textproduct: Seattle/Tacoma

UPDATE

High wind watches have been upgraded to warnings across much of the interior lowlands of western Washington. For the foothills, we opted to go with advisories and begin them early tomorrow morning as gap winds with gusts of up to around 45 mph will be possible for the Cascade valleys and foothills. Guidance has been consistent on the coast staying below thresholds, so the watch has been cancelled there. There may be some gusts up to 45 mph in the morning with the easterlies, but the actual low core is not compact enough to forecast significant winds on the coast tomorrow afternoon. We will continue to watch as this low feature currently offshore of San Francisco continues to work its way NNW overnight into tomorrow morning. For more details on the forecast and the dynamics at play, see below. 62

SYNOPSIS

Quiescent weather is expected this afternoon, with more activity expected tomorrow. A deepening low pressure system will be moving up along the coast into western Washington tomorrow, favoring strong wind potential in Puget Sound and along the coast, in addition to some rainfall. Unsettled weather is expected again this weekend and into early next week.

SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/

The primary weather feature to watch in the short term will be the low pressure system moving up from the south tomorrow. As it moves north and strengthens, wind gusts have increased in the forecast though uncertainty still remains. For more on that, see the wind section of this discussion here:

Wind: A compact low pressure system developing along the northern California coast this evening will lift northward toward the Pacific Northwest on Wednesday. Models continue to struggle with the path and subsequent strength of the surface low as it approaches Western Washington midday Wednesday. Morning runs of the HREF and REFS continued to show mean gusts of 45-50 mph through Puget Sound as well as coastal areas by early to mid afternoon Wednesday with 90th percentile data showing gusts in the realm of 60 mph. As with many high wind producing storm systems (i.e. last week), deference is also given to the rate of change of surface pressure and topographically driven contributions. As of this morning, isallobaric plots from both the GFS and NAM were still showing 8 to 12 millibars of pressure rise in just 3 hours through a swath of the coast and Puget Sound on Wednesday afternoon. Funneling that through a north/south oriented channel like Puget Sound can act to enhance the wind...especially when 850 millibar winds are roughly from the same direction at 60+ knots. Given the ongoing uncertainty, a high wind watch has been issued for much of the lowlands and will let following shifts decide on warnings versus advisories. 27

In addition to the wind, there will also be some rain with this system, but amounts aren't looking particularly impressive. A few tenths of an inch up to a quarter inch throughout Puget Sound and around an inch in the north Cascades and portions of the Olympics. In the context of the last few weeks, certainly not impactful rain amounts. Additional, light rain amounts are expected Thursday, with calmer weather expected over the weekend.

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LONG TERM /FRIDAY THROUGH TUESDAY/

Upper level ridging appears to be the favored forecast solution late in the week into the weekend, which would be a welcome break from the last few weeks of intense activity. This reprieve will be relatively short lived as unsettled weather is favored in the week two timeframe from the CPC. There is potential for more atmospheric river activity in this period, but there is not solid confidence on this as of now.

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AVIATION

VFR cigs prevail for all terminals this evening with light ENE winds, except for OLM with patchy low clouds. Rain will be reintroduced to most terminals late tonight around 09Z. Locations that could see reductions overnight to MVFR and/or IFR include OLM and PWT for stratus and low cigs. ESE winds will increase tomorrow morning generally around 12Z, but there is a sharp increase and direction change to southerly in the early afternoon that will affect all terminals. High end gusts in Puget Sound could near 45-50 kts between 20Z-01Z.

KSEA...VFR and light winds presently at the terminal. These conditions are likely to linger throughout the evening, with an expected increase in the ESE winds to 8-10 kts beginning after 06Z. Rain chances return after 09Z as the next system approaches. Expect ESE gusts to 20-25 kts to begin after 14Z, and a rapid shift and increase to S winds after 19Z. Confidence has grown in gusts reaching 45-50 kts at the terminal tomorrow afternoon.

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MARINE

A strong surface low pressure system is expected to lift northward across the coastal waters or Olympic Peninsula on Wednesday. This is expected to produce gales across nearly all area waters. A broad surface trough will remain to the southwest of the area and high pressure over interior British Columbia for a period of northerly flow on Thursday into Friday. Surface high pressure will build over the waters this weekend for a quieter period of weather.

Seas will subside below 10 feet tonight into Wednesday. Seas may briefly approach 10 feet Wednesday afternoon into Wednesday night, but generally are expected to remain below 10 feet into the end of the week.

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HYDROLOGY

Rivers across western Washington continue to trend downward as precipitation across the region lightens up. Additional rainfall throughout the week is not expected to cause any additional rises to area rivers. Additionally, the threat of landslides has decreased across western Washington.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Coastal Flood Advisory from 6 AM to noon PST Wednesday for City of Seattle-Downtown Everett / Marysville Area-Island County-Lowlands of Pierce and Southern King Counties- Lowlands of Western Skagit and Northwestern Snohomish Counties-Lowlands of Western Whatcom County-San Juan County-Shoreline / Lynnwood / South Everett Area.

High Wind Warning from 1 PM to 7 PM PST Wednesday for Downtown Everett / Marysville Area-Island County-Lowlands of Western Skagit and Northwestern Snohomish Counties- Lowlands of Western Whatcom County-Port Townsend Area-San Juan County.

Wind Advisory from 4 AM to 7 PM PST Wednesday for Foothills and Valleys of Central King County-Foothills and Valleys of Pierce and Southern King Counties-Foothills and Valleys of Snohomish and Northern King Counties-Foothills and Valleys of Thurston and Lewis Counties-Foothills and Valleys of the North Cascades.

High Wind Warning from 10 AM to 7 PM PST Wednesday for City of Seattle-Eastern Kitsap County-Eastside-Lowlands of Lewis and Southern Thurston Counties-Lowlands of Pierce and Southern King Counties-Middle Chehalis River Valley- Northern Hood Canal-Olympia and Southern Puget Sound- Shoreline / Lynnwood / South Everett Area-Southern Hood Canal-Willapa and Black Hills.

PZ...Gale Warning from 1 AM Wednesday to 1 AM PST Thursday 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-West Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory from 1 AM Wednesday to 1 AM PST Thursday for Central U.S. Waters Strait Of Juan De Fuca.

Gale Warning from 9 AM Wednesday to midnight PST Wednesday night for Admiralty Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands.

Gale Warning from 9 AM to 10 PM PST Wednesday for Puget Sound and Hood Canal.