textproduct: Seattle/Tacoma

SYNOPSIS

Unsettled conditions continue into tonight as an upper low moves across the Pacific Northwest. Cool, dry conditions Thursday and Friday. An upper low will slide southwards along the west coast over the weekend, resulting in the next chance for precipitation.

SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/

Scattered showers continue this afternoon in a post-frontal airmass. Some instability, generally ranging 200-600 J/kg, will continue to result in localized thunderstorms into this evening, particularly along the Olympic Peninsula, as well as in the convergence zone that has developed over King/Kitsap Counties. Small hail, brief gusty winds, and lightning may occur within any thunderstorm. The convergence zone will slowly progress southwards across King County into this evening. Showers will also result in additional snow accumulation at times for the mountains. Snow levels will remain around 1500 to 2000 feet into tonight, with snow for the Cascades Passes. Brief, heavy snow rates can occur in heavier showers as well. In general, precipitation tapers off tonight, with drier conditions developing by Thursday morning.

Overnight temperatures will fall into the 30s for most locations into Thursday morning. In addition to the colder temperatures, skies will clear for the Olympic Peninsula, as well as portions of south Sound. This will help promote frost development into Thursday morning. Have issued a Frost Advisory for many lowland locations across the Olympic and Kitsap Peninsulas. There also remains higher probabilities (over 70 percent) for freeze conditions for Olympia southwards and along the Southwest Interior, for that reason, upgraded to a Freeze Warning for these locations. Will need to monitor the potential for additional Frost Advisories elsewhere, however, lingering clouds are expected for a good portion of the central and north Puget Sound, which may limit any frost. Otherwise, any lingering clouds Thursday morning will clear for some sunshine in the afternoon. Dry conditions then continue through Friday with ridging building into Western Washington.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

An upper low will approach the Pacific Northwest Saturday, and then slide southwards along the west coast early next week. There remains some uncertainty in regards to the eastward extent of the associated front over the weekend, which would impact the potential for additional rainfall, primarily on Sunday. At this time, the highest chances for rainfall remain along the Olympic Peninsula, but precip potential exists elsewhere as well. Either way, this system will remain rather far offshore, and impacts are expected to be limited. Slight precip chances remain into midweek as the upper low moves onshore, with the bulk of the energy focused south of Western Washington. Seasonable temperatures look to return early next week. JD

AVIATION

Ceilings have been trending lower than guidance at some terminals. Therefore, seeing a mix of VFR and MVFR this afternoon and likely to see some bouncing around in between categories at times. Latest radar imagery shows convection well underway this afternoon. The chance of isolated thunderstorms still remains below 20-30%. The time frame for thunderstorms is between now and 02Z. Threats with thunderstorms may include lightning, heavy downpours, small hail, gusty/variable winds. In addition, a convergence zone has developed over PAE this afternoon and is expected to sink southwards and impact both BFI and SEA. High-res guidance suggests embedded thunderstorms are possible with the convergence zone as it moves over the terminals. Winds will shift northerly as well between 23Z-03Z with sustained speeds of 8-15 kt. Shower activity will taper off tonight. Light winds 5 kt or less expected tonight into Thursday morning. MVFR cigs possible (20-40% chance) between 12Z-18Z Thursday.

KSEA...Ceilings have been bouncing between VFR and MVFR this afternoon, with some lower cloud decks lingering around. Isolated thunderstorms remain possible through around 03Z. A convergence zone is expected to impact the terminal this evening as early as 23Z, with winds expected to shift northerly around 23Z and continuing through 03Z with speeds 10-15 kt and gusts to 20-25 kt. Winds will then shift southerly after 03Z with speeds around 5 kt. Guidance continues to highlight a slight chance of MVFR cigs between 12Z-18Z, with current probabilities around 20-30%.

29

MARINE

Isolated thunderstorms remain possible over area waters today. Main concerns with the thunderstorms are erratic/gusty winds, lightning, and heavy downpours. Small Craft Advisories continue in effect for the coastal waters for the combination of steep seas 8-12 ft with a dominant period of 9-10 seconds and winds. Although seas have been hovering slightly below 7-9 ft with the latest obs, seas are expected to build later this evening to 10 ft and thus the Small Craft Advisory for Grays Harbor Bar has been extended through Thursday morning. Thunderstorm activity expected to taper off tonight. High pressure will rebuild over area waters on Thursday and seas are expected to decrease below 10 ft, but still looking to remain steep through Thursday night. A westerly push is expected through the Strait of Juan de Fuca on Thursday. However, current probabilities are between 30-45% of winds exceeding 21 kt. High pressure will weaken on Saturday and the next system is expected to impact area waters this weekend. Latest guidance suggests winds and waves will remain below small craft thresholds at this time.

29

HYDROLOGY

No river flooding is expected during the next seven days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Storm Warning until 11 PM PDT this evening for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and Northern King Counties-Cascades of Southern King County.

Frost Advisory from 1 AM to 9 AM PDT Thursday for Eastern Kitsap County-Eastern Strait of Juan de Fuca-Foothills of the Western and Southern Olympic Peninsula-Lake Crescent Area Including US 101-Lowlands of Pierce and Southern King Counties-Northern Hood Canal-Port Townsend Area.

Freeze Warning from 1 AM to 9 AM PDT Thursday for Lowlands of Lewis and Southern Thurston Counties-Middle Chehalis River Valley-Olympia and Southern Puget Sound-Southern Hood Canal-Willapa and Black Hills.

PZ...Small Craft Advisory until 5 AM PDT Thursday for 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-Grays Harbor Bar.