textproduct: Seattle/Tacoma

SYNOPSIS

A stationary front over the Columbia River will begin to move out of the region this weekend. Widespread precipitation will decrease in coverage into Saturday, with a convergence zone continuing to affect portions of the Cascades and adjacent foothills. Warmer temperatures are on the way next week, although the pattern will continue to remain unsettled with additional systems passing through with more widespread rain.

SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/

The stalled frontal boundary responsible for the persistent rain and snow of the past 36 hours is finally getting shoved southeastward this evening as a trailing upper trough axis swings onshore. Precipitation will gradually taper off from north to south overnight with only a few rain and/or snow showers continuing into Saturday morning. Temperatures, particularly in outlying areas away from Puget Sound, will flirt with the freezing mark by morning. So any snow and moisture that remains could lead to some slippery road conditions. Drier conditions are expected areawide Saturday afternoon into Saturday night as weak upper ridging noses into the area. After a chilly start, much of Sunday looks dry before precipitation starts to creep back into the picture Sunday night into Monday ahead of an approaching warm front. By Western Washington standards, the difference in temperatures between this weekend and early in the coming week will be a sort of weather whiplash as we warm back well into the 50s...perhaps approaching 60 in spots with snow levels skyrocketing well above the passes. No forecast updates planned at this time. Previous discussion with updated aviation portion. 27

The synoptic pattern remains relatively intact, with a stationary front along the Columbia River, and a strong zonal upper level jet over the state expanding over the northern CONUS. A shallow shortwave trough will pass through Saturday afternoon, with a ridge beginning to build over the Pacific late this weekend.

Precipitation: Widespread stratiform showers continue on radar this afternoon. The precipitation has transitioned from snow to rain in spots as temperatures struggle to climb into the upper 30s this afternoon (many spots will likely remain in the mid 30s). The ensembles have the stratiform precipitation diminishing from north to south this evening. A convergence zone is expected to setup across the foothills/west slopes of the Cascades tonight into the first part of Saturday. The overall moisture output from this convergence zone will be significantly less than the precipitation past 24 hours. The temperatures tonight however will be a couple degrees cooler in spots, with lows right around freezing (areas closest to water will see lows in the mid 30s, and mountain areas with fresh snow will see lows in the teens).

For impacts: the heavy snow threat has ended for the Olympics this afternoon, and the winter storm warning was subsequently cancelled. Snow showers will continue, but are not expected to produce any significant impacts. For the Cascades, heavy snow is expected to continue on the west slopes through tonight. Travel continues to remain dangerous in the passes with several roadways experiencing closures at times today. The winter storm warning will continue into Saturday morning, with travelers encouraged to check road conditions on WSDOT before traveling. Hi-res ensemble guidance was keeping a medium to high chance of central Cascade foothill and valley locations receiving an additional 1-2 inches of snow through Saturday morning with the convergence zone. Given the lower snow levels in this area, and the impacts from this morning's snow, the winter weather advisory was extended through 11 AM for the King/Pierce County areas, and expanded to include a portion of the foothills extending into southern Snohomish County. Snow in these areas will be dependent on the location that the convergence zone sets up. Remaining lowland areas have a low probability of seeing any accumulating snow (let alone a trace), but may still see snowflakes come down tonight for any shower that moves through.

The precipitation will wrap up Saturday evening, with a break in any moisture for the majority of the day until Sunday evening (a 30% chance of snow showers will continue in the mountains). Temperatures will warm considerably from Friday into Saturday and Sunday, with highs returning into the mid 40s. Lows will still remain around freezing for most through the weekend. Winds will vary at times between the south and north, but generally remain around 5 to 10 mph.

