textproduct: Seattle/Tacoma

SYNOPSIS

Increasing clouds ahead of an approaching weather system expected to move onshore overnight into Friday. The large scale pattern looks to remain active with cooler and unsettled weather with periods of precipitation over the region through next week.

SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/

Afternoon sunshine for many areas except for very persistent fog over the Hood Canal area this afternoon. Increasing high- and mid-level clouds working into the Olympic Peninsula ahead of an approaching front will spread inland this afternoon and evening. Widespread precipitation ahead of the front will spread inland late this evening into Friday - falling as rain over the lowlands and needed snow in the mountains. Snow levels will lower to around 2500-3000 feet by Friday afternoon - allowing snow accumulation at all of the Cascade passes. Most likely amounts in the 4 to 6 in range with slightly more, 6 to 8 inches around Mt. Baker. This system isn't expected to bring much wind to western Washington - breezy at times Friday - however winds over the offshore waters will allow waves along the Pacific Coast to build to roughly 15 to 18 feet on Friday, and beachgoers should use caution and remain aware of the unsettled seas.

By Saturday precipitation will taper off with cooler temperatures settling into the area with snow levels dipping to 1800-2500 feet by Saturday afternoon. Additional snow accumulations over the mountains will be light. Temperatures will peak in the mid to upper 40s with mostly cloudy skies.

LONG TERM /SUNDAY THROUGH THURSDAY/

Ensembles continue to consistently highlight an upper level trough digging south and west of 130W into the offshore waters of California by Sunday afternoon. This leaves much of the Pacific Northwest under split flow aloft, with most variability in the forecast north of the split in western Canada and southern Alaska. An upper level shortwave trough forecast to move south out of the Alaska interior Sunday evening will break the split flow over the Pacific Northwest going into early next week Clustering analysis hints at two scenarios, one where this upper level trough digs southward over the Northeast Pacific, and the second taking the upper level trough southward along the British Columbia coast and more directly into western Washington and Oregon. The latter scenario is forecast by 44% of the ensemble members, represents the wetter scenario for much of the Pacific Northwest. Precipitation with the system is expected to start to move onto the coast early Monday morning, with cooler temperatures over the interior lowlands it is possible that this precipitation could fall as a rain snow mix as it begins to spread inland through the morning. Interestingly, the scenario with the upper level trough over western Washington would also be less likely to result in less southerly flow ahead of the front - helping keep temperatures a little cooler as that precipitation initially moves inland. In either scenario, temperatures are not expected to be cool enough for more than a rain snow mix, with no accumulation at low elevations. Snow levels around 1500-2000 feet will allow for additional accumulations over the higher terrain steady. The pattern looks to remain cool an active through the remainder of the week, with a mean upper level trough over or along the west Coast. Good news for our mountain snowpack.

AVIATION

Light westerly flow aloft will become southwesterly tonight as the upper level ridge over the area exits to the south ahead of frontal system approaching the area. Areas of patchy fog stubbornly lingering along Hood Canal this afternoon will continue to shrink into the evening. Increasing high and mid level clouds will spread through the area late this afternoon and evening ahead of that system. Widespread VFR conditions will persist through this evening with light rain along the coast around 05Z and into the interior by 09Z. Ceilings will deteriorate all areas overnight tonight with widespread MVFR ceilings across the region by Friday morning. Heaviest rain looks to be with the front late morning on the coast and early afternoon through the interior. Low level winds will remain light south-southwesterly this evening becoming more westerly along the coast behind the front and southerly over the interior.

KSEA...VFR conditions evening with increasing high and mid level clouds. Ceilings will lower tonight ahead of an approaching front, with MVFR ceilings in areas of light rain after 12Z Friday. Surface winds S/SW 5 to 7 knots, easing to 4 to 6 kt this evening then strenthen to 6 to 8 knots late Friday morning.

MARINE

A weak surface ridge will maintain over area waters for tonight. A front will move onshore on Friday with a secondary system to follow on Saturday. Pre-frontal southerlies on Friday may be enough to bring SCA gusts for the Northern Inland waters during the morning hours. As a result, a SCA has been issued as hi-res guidance suggests a 50-60% chance of gusts above 21 kt. Surface high pressure briefly builds into British Columbia Saturday night and Sunday turning the flow northerly. A series of troughs digging southward across the coastal and offshore waters will keep conditions unsettled early next week with the potential for more headlines.

Seas will build over 10 feet this evening remain in the 10 to 15 foot range into the early weekend. The outer coastal waters may briefly touch 16 ft Friday morning. They may briefly subside below the 10 foot threshold Sunday into early Monday before building back into double digits again by Tuesday.

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HYDROLOGY

No river flooding is expected over the next 7 days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory from 7 PM this evening to 4 PM PST Friday for Grays Harbor Bar.

Small Craft Advisory until 4 PM PST Friday 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.