textproduct: Seattle/Tacoma

SYNOPSIS

The transition to active weather is underway as a series of systems will reintroduce rain and windy conditions today through the end of the week. A more significant atmospheric river event is gaining forecast traction early next week for more widespread rain and potential hydrologic impacts. Coastal flooding due to higher astronomical tides will remain minor into early next week.

SHORT TERM /TODAY THROUGH SATURDAY/

The upper level ridge that has kept conditions mild is beginning to flatten, allowing for the first in a series of weather events to move in this morning for increasingly widespread rainfall. Expected rainfall through Saturday afternoon will be between 2-4 inches in the Cascades, with isolated peaks to 5 inches. A quarter to half an inch of rain is expected throughout Puget Sound. The warm front will keep rain in the forecast today, with the cold front catching up Friday into the weekend for additional rain through the weekend. Snow levels will start off high now through Friday, but lower to around 4000 feet on Saturday for snow accumulation in the Cascades. Storm total snow from late Friday night through late Saturday night show around 10-12 inches for Stevens Pass with higher amounts into the North Cascades and Mount Baker area. Much lighter amounts are forecast for Snoqualmie Pass on account of the higher snow levels. Winds will increase Friday afternoon into Saturday with the cold front passage and be the strongest along the immediate coast, Strait of Juan de Fuca and portions of the Chehalis Valley. Expect gusts of 20 to 30 mph through early Saturday morning.

Highs today will be in the upper 40s, with the high temperatures climbing back into the 50s as the rain arrives Friday into the weekend. Lows will be in the 40s.

The Coastal Flood Advisory will remain in effect today as inundations of around 2 feet above ground level are possible for the Grays Harbor County coastal areas, including Westport. High tide will occur around 1130 AM today and 1200 PM on Friday. Minor coastal flooding may occur on Friday along the northern coast, but this will be evaluated again in the next forecast cycle.

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

The long term forecast is certainly where it gets increasingly interesting. Another weather system will move over a flattened NE Pacific ridge, bringing about more rain Sunday into Monday. An additional 1 to 2 inches of rain are likely in the mountains, with another half to three quarters of an inch in the lowlands.

Both the deterministic and ensemble model solutions are keying in on a significant atmospheric river event early next week. A prolonged fetch of exceedingly moist air will take aim at the PNW on Monday and last into Wednesday. While the track and duration of the atmospheric river could fluctuate a bit between now and then, the amount of moisture arriving next week is enough to signal 5 to possibly 10 inches of rain in the Cascades, and 2-4 inches in the lowlands. More information can be found in the hydrology section of this discussion, but will certainly have the capability to be impactful from a hydrologic standpoint. Winds will rise as this system moves through the area, as well as higher snow levels by nature of it being an atmospheric river. This event will be very closely monitored and forecast to get a better sense of the range of outcomes and expected rainfall as it gets closer.

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AVIATION

Northwest flow aloft as a warm front moves through the area. A mix of MVFR/IFR/LIFR cigs early this morning with rain and drizzle. MVFR/IFR cigs expected for much of today, continuing into tonight, as the front stalls over the area. In addition, substantial low level moisture will result in lowered vsbys, ranging between 2 to 4 statute miles mainly from 17Z onwards into this evening. Light winds this morning will increase from the S/SE this afternoon, mainly between 5 to 10 kts.

KSEA...IFR cig this morning. Ceilings will likely fluctuate between IFR and low- end MVFR throughout this afternoon. Vsbys will also lower near 2 to 4 statute miles from 17z through the afternoon with rain at times. S/SE winds continue through today, remaining generally around 4 to 6 knots.

JD/Mazurkiewicz

MARINE

A warm front will move into the waters today for increasing south to southeast winds, mainly for the Coastal Waters. Brief Small Craft Advisory wind gusts are possible for the Coastal Waters late this morning, but winds are primarily expected to remain below SCA thresholds. A stronger front will move through late Friday into Friday night, resulting in increased west winds. Latest probabilities of gales for the Coastal Waters peak around 50 to 80% for the outer Coastal Waters, northern inner Coastal Waters, and western Strait of Juan de Fuca. For this reason, have issued a Gale Watch Friday afternoon through Friday night. A stronger westerly push through the Strait of Juan de Fuca is expected late Friday through Friday night, and another Gale Watch has been issued for the central and eastern Strait of Juan de Fuca. Onshore flow will increase elsewhere as well, with additional SCA headlines possible for other waters beginning Friday afternoon. Additional weather systems will then move through over the weekend into into next week, which will result in further increased winds and additional headlines.

Combined seas will remain 5 to 7 feet through tonight. Seas will build to 10 feet Friday afternoon, and peak Friday night around 13 to 17 feet. The period will also range 10 to 13 seconds late Friday into Friday night. Seas will briefly subside below 10 feet on Sunday, before building above 10 feet later Monday into Tuesday. JD

HYDROLOGY

A series of wet weather systems forecast to reach the area will eventually increase the threat of river flooding this weekend, especially into the first half of next week. West to northwesterly flow aloft with the system on Friday will put the area of greatest precipitation in the North Central Washington Cascades. Snow levels, at this time, will be rising over 5500 to 6000 feet.

As the flow aloft becomes more westerly to southwesterly the flood threat will shift to the flood prone Skokomish River late in the weekend.

There's still a wide variance in the possible precipitation amounts with an atmospheric river setting up through the first part of next week. It is still within the realm of possibility that we could have a widespread river flooding event on our hands beginning next Tuesday/Wednesday, with multiple rivers reaching minor and moderate flood stages. Uncertainty continues to remain in regards to the exact flood levels and stages through next week, and as such it will be critical to monitor the forecasts through the next seven days.

The landslide threat is minimal the next few days. The series of wet systems will eventually take its toll with the landslide threat increasing slightly this weekend and rapidly early next week.

JD/Felton

SEW WATCHES/WARNINGS/ADVISORIES

WA...Coastal Flood Advisory until 3 PM PST this afternoon for Grays Harbor County Coast.

Coastal Flood Watch from Friday morning through Friday afternoon for Grays Harbor County Coast.

PZ...Gale Watch from Friday afternoon through late Friday night for West Entrance U.S. Waters Strait Of Juan De Fuca.

Gale Watch from Friday afternoon through Saturday morning for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.

Gale Watch from Friday morning through late Friday night 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 Point Grenville To Cape Shoalwater 10 To 60 Nm.