textproduct: Seattle/Tacoma

SYNOPSIS

A warm front will produce lowland rain and mountain snow becoming mostly rain late tonight into Wednesday morning. A second system will bring another round of precipitation Wednesday afternoon into Thursday with some potential for breezy conditions Thursday afternoon and evening. The pattern looks to trend drier and eventually cooler Friday into early next week.

SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/

A warm front is lifting northward across Western Washington tonight. The 00Z KUIL sounding showed a sliver of warmer air above 3500 feet and temperatures are creeping upward at Hurricane Ridge. We're starting to see a surface reflection of the front now with southerly winds picking over the Southwest Interior. All of this amounts to rising snow levels along with the previously advertised threat of freezing rain in the Cascade passes later tonight. Thus far, Steven Pass and Snoqualmie have picked up 9 and 4 inches of snowfall respectively. We're still on track to see another brief lull in precip during the daylight hours of Wednesday before another frontal system spreads additional precip across the area Wednesday night through much of Thanksgiving. As discussed last night around this time, this system will be weakening as it arrives. So, overall, wind won't be much of a concern with this system. It will, however, keep easterly flow going in the Cascade passes which has a way of producing a wintry mess of conditions. Please keep that in mind if you have travel plans that include them both Wednesday night and/or Thanksgiving Day. Previous discussion follows with update to the aviation portion. 27

Precipitation associated with the first of the two systems expected over the next few days has begun to move inland this afternoon. Snowlevels at or around 2500 feet this afternoon, as expected has helped that precipitation fall as snow at all the passes this afternoon with anywhere from 1 to 3 inches accumulating as of 2 PM between Stevens and Snoqualmie passes. This is expected to continue, with a good chance at a period of mixed precipitation, including freezing rain at both Stevens and Snoqualmie Passes tonight as conditions warm aloft and cold easterly flow persists at the passes themselves. Low level easterly flow at the passes should ease early Wednesday morning allowing precipitation to fully transition to rain.

Overall the precipitation forecast remains consistent with a good chance at a break in the day Wednesday, before warm frontal precipitation pushes back into the area from the southwest later in the afternoon and evening. This is associated with a developing system over the eastern Pacific that ensembles have been showing moving inside of 130W Thursday morning. The ensembles have trended further south with the track and strength of the surface low. There is increasing confidence in the low weakening as it approaches the coast, with still some variability on the ultimate track, which is now trending somewhere from the northwest Oregon coast and the central Washington coast. Overall this system will boost precipitation again Wednesday night through much of Thursday with snow levels above 5500 feet and as high as 7000 feet at times. In addition, as the surface low tracks closer to the coast expect widespread breezy conditions with gusts to 30 mph a good bet. Precipitation may linger a bit later into Thursday evening, but overall storm total amounts remain consistent with earlier forecasts.

LONG TERM /FRIDAY THROUGH TUESDAY/

The overall pattern will trend drier into Wednesday, with ensembles in solid agreement of a high amplitude ridge developing over the eastern Pacific and becoming the dominant feature in the overall pattern through the weekend. There is some uncertainty regarding the potential for a weak system affecting western Washington late in the weekend - but given the pattern this would be a brush-by at best. Overall expect drier conditions with a trend to cooler Sunday morning into Monday with 70% chance of some lowland locations - especially south of Puget Sound along the I-5 corridor dipping down to near freezing. Otherwise expect near normal high temperatures and slightly below normal temps for most locations into early next week.

AVIATION

Westerly flow continues aloft as a warm front lifts northward across Western Washington. This will continue to produce widespread IFR or occasional LIFR ceilings along with surface visibilities reduced to 2 to 4 statute miles at times. Warm frontal precipitation will decrease overnight, but poor ceilings and occasional reduced surface visibility is likely to persist through much of Wednesday morning. Some brief, albeit marginal, improvement is expected by Wednesday afternoon for interior terminals, but ceilings are expected to deteriorate back to low MVFR in increasing precip across the area Wednesday evening as another frontal system approaches.

KSEA...Precipitation will ease overnight, but ceilings are likely to fluctuate between IFR or low MVFR through at least 18Z Wednesday with surface visibility dropping to 3-5SM at times. This is pretty typical with the passage of a warm front. Confidence in significant improvement for Wednesday afternoon is not high. Increasing low level easterly flow may help erode the low ceilings briefly Wednesday afternoon, but the arrival of the next system will bring low MVFR ceilings back into the terminal by late evening. Surface winds E/SE 5 to 8 knots veering more southerly overnight, then backing to easterly again Wednesday afternoon. 27

MARINE

A small craft advisory remains in the effect for the coastal waters and portions of interior waters as a frontal system moves across the area. Winds likely peak this evening, reaching back beneath advisory thresholds by early morning Wednesday. However, may need to extend the SCA for the coastal zones due to seas (either remaining above 10 ft or due to steepness - both are equally possible given the latest trends and guidance). A bit of a lull in wind follows later Wednesday, but another strong front will bring another round of advisory strength winds Thursday. Late in the week, high pressure begins to rebuild over the coastal waters into the weekend. Will need to monitor the strength of any northeast flow developing, but at this point strong Fraser outflow isn't particularly likely. Cullen

HYDROLOGY

The first in a series of system has moved widespread precipitation into the area today, with a second system bringing another round Wednesday evening through Thursday. Snow levels especially Wednesday through Thursday evening will remain above 5000 feet. The flood threat for most rivers remains minimal, with the primary focus remaining the Skokomish. The current forecast has river levels cresting just below flood stage Thursday morning, and this may hinge on the second push of moisture Wednesday into Thursday evening. The flood potential Outlook for Mason County continues to look on track and will remain unaltered at this time.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory until 4 AM PST Wednesday for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and Northern King Counties-Cascades of Southern King County.

PZ...Small Craft Advisory until 1 AM PST Wednesday for Admiralty Inlet-Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm- East Entrance U.S. Waters Strait Of Juan De Fuca.