textproduct: Seattle/Tacoma

SYNOPSIS

A strong upper level ridge will move onshore this weekend into early next week. A heat advisory continues for much of western Washington for widespread moderate HeatRisk as highs climb into the 80s and 90s on Sunday and Monday. Fire danger will also become elevated as relative humidities dip with offshore flow. The high will weaken midweek for increasing onshore flow and cooler temperatures.

SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/

Strong upper level ridging continues to build into the region this evening. Thermally induced low pressure over Western Oregon tonight will continue to expand north and westward on Saturday with the low level flow flipping weakly offshore. NBM high temperatures in the short term, particularly around central Puget Sound, looked a bit on the low side. So, high temperatures were given a modest boost. This puts Seattle metro in the mid 80s tomorrow and a degree or two either side of 90 for Sunday/Monday with regional high temperature records in jeopardy. The rest of the forecast is unchanged at this time. Previous discussion follows with updates to marine/aviation portions. 27

It's a dry and mostly sunny day across much of western Washington this afternoon. Satellite has a few areas of clouds over the Cascades, and Olympics, and some low stratus around Neah Bay. Quite a few places in the lowlands have climbed into the 70s this afternoon. A few locations west of Olympia/Chehalis Valley may track towards 80 this afternoon. Otherwise, only minor HeatRisk today with the temperatures, with winds out of the north around 5-10 mph. Lows tonight drop down to around 50 to 55.

The main story remains warm, and dry air going into the weekend, and into Monday across western Washington. An upper level ridge building offshore will begin to move inland this weekend, with the axis overhead by Sunday evening. A thermal trough is also expected to build along the coast, which will in turn make the flow more offshore in the overnight periods. Winds will turn more out of the east, but most places will see light winds overnight (with the exception of the west Cascades and Olympic slopes where a few gusts up to 20 mph may occur). During the days, the winds will turn to the north/northwest 5 to 10 mph, with Saturday being breezy in much of Puget Sound as winds pick up to 15-20 mph in the afternoon.

Now for the heat: Saturday will be the cooler of the two weekend days, but will still be quite a bit warmer to Friday, as highs approach the mid 70s into the upper 80s (with a couple low 90s in areas west of Olympia). HeatRisk increases to moderate for a large swath of the south/southwest interior, but remains minor for the remainder of the lowlands. Lows will drop into the mid and upper 50s Saturday night into Sunday. Sunday will be the warmest day of the weekend, with highs around 80 to near 100 in the southwest interior. All of the lowlands will see moderate HeatRisk impacts, with a few isolated instances of major HeatRisk (but this has decreased slightly to 20-40% with the latest forecast update due to low temperatures dropping a couple degrees to around 60). Nevertheless, the Heat Advisory will continue for the emphasis of those spending time outside (who are sensitive to extreme heat) to take breaks in the air conditioning/shade, and staying hydrated.

HPR

LONG TERM /MONDAY THROUGH FRIDAY/

Surface thermal trough deepens inland with highest surface temperatures on Monday. Widespread moderate HeatRisk remains likely on Monday, with more highs likely to break 90 in the interior. Monday evening, this thermal trough will begin to weaken and disorganize. Overnight temperatures have cooled a couple degrees, but will still likely range from the upper 50s to low 60s. Tuesday will be much cooler due to this weakening of the thermal trough, bringing winds with a northwesterly component. A few gusty winds will be possible on Tuesday. The upper level ridge that had built over Western Washington shifts inland on Tuesday, causing temporary zonal/onshore flow until a upper level trough pushes in on Thursday, keeping temperatures cool through the end of the week.

HPR/NW

AVIATION

Upper level ridging centered just offshore will continue to amplify into Saturday with northwest flow aloft over Western Washington. The low level flow will be northerly or weakly offshore into Saturday afternoon. The air mass is dry and stable with clear skies and VFR conditions in place.

KSEA...Clear skies. Surface winds northerly 7 to 10 knots will become northeasterly 5 to 7 knots overnight then rise again to northerly 8 to 12 knots Saturday afternoon.

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MARINE

Broad surface high pressure will remain centered over the offshore waters into early next week. Thermally induced low pressure over Western Oregon will expand north and westward on Saturday for periods of light offshore flow Saturday into Sunday. The thermal trough will shift into the interior of Western Washington Sunday night with onshore flow increasing across the coastal waters. A typical summertime pattern of varying degrees of onshore flow will be in place early next week with high pressure offshore and lower pressure across the interior of the Pacific Northwest. Weak systems passing well north of the region will induce enough onshore flow for possible gales in the central and east strait Monday evening and again on Tuesday.

Coastal seas will remain well under 10 feet through the weekend before building to around 9 or 10 feet on Tuesday.

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FIRE WEATHER

Drying trend over the area the next few days with elevated fire weather concerns beginning Saturday and lasting into Tuesday. Upper level ridge building this weekend with the low level flow turning offshore Saturday night. Highs warming into the 80s Saturday, 80s to lower 90s Sunday and for the interior 80s to mid 90s Monday. Relative humidity values lowering in the drier locations, like the Southwest Interior, into the teens both Sunday and Monday afternoon. Even though fine fuels have not reached critical thresholds, this rapid drying will create elevated fire concerns especially for dry grassy/shrubby areas, as well as piles of dead fuels. Offshore flow Sunday into Monday will prevent good relative humidity recoveries overnight. Relative humidities will improve with the return of onshore flow Tuesday. Winds will become particularly gusty, with portions of the coast/mountains/south interior seeing west to northwest wind gusts up to 20 to 30 mph. Low level onshore flow and cooler temperatures Wednesday and Thursday will ease fire weather concerns the middle of next week. Felton/HPR

SEW WATCHES/WARNINGS/ADVISORIES

WA...Heat Advisory from 11 AM Sunday to 5 AM PDT Tuesday for City of Seattle-Downtown Everett / Marysville Area-Eastern Kitsap County-Eastern Strait of Juan de Fuca-Eastside- 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- Foothills of the Western and Southern Olympic Peninsula- Island County-Lake Crescent Area Including US 101-Lower Chehalis River Valley-Lowlands of Lewis and Southern Thurston Counties-Lowlands of Pierce and Southern King Counties-Lowlands of Western Skagit and Northwestern Snohomish Counties-Lowlands of Western Whatcom County- Middle Chehalis River Valley-Northern Hood Canal-Olympia and Southern Puget Sound-Port Townsend Area-San Juan County-Shoreline / Lynnwood / South Everett Area-Southern Hood Canal-Willapa and Black Hills.

PZ...None.