textproduct: Seattle/Tacoma

SYNOPSIS

An atmospheric river will impact the region through much of the week, bringing multiple rounds of rain and warmer temperatures to western Washington. Higher snow levels and periods of moderate to heavy rain will bring rises to area rivers this week, with river flooding possible. Wet and unsettled conditions look to continue into next weekend as another system moves into the region.

SHORT TERM /TODAY THROUGH WEDNESDAY/

A warm front offshore, associated with an atmospheric river aimed at British Columbia, will continue to stream precipitation into western Washington this morning. Snow continues to fall across portions of the Central and North Cascades early today, but expect a gradual transition over to rain as conditions warm and snow levels rise to 8000-10000 ft. Latest observations indicate that snow has already transitioned over to rain at Snoqualmie Pass early this morning, so have allowed for the Winter Weather Advisory to expire for the Central Cascades. With snow levels being the last to rise across the Northern Cascades, have maintained the Winter Weather Advisory for the Cascades of Skagit and Whatcom counties through mid morning. Precipitation will continue through the morning hours as the front lifts across the area. A brief break is expected this afternoon and early evening in the wake of the front before the moisture axis over British Columbia gradually sinks southward and brings the next round of rain to the area tonight into Tuesday. Temperatures will be warm and mild across the region today, with afternoon highs expected to be in the 50s to low 60s (across the southern Sound). Conditions will be breezy at times today into Tuesday, with wind gusts to 25-35 mph possible. Breeziest spots will be areas along the coast and across the North Interior.

Another round of widespread rain will spread over the area on Tuesday as the moisture axis of the atmospheric river gradually sinks southward across the area. Periods of rain will continue into Wednesday as the axis remains positioned over the region. For western Washington, the highest QPF remains concentrated across the northern two-thirds of the region (primarily along the Coast, Olympics, and the Cascades from King County northward). Rises will occur on area rivers, with the main concern throughout the week being potential hydrological impacts from higher snow levels and from rain falling on recent snow in the mountains. For more info on river flooding, see the hydrology section below. Temperatures will remain warm and mild Tuesday and Wednesday, with afternoon highs expected to be in the 50s and overnight lows expected to be in the upper 40s to low 50s.

LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/

The warm and wet pattern will persist through much of week, with additional periods of rain expected through Friday before the plume of moisture pushes southward. The heaviest precipitation will remain focused across the Cascades from King County northward, across the Olympics, and along the Pacific Coast for the second half of the week. Ensembles then hint at a cooler trend commencing next weekend as the atmospheric river exits the area and another system moves in from the northeastern Pacific. While there is still some disagreement to just how wet and cool conditions will get over the weekend, snow levels look to drop back down between 2000-3000 ft. Afternoon highs over the weekend will be in the 50s, but overnight lows look to drop back into the 30s.

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AVIATION

Scattered showers will continue today, with more reaching the inland terminals by late morning as FROPA cause wind speeds to pick up for the second half of the TAF period. Areas generally from KSEA north will see speeds between 10-15 kts by this afternoon. Cigs will see some improvement as well, with VFR conditions returning. This FROPA will allow for a brief break in rain, but rain returns tonight as the frontal boundary sags south into western WA, bringing more MVFR conditions.

KSEA...VFR at the terminal early this morning. More rain is pushing inland, and -RA will return after 17Z, lingering into the afternoon. As the front moves through, expect southerly winds to increase to 8- 10 kts after 17Z, lingering until around 00Z-02Z. After the rain is forecast to end or become more isolated after 20Z, MVFR cigs should return to VFR. VCSH returning after 04Z this evening, though conditions look to remain VFR at this time.

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MARINE

Southerly flow will increase over the waters tonight and Monday as a warm front lifts north into B.C. Seas will remain elevated through Tuesday. Southerly flow will prevail through Thursday generally within 10-20 kt range. Grays Harbor Bar conditions will be close to SCA criteria come Tuesday as the seas approach ten feet. For the Small Craft conditions in Puget Sound, expect the more elevated winds to occur from Seattle north. For the southern Puget Sound, winds will be slightly less than in the aforementioned areas.

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HYDROLOGY

River flooding continues for the Chehalis River at Porter, which is currently cresting and expected to recede today, before falling below flood stage this afternoon.

An atmospheric river impacting the region this week will bring additional flooding concerns as it brings periods of moderate to heavy rain to western Washington. While the bulk of the moisture will initially be focused on British Columbia today, it will gradually sink southward into our area on Tuesday and remain over the region through much of the week. Highest QPF amounts will remain focused on the coast, the Olympics, and the central and northern Cascades. The combination of higher snow levels and rain on recent snows in the mountains will lead to rises on area rivers, especially those flowing off the central and northern Cascades. Rivers will continue to run high through the end of the week, before receding over the weekend.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory until 9 AM PDT this morning for Cascades of Whatcom and Skagit Counties.

Flood Watch from Tuesday afternoon through Wednesday afternoon for Foothills of the Western and Southern Olympic Peninsula-Middle Chehalis River Valley-Olympia and Southern Puget Sound-Olympics-Southern Hood Canal-Willapa and Black Hills.

PZ...Small Craft Advisory until 5 AM PDT Tuesday 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-West Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 5 AM PDT Tuesday for Admiralty Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca- Northern Inland Waters Including The San Juan Islands- Puget Sound and Hood Canal.