textproduct: Seattle/Tacoma

SYNOPSIS

Showers give way to mostly dry and clearing conditions tonight. Cool temperatures Wednesday morning with areas of fog. Increasing easterly gap winds during the day Wednesday. A weak front moves in Wednesday night, but splits as it does so, bringing light precipitation. Additional systems reach the area Friday and into this weekend, with the potential for more substantial rain and accumulating snow in the mountain passes Sunday into early next week. A cooling trend looks to start next week.

SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/

An upper level trough axis has moved east of the Cascades this evening. Skies have begun to clear across much of the lowlands. But light surface winds, a long night ahead, and plentiful low level moisture are expected to result in widespread low cloud development overnight along with areas of fog by morning. Apart from monitoring of fog development overnight, current forecasts have trends covered well. Previous discussion follows with update to aviation portion. 27

Radar imagery and surface analysis show a few very isolated showers, mostly across the northwest interior. Shower coverage will decrease tonight as a brief upper level ridge and associated surface high pressure system moves across the region. High pressure will quickly move inland tonight. As such, clouds will clear tonight. This, along with mostly light to calm winds at least early in the night will allow for areas of dense fog to develop into early Wednesday morning. The best chances of seeing fog will be across the Southwest Interior up through the Kitsap Peninsula and up towards the Seattle metro, in western Whatcom county, and along the Strait of Juan de Fuca. Satellite trends will be monitored tonight for the potential for any advisories. Additionally, low temperatures will likely drop a decent amount, reaching into the low to mid 30s, to near 40 in the urban metro.

High pressure will remain over eastern Washington/southeastern British Columbia on Wednesday while a frontal system will approach the coast Wednesday night. Conditions will be dry during the day Wednesday. However, winds will begin to increase early Wednesday morning as offshore gradients increase. Winds look to peak early Wednesday afternoon. Maximum gusts through the gaps in the terrain may reach 25-35 mph, with locally up to 40-45 mph near North Bend. Since this is one of the few times this is expected to peak during the day, a wind advisory will be issued mainly for the North Bend area.

Rain chances will begin to increase along the coast after around midnight Thursday, but as the front moves inland, it quickly begins to split and weaken as upper level support decreases. As such, rain will devolve rather quickly into scattered showers for the rest of the area Thursday morning. Precipitation accumulations will be small to negligible. Showers taper off Thursday night. Temperatures on Thursday will be a bit warmer, with highs in the low to mid 50s and lows in the upper 30s to low 40s.

Along the coast, large long period waves will arrive Thursday afternoon. However, currently guidance is suggesting that waves will be just short of high surf advisory criteria. Will continue to monitor trends as the waves develop and move into the area. Localized beach erosion and significant run-up may be possible on Thursday.

LONG TERM /FRIDAY THROUGH TUESDAY/

Precipitation chances reenter the forecast on Friday for the far northwest portions of the area as a system tracks across southern British Columbia. This will try to slowly sink southward on Saturday before an upper level trough moves into the area Sunday into Monday, which will bring the best for more substantial precipitation area-wide. Lowering snow levels will likely result in some accumulating snow at the passes. There are still considerable differences in the exact evolution of the weather this weekend, and the ensembles are still showing about a 50/50 split between between a cooler and perhaps drier solution vs warmer and wetter solution. What does look more likely is next week marking the start of a cooling trend through the week with highs back in the 40s and lows in the 30s.

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AVIATION

An upper trough axis has moved east of the Cascades with north to northwest flow aloft developing over Western Washington as a weak upper ridge builds into the area. Weak surface gradients will become increasingly offshore during the day on Wednesday. Evening surface observations show little temperature/dew point separation. With higher level clouds clearing out, low clouds and fog are already beginning to form and will become progressively more widespread overnight into Wednesday morning. IFR or LIFR ceilings with areas of reduced surface visibility are expected 11Z-16Z for many interior lowland terminals. Low level offshore flow will begin to develop on Wednesday and this should help gradually erode the low clouds for a return to VFR most areas by midday Wednesday.

KSEA...Weak surface winds overnight combined with some clearing and little temperature/dew point separation lend a good degree of confidence into the development of low clouds and fog. Satellite trends and regional observations point toward fog formation as opposed to stratus. Will incorporate this into the 06Z TAF package. Fog and low clouds may hang on into late morning before low level easterly flow breaks through near-surface inversions. Surface winds light N/NE this evening will become light and variable overnight then becoming E/NE 5 to 8 knots toward late morning...rising to 10 to 15 knots at times toward 00Z. 27

MARINE

Marine conditions look to remain rather tranquil until Wednesday morning as high pressure moves inland and a frontal system approaches the coastal waters from the west. Offshore gradients increase, which will bring Small Craft Advisory winds to the outer coastal waters as well as through the western Strait of Juan de Fuca. Winds will ease overnight Wednesday into Thursday morning as the front moves inland. Additional systems look to move across the northwest Friday into Saturday with potentially a stronger system Sunday into Monday, but there remain significant differences between model outcomes.

Seas around 7 to 9 ft will begin to rise Thursday as a long period swell group arrives to the west coast. Wave heights will increase to 16 to 18 ft late Thursday (with a dominant period of around 15 to 16 s) and will slowly ease to around 10 to 12 ft Friday and remain around there through the weekend into early next week.

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HYDROLOGY

No flooding is expected through the week ahead. Stronger weather systems moving into the region by early next week could bring periods of heavier rain and will be the next time frame to watch with respect to hydrologic concerns.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Wind Advisory from 10 AM to 10 PM PST Wednesday for Foothills and Valleys of Central King County.

PZ...Small Craft Advisory from 7 AM to 4 PM PST Wednesday for Coastal Waters From Cape Flattery To James Island Out 10 Nm-West Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory from 7 AM to 10 PM PST Wednesday for Coastal Waters From Cape Flattery To James Island 10 To 60 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