textproduct: Seattle/Tacoma

SYNOPSIS

Another atmospheric river will bring several rounds of precipitation across the region this week. The first system will arrive tonight through Monday, with rain and gusty southwest winds. Another system Tuesday into Wednesday will bring in slightly cooler air, and accumulating mountain snow. The unsettled weather will continue through the end of the week. The threat of river flooding is expected to continue through the week with the additional precipitation.

SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/

No significant changes made to the forecast this afternoon. The previous discussion can be found below with updates to the aviation section:

The atmospheric river pattern remains in place across the Pacific Northwest. A strong 300 mb jet continues to transport moisture into the region, with the nose of this jet expected to remain along the WA coast through much of the week. A frontal system with this synoptic setup will swing through the region tonight into Monday. This first system is expected to be the warmest of the systems passing through this week, with snow levels hovering around 8,000 to 9,000 ft. The rain will arrive late this evening into Monday, with the hi-res guidance showing more of a convective type mode with the showers. There is also a 20-30% chance of thunder (especially along the coast), which may produce locally heavier amounts (up to 0.25-0.50"/hr). The rain with this round will wrap up Monday evening. All set and done, this round of rain is expected to 0.50-1" of rain in the lowlands, 1-2" along the coast, and 2-3" in the mountains with potential for locally higher amounts exceeding 4". This will cause additional rises in streams and rives (see hydrology section for more details). In addition to rain, a wind advisory continues from tonight through tomorrow evening for breezy southwest winds of 25 to 30 with gusts to 45 to 55 mph. The highest wind gusts are expected along the Pacific coast, although the threat remains nil at the present time for seeing wind gusts exceed 60 mph (high wind warning threshold).

Another system with this atmospheric river will arrive Tuesday into Wednesday. This system will be cooler, as snow levels drop Tuesday night into Wednesday down to around 2,000 ft. The trickiest part of the forecast is determining how quickly the snow levels will drop. Models show more of a delay in the drop of the snow level (occurring Tuesday night/Wednesday morning). The warm air mass may cut into the snow accumulations, and this may delay the cold winds behind the cold front bringing the snow levels down. There is still a 50% chance that Snoqualmie Pass could see over 12" of snow through Wednesday evening (and it's a 90% chance for Stevens Pass). QPF in the lowlands varies quite a bit, but the mean NBM guidance gives a 48 hour total through Wednesday night of just over an inch in the Puget Sound/Northwest Interior, to 1.6" in the south interior. Continue to monitor the forecast as confidence increases on potential impacts with this system.

Additionally, waves are expected to increase along the coast at times Monday through Wednesday, with waves approaching 15 to 17 ft. While the periods will likely carry some of the energy to the coast, the combination of the waves heights and periods fall short of criteria for issuing a high surf advisory at this time.

High temperatures will remain above average through Tuesday, with highs remaining in the mid to upper 50s.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

The Tuesday system continues into Wednesday with precipitation decreasing Wednesday evening. A third system appears likely Thursday into Friday, with a similar amount of precipitation to the previous two systems. The temperatures will cool to around average with highs in the mid to upper 40s (lows dipping into the mid and upper 30s). Ensembles favor a more active weather pattern continuing into next week (although some splits in solutions).

HPR

AVIATION

Southwesterly flow aloft increasing in strength after 12Z Monday morning as the next in a series of systems enters W WA. Surface winds will experience a similar strengthening overnight, becoming gusty by early Monday morning. Surface wind speeds mostly in the neighborhood for 5-10 kts through 08-10Z late tonight/early Monday morning before ramping up 10-15 kts with gusts up to 25 kts. Direction will remain southerly for most locations throughout the TAF period, but speeds are expected to increase again in the mid to late morning hours to 20-25 kts with gusts up to at least 35 kts possible.

Cigs a mix of VFR to MVFR this evening but will gradually sink as rain becomes more widespread with incoming system. Widespread MVFR conditions should be in place for most terminals by 12Z, with locations more prone to lower cigs likely reaching IFR conditions by that point. Cigs sink further in the late morning before aforementioned higher wind speeds facilitate a degree of mixing. This may allow for some slight recovery in the afternoon into high- end MVFR to low-end VFR before persistent rainfall drags them back down into solid MVFR conditions in the evening.

