textproduct: Seattle/Tacoma

SYNOPSIS

A frontal system will move southwards across Western Washington into Sunday for increasing precipitation. An upper trough will move inland on Monday for additional light showers. A wetter system will move through Wednesday and Thursday for heavier precipitation, mountain snowfall, and locally breezy winds. Cooler temperatures likely by late week.

SHORT TERM /TODAY THROUGH MONDAY NIGHT/

A frontal system remains stalled over British Columbia this evening, with latest radar showing the main band of precipitation mainly focused along the northern tier of Olympic Peninsula and Whatcom County (mainly the Cascades) tonight. A cold front will the slide southeast across Western Washington late tonight into Sunday bringing steadier rainfall to the lowlands. Southerly winds have started to increase tonight. Gusts are expected to primarily range between 20 to 35 MPH for most areas and be the strongest near Whidbey Island and near Bellingham. Overall, the forecast largely remains on track this evening, with no significant updates needed. The remainder of the previous discussion follows below, with updates made to the aviation section. 14

Precipitation will be mainly confined to the Cascades Sunday afternoon in a post- frontal airmass. In addition, a convergence zone will develop around midday Sunday over King and Snohomish Counties, resulting in continued showers at times, especially from Everett to SeaTac. Snow levels will also lower near 2000 to 3000 feet by Sunday evening, resulting in accumulating snow for the Cascade Passes at times through Monday. Snowfall accumulations Sunday into Monday are expected to range between 3 to 6 inches for Stevens and Snoqualmie Pass, with locally higher amounts in the convergence zone. If you're traveling over the Passes late Sunday through Monday, monitor the weather conditions and pass information.

Another round of light precipitation is expected elsewhere on Monday as the upper trough moves inland. However, amounts are expected to remain light. Temperatures will cool on Monday with highs in the mid to upper 40s.

LONG TERM /TUESDAY THROUGH FRIDAY/

A brief break in the precipitation on Tuesday morning as ridging begins to build across the Western US. However, a warm front will approach Western Washington late Tuesday, resulting in increasing precipitation. This will begin a period of wetter weather that will continue through Thursday as the next weather system taps into subtropical moisture. Periods of lowland rain are expected Wednesday through Thursday with the frontal system.

In addition, snow levels will remain lower at the onset of the precipitation, likely ranging between 2000 to 3000 feet, resulting in snow over the Cascade Passes Tuesday night into Wednesday morning. Snow levels will then rise to near 4500 to 5500 feet by late Wednesday. However, east flow at the surface may result in a brief period of wintry mix or freezing rain for the Cascade Passes on Wednesday. There remains uncertainty in the orientation of the warm front midweek and how quickly temperatures warm over the Cascades. Either way, snow at the onset (and potentially brief freezing rain) may result in difficult pass conditions midweek. If you are traveling over the Passes beginning late Tuesday for the holiday, continue to monitor the weather forecast.

Uncertainty in ensembles begins to increase Friday, however, a signal for continued troughing will allow for unsettled weather late week, and have kept NBM POPs at this time. Otherwise, temperatures will warm a few degrees above normal Wednesday and Thursday with highs in the low to mid 50s. JD

AVIATION

Terminals across the area are primarily VFR to MVFR tonight. Expect more widespread MVFR conditions to develop overnight and to persist through the day on Sunday as a frontal system sags southward and brings the next round of more widespread rain to the area terminals. Localized IFR conditions will be possible at times, primarily between 10Z-20Z Sunday as the rain moves through. At this time, PWT and OLM still have the highest chances of seeing conditions lower to IFR, with guidance indicating roughly a 35-40% chance of occurrence. The remainder of the terminals have roughly a 10-25% chance of IFR conditions developing on Sunday. Light showers will continue to move into the area ahead of the front tonight, but expect the main band of rain to gradually sink southeastward across the area late tonight into early Sunday. More widespread rain will likely reaching the interior Puget Sound terminals between 10Z-14Z.

Southerly winds have increased this evening and will remain breezy into Sunday morning, with sustained speeds generally between 8-14 kt and gusts to 20-25 kt. Winds will ease again after 18Z Sunday. A convergence zone may develop behind the front, likely between 18-20Z around PAE, and sink southward to BFI and SEA around 21Z.

KSEA...MVFR conditions persist at the terminal tonight and are expected to continue into Sunday as a frontal system sags southeastward through the terminal. There is a small chance (20-25%) of IFR conditions developing at the terminal between 12Z-21Z as the main band of rain pushes through. Southerly winds have increased to 8-12 kt this evening and may be breezy with gusts to 20 kt at times through 18Z Sunday. A convergence zone may shift southward into SEA, likely between 21-23Z and may briefly bring a period of N/NE winds and lower cigs/vis to the terminal.

29/14

MARINE

A weather system will move across the waters through Sunday, bringing increasing winds. Small Craft Advisories are in effect for all waters into Sunday due to the increased winds. Gusts are expected to range between 20 to 30 kts for most waters into Sunday morning, with brief gusts near 35 kts possible for Admiralty Inlet and Bellingham Bay. Winds will then transition more westerly on Sunday, with SCA west winds through the central and eastern portions of the Strait of Juan de Fuca. There may also be brief gale force wind gusts in the eastern Strait of Juan de Fuca Sunday afternoon, however, winds are expected to predominantly remain below 35 kts. Winds will then slowly subside Sunday night into Monday. South winds will increase again on Tuesday as another frontal system moves into the waters. Winds will likely remain elevated through at least Thursday as the system stalls over the area. Additional headlines are expected late Tuesday through Thursday.

Seas of 10 to 13 feet will continue through Sunday before subsiding below 10 feet Monday into Tuesday. Seas are expected to build above 10 feet, especially for the outer Coastal Waters, midweek with the next weather system. JD

HYDROLOGY

A weather system will move across western Washington tonight into Monday for lowland rain and mountain snowfall. No river flooding is expected during this period, but rivers will rise. Heavier precipitation is likely beginning late Tuesday through Thursday next week. Snow levels will also rise to 5000 to 6000 feet by Thursday, resulting in additional rises on rivers. At this time, the Skokomish River in Mason County is forecast to approach flood stage midweek. Although no additional river flooding is forecast at this time, rivers will continue to be monitored. JD

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 4 PM PST Sunday for Grays Harbor Bar.

Small Craft Advisory until 10 PM PST Sunday for West Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory from 7 AM Sunday to 7 AM PST Monday for Central U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 7 AM PST Monday for East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 7 AM PST Sunday for Admiralty Inlet- Northern Inland Waters Including The San Juan Islands- Puget Sound and Hood Canal.

Small Craft Advisory until 10 AM PST Monday 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.