olympia. Fog and low stratus will become more widespread once again tonight due to light surface flow and mainly clear skies this evening. Will need to monitor the potential for dense fog, in addition to freezing fog, with temperatures near to below freezing away from the water. Otherwise, dry conditions expected into Thursday morning.

A weakening frontal system will move into the area Thursday afternoon into Friday for lowland rain at times. Although snow levels will rise to near 5000 to 5500 feet for the central Cascades Thursday night into Friday, surface temperatures will remain colder, along with light east flow at the surface at the Cascade Passes. This increases concerns for freezing rain potential, especially at Snoqualmie Pass, Thursday evening through Friday morning. There remains some uncertainty as to the precipitation type, as well as temp profiles during this period. However, both HREF/REFS suggest 50-80% probabilities of 0.1" of freezing rain during this period. As such, have issued a Winter Weather Advisory for the Snoqualmie Pass region, with the highest probabilities east of Tinkham Road along the I-90 corridor. A wintry mix will also remain possible at Stevens Pass during this period, but less confidence for US-2 at this time. Otherwise, high temperatures will remain on the chillier side in the mid 40s on Thursday, warming into the low 50s for some areas by Friday.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

A deep upper low will approach the west coast Saturday with continued troughing offshore into early next week. A compact surface low pressure system will move northwards along the coast later Saturday into Sunday. There remains uncertainty into both the strength and track of this compact low during this period. However, the majority of ensembles do track the compact system along the Washington coast during this period, with variation in strength. The main concern is surrounding the potential for gusty winds. This uncertainty in wind gusts can be seen well in the 10-90th spread in 24-wind gusts from NBM, ranging 20-45 MPH for Puget Sound, and and 20-55 MPH for the coast and northern interior. As such, the wind potential will continue to be refined over the next few days.

Coastal flooding is also expected during high tides Saturday and Sunday, both for the coast and interior. This is due to higher astronomical tides, as well as the combination of lower pressure (from the compact low) and increased winds and waves. At this time, minor to moderate coastal flooding is expected for both Saturday and Sunday. Major coastal flooding cannot be ruled out either for the coast this weekend, particularly due to the stronger winds and increasing seas. Water levels look to rise up to a maximum of around 2 to 2.5 feet above ground level for most locations along the water this weekend.

Otherwise, continued periods of lowland rain and mountain snow (or brief wintry mix) are expected Saturday into Monday. Confidence lowers in the pattern Tuesday into midweek, however, ensembles suggest another potential frontal system with more westerly flow aloft. JD

AVIATION

An upper level ridge will continue to deamplify over the region with light southwesterly flow aloft. Surface flow remains light east/southeasterly to calm. Mostly calm winds will prevail again tonight with offshore flow continuing. Clear skies prevail over much of western Washington this afternoon, with the notable exceptions of the Strait of Juan de Fuca coast and the south-central Puget Sound where clouds very slowly continue to evaporate. While clearing is now starting to occur over Seattle proper, it's tough to say if it will be enough to fully clear out over Bremerton or Shelton. It may just be for a short time before another round of fog is expected to redevelop this evening. Models remain aggressive in redeveloping fog as early as 00-03Z this evening with more widespread fog by 06Z tonight. The question will be how early the fog clears out tomorrow morning. A weak disturbance will be coming up from the south on Thursday, bringing in wind and some light rain that should help to mix out the fog. The earlier solutions have fog clearing out pretty early in the morning, while others keep it going through at least mid-morning. It will depend on how much this incoming disturbance is able to push away the fog. Low ceilings, however, will more likely persist later into the morning. Light rain will begin to move in across the southern half of the area by the end of the TAF period.

KSEA...Low clouds continue at times as of 23z. A brief period of VFR conditions may occur through sunset before continuous fog is expected to quickly return with LIFR conditions. There is low confidence in the timing of improvement. Models are hinting at some improvements in visibility by early morning/15Z or so, but low ceilings will likely persist through at least 18Z until the next system arrives Thursday afternoon. JD/62

MARINE

A weak weather system will move northwards into the area Thursday into Friday with light southeast winds during this period. A stronger system is expected over the weekend, resulting in increased winds Saturday through Sunday. Headlines are expected during this period with the increasing winds. In addition, there is also a 60 to 80 percent probability of gale force wind gusts along the Coastal Waters Saturday and Sunday, and will need to monitor for potential Gale Watches. Winds then look to become lighter early next week.

Fog also continues around Puget Sound this afternoon, mainly south of Everett. Will need to monitor for the potential of widespread dense fog again this evening through tonight for Puget Sound.

Seas of 5 to 7 feet will continue through Friday before building to 10 to 14 feet Saturday and Sunday. JD

HYDROLOGY

Multiple frontal systems will move across Western Washington later Thursday through the weekend. The Skokomish River will rise during this period, and is currently forecast to exceed minor flood stage on Sunday. No additional river flooding is expected elsewhere into next week. JD

SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory from 1 PM Thursday to 7 AM PST Friday for Cascades of Southern King County.

PZ...None.