KSEA...Low end VFR conditions in place likely to hang on past midnight, lowering to MVFR by 10Z and bottoming out around 1000 ft in the late morning and early afternoon. As mentioned above, high winds may allow for some afternoon improvement and a return to low- end VFR. MVFR conditions re-emerge in the evening thanks to plenty of low-level moisture. Southerly surface winds will increase throughout the TAF period, ranging 5-10 for much of tonight before ramping up to 10-15kt with gusts up to 25kt by 10Z, climbing again to 20-25kt with gusts to 35kt by 18Z. Some easing in the afternoon but not enough to escape previous range. Main easing of winds will wait until Monday evening, but even then still ranging 10-15kt.

18

MARINE

A front traverses across the coastal waters and seas late tonight through Monday and bring strong winds. A Gale Warning remains in effect for the coastal waters, the west entrance Strait of Juan de Fuca and the corridor extending from Puget Sound/Hood Canal north through the northern Salish Sea. Probabilities of the gale force winds are now 70%-80% across Puget Sound/Hood Canal through the northern Salish Sea. As for the coastal waters, probabilities of gale force winds are above 90%. Seas will also build to 18-20ft beginning late tonight through at Monday and remain elevated through the week, especially as another weather system moves through on Wednesday.

HYDROLOGY

Most rivers will continue to recede through tonight. Atmospheric river arriving Monday morning with rivers rising again by late morning. Even with the rises most of the rivers staying below flood stage Monday. Rivers going over flood stage will be the ones that will have not dropped below much below flood stage over the weekend like the Skagit, Cedar, White and Cowlitz below Mayfield Dam.

Rain easing Monday night into Tuesday before the tail end of the atmospheric river reaches the area later Tuesday into Wednesday increasing precipitation rates. This second round of precipitation will drive some rivers above flood stage with major flooding possible along the Skagit and lower reaches of the Snoqualmie.

Cooler air moving into the area Tuesday night adds some uncertainty to the river levels. Current forecasts have snow levels dropping to 2000 to 2500 feet by Wednesday morning. There is more moisture with this second surge of the atmospheric river but the cooler air will keep some of the moisture in the mountains in the form of snow. This combination makes for tricky river forecasting.

A flood watch is in effect from Monday morning through Thursday afternoon across the lowlands. Felton

SEW WATCHES/WARNINGS/ADVISORIES

WA...Wind Advisory until 10 PM PST Monday for City of Seattle- Downtown Everett / Marysville Area-Eastern Kitsap County- Eastside-Foothills of the Western and Southern Olympic Peninsula-Grays Harbor County Coast-Island County-Lower Chehalis River Valley-Lowlands of Lewis and Southern Thurston Counties-Lowlands of Pierce and Southern King Counties-Lowlands of Western Skagit and Northwestern Snohomish Counties-Lowlands of Western Whatcom County- Middle Chehalis River Valley-Northern Hood Canal-Northern Washington Coast-Olympia and Southern Puget Sound-Port Townsend Area-San Juan County-Shoreline / Lynnwood / South Everett Area-Southern Hood Canal-Western Strait of Juan de Fuca-Willapa and Black Hills.

Flood Watch from Monday morning through Thursday afternoon for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and Northern King Counties-Cascades of Southern King County-Cascades of Whatcom and Skagit Counties-City of Seattle-Downtown Everett / Marysville Area-Eastern Kitsap County-Eastern Strait of Juan de Fuca-Eastside- Foothills and Valleys of Central King County-Foothills and Valleys of Pierce and Southern King Counties- Foothills and Valleys of Snohomish and Northern King Counties-Foothills and Valleys of Thurston and Lewis Counties-Foothills and Valleys of the North Cascades- Foothills of the Western and Southern Olympic Peninsula- Grays Harbor County Coast-Lake Crescent Area Including US 101-Lower Chehalis River Valley-Lowlands of Lewis and Southern Thurston Counties-Lowlands of Pierce and Southern King Counties-Lowlands of Western Skagit and Northwestern Snohomish Counties-Lowlands of Western Whatcom County-Middle Chehalis River Valley-Northern Hood Canal-Northern Washington Coast-Olympia and Southern Puget Sound-Olympics-Port Townsend Area-Shoreline / Lynnwood / South Everett Area-Southern Hood Canal-Western Strait of Juan de Fuca-Willapa and Black Hills.

PZ...Gale Warning until 4 PM PST Monday 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-Northern Inland Waters Including The San Juan Islands-Puget Sound and Hood Canal- West Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 10 PM PST this evening 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.