textproduct: Seattle/Tacoma

UPDATE

Showers continue this morning across the Puget Sound region, Chehalis River Valley and Cascade foothills/valleys. Some snow has mixed down into lowland areas but with no reports of significant accumulation. Showers are likely to persist into the afternoon as an upper-low swings out of BC and sideswipes the region. Hi- res guidance mainly place these features over areas of terrain such as the Cascades and Olympics/Olympic foothills near Lake Cushman where snow will likely be the dominant precipitation type into the evening. The previous discussion is below along with an updated aviation section:

SYNOPSIS

A trough and upper level low will move inland today into Friday, with rain and snow showers diminishing through today. A brief ridge will dry the region out later today into the first part of Friday. A blast of cold air from the Fraser River will keep temperatures chilly next couple of mornings, with wind chills into the low 20s and teens in many locations. Another low will dig down offshore from the Gulf of Alaska. The pattern will become active again this weekend into next week, with several rounds of rain, mountain snow, and gusty winds.

SHORT TERM /TODAY THROUGH SATURDAY/

A stacked upper level trough with an upper level low was located over the British Columbia/Washington border. This trough will spin an additional shortwave over the region this morning, which will interact with the stationary front over the central part of the state. The rain/snow showers in Snohomish County have diminished, and remnants of the complex from earlier are moving westward over north Clallam County this morning. The focus turns towards the south interior/south Puget Sound where additional bands of showers are moving north. The temperatures remain a few degrees above freezing in the Chehalis Valley/I-5 corridor (and this has kept the precipitation as rain). To the west in the Black Hills, it is closer to freezing and WSDOT cameras indicate snow on the sides of the roads on SR-8. To the north, northeast winds have picked up, bringing cooler Fraser River air into the north interior (down to as far as Clallam County). The cold weather advisory continues through late this morning for west Whatcom, San Juan, north Clallam, and also west central Snohomish Counties for wind chills in the low 20s to teens this morning.

The precipitation chances decrease substantially going into the afternoon as the trough moves east. This will be replaced with a brief ridge that passes overhead late tonight/Friday morning. Some cloud-free blue sky is possible later in the day (especially in the north interior and coast) - although highs will likely struggle to reach 40 for many today. The clear skies however will result in cool temperatures tonight as the Fraser air is reinforced by north flow behind the trough (though the outflow will weaken tonight). Wind chills/apparent temperatures are likely to become brisk again in the mid 20s, and may spread to other parts of the interior.

Friday will see a marginal warm up (highs returning into the low to mid 40s). The attention turns to the next low dropping down into the region from the Gulf of Alaska. Precipitation chances will increase for the Pacific Coast and Mountains late Friday into Saturday. As warmer air is brought in Saturday (highs closer to 50s), lowlands will only be expected to see rain with any showers, while mountains will see snow above 2,000 ft for the Cascades, and 3,000 ft in the Olympics. Precipitation amounts will be light for the day Saturday. Breezy offshore winds are expected Saturday, primarily in the Cascades and the Pacific Coast. Most likely gusts will peak around 25 to 35 mph. Lastly, seas are expected to increase offshore, with the potential for high surf conditions for beaches along the Pacific Coast Saturday and Sunday.

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

Most ensembles keep the low offshore this weekend into next week, with multiple rounds of precipitation continuing the remainder of the weekend into next week. Highs will remain in the low to mid 50s through this part of the forecast. The moisture content of the precipitation will increase on Monday and Wednesday. Hydrologic flooding impacts are not expected, but there remains potential for mountain snow with this system. There is lower confidence in heavier snowfall rates, as well as snow levels with the warmer air in place, but large-scale impacts are not expected with the snow being spread across several days (Wednesday is the only day where the snow may be heavy enough to produce possible travel impacts).

HPR

AVIATION

Southwesterly flow aloft will gradually become more northwesterly today as an upper level low continues to sink southward and then push inland south of the area. Some showers will across the southern portion of the area, tapering quickly through the morning. Some additional showers may wrap around the low and bring isolated rain or rain/snow again late in the day (again mostly south of KSEA/KBFI). This will maintain MVFR conditions across the south and VFR conditions elsewhere. Breezy Fraser outflow winds continue to impact KBLI this morning, but weaken starting 18-20z. Lower ceilings again develop across the south overnight, but northerly flow will aid in drying and could keep ceilings closer to 3000 ft or higher overnight.

KSEA...VFR conditions with shower activity shifting south. Will see gradually decreasing clouds. Surface winds northerly at around 10 kt through the day, veering to southeast and easing this evening through the night. This brings the likelihood of MVFR stratus return again after 12z Friday.

MARINE

A low pressure system located just south of the coastal waters will continue to move away from the area this morning. Meanwhile, strengthening high pressure over the interior of British Columbia overnight will continue to promote breezy Fraser Outflow across the Northern Inland Waters and Eastern Strait and easterly flow through the central and western Strait this morning. Fraser Outflow will ease late this morning as the high over British Columbia weakens, allowing for winds to ease below small craft criteria. Seas across the coastal waters will range between 5-8 ft.

Light offshore flow will then continue into early Friday with a broad surface trough over the coastal waters. A stronger system will move into the area waters over the weekend, increasing offshore flow and likely bringing southeasterly gales to the coastal waters. Seas look to build to 14-18 ft, though latest probabilistic guidance suggests that there is roughly a 40-50 percent chance of coastal seas building to greater than 18 ft. The pattern looks to remain active heading into next week, with additional systems moving across the area waters likely bringing additional rounds of headlines.

14

HYDROLOGY

No river flooding is expected over the next seven days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Cold Weather Advisory until noon PST today for Lowlands of Western Whatcom County-San Juan County.

PZ...Small Craft Advisory until 10 AM PST this morning for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands-West Entrance U.S. Waters Strait Of Juan De Fuca.