LONG TERM /MONDAY THROUGH FRIDAY/

There are strong signals that the following week will continue to remain active, as ensembles continue to point to a warmer and wet pattern continuing into next week. Weak atmospheric river conditions are expected to continue as the jet stream remains on the west coast, but shifts north into northern WA/southern B.C. next week. The main concern will be potential hydro impacts from higher snow levels from the warm air moving in, and some snow melt from recent snows. See the hydrology section below for additional details on river flooding. The precipitation is primarily focused in the mountains, and the coast. Temperatures will climb through the week, with highs approaching upper 50s to 60 towards Wednesday through Friday.

HPR

AVIATION

Westerly flow aloft will continue into Saturday before flow transitions more NW Saturday afternoon. Precipitation associated with the frontal system that was stalled over Western Washington continues to slide southwards this evening, and mainly is impacting KSEA southwards as of 03z. Additional showers will develop tonight into Saturday with weak convergence over central Puget Sound. Snow or rain/snow mix will be likely in showers, however, accumulations are not expected on paved surfaces. Due to abundant surface moisture, cigs and vsbys are expected to remain deteriorated for most terminals tonight, especially for KPAE southwards. LIFR/IFR are expected around Puget Sound with MVFR/IFR elsewhere. VFR cigs will continue for the coast. Further improvements to widespread VFR are then forecasted by midday Saturday with more SCT/BKN conditions. Light winds this evening will increase from the west Saturday morning with increasing onshore flow.

KSEA...Steadier snow will subside between 0400z to 0430z as precipitation continues to taper off and move south of the terminal. Weak convergence zone activity will support isolated snow or rain/snow showers tonight (mainly after 07-08z) into Saturday morning before drier conditions return after 15z. Cigs and vsbys will remain deteriorated tonight with light surface winds. Primarily LIFR conditions through 15z with brief IFR cigs at times into Saturday morning. FG also likely to develop overnight and continue through 13-16z Saturday. Cigs will improve to MVFR mid Saturday morning before returning to VFR generally after 18-19z. Light, variable winds will become light S/SW tonight with light S/SE winds on Saturday. JD

MARINE

A stalled front near the Oregon border this afternoon will shift southeastward tonight and Saturday as a surface ridge builds into the coastal and offshore waters. This will lead to increasing onshore flow over the coastal waters and in the Strait of Juan de Fuca tonight into Saturday. The ridge will shift onshore Saturday night and Sunday as a strong warm front approaches the offshore waters. The warm front will lift northward across area waters Monday into Tuesday for increasing winds and seas.

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HYDROLOGY

Flood Watches remain in effect for SW Washington including Grays Harbor, Thurston and Lewis counties due to potential flooding. Both the Newaukum River near Chehalis and the Skookumchuck River near Bucoda have reached flood stage, with the Chehalis at Grand Mound following early Saturday. Rivers will crest this weekend.

An atmospheric river is expected to move inland to the north early next week. This could bring additional flooding concerns to portions of the area as several rivers are forecast to reach Action Stage. For now, the only river forecast to reach flood stage is the Skokomish River.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Storm Warning until 5 AM PDT Saturday for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and Northern King Counties-Cascades of Southern King County.

Flood Watch through Sunday afternoon for Cascades of Pierce and Lewis Counties-Foothills and Valleys of Thurston and Lewis Counties-Foothills of the Western and Southern Olympic Peninsula-Grays Harbor County Coast-Lower Chehalis River Valley-Lowlands of Lewis and Southern Thurston Counties-Middle Chehalis River Valley-Olympia and Southern Puget Sound-Olympics-Willapa and Black Hills.

Winter Weather Advisory until 11 AM PDT Saturday for Foothills and Valleys of Snohomish and Northern King Counties.

Winter Weather Advisory until 11 AM PDT Saturday for Foothills and Valleys of Central King County-Foothills and Valleys of Pierce and Southern King Counties.

PZ...Small Craft Advisory from 11 PM this evening to 11 PM PDT Saturday for Central U.S. Waters Strait Of Juan De Fuca- 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 from 11 PM this evening to 3 PM PDT Saturday for East Entrance U.S. Waters Strait Of Juan De Fuca.