textproduct: Seattle/Tacoma

SYNOPSIS

A building upper level ridge will bring a return of warmer temperatures and continued dry conditions. The dry conditions will elevate fire weather concerns through midweek. The ridge will weaken late this week and a frontal system is expected to bring cooler temperatures and measurable precipitation to most of the area by the end of week.

SHORT TERM /TODAY THROUGH TUESDAY/

Forecast is on track for the morning update with minor adjustments to the aviation discussion. -HPR

Onshore flow is weakening across the area early this morning. As a result, low cloud coverage will be less extensive across the interior and will burn back to the coast earlier than yesterday. This along with an upper ridge rebuilding back into the region will give high temperatures across Western Washington a nudge upward several degrees this afternoon. The warming trend kicks into high gear on Monday as 500 millibar heights approach the 580 decameter threshold and thermally induced low pressure at the surface expands into the region. Much like yesterday, most of the models keep the axis of the thermal trough over the interior with light onshore gradients. The 50th percentile NBM used in the forecast largely remains disconnected from this fact and continues to suggest warmer temperatures for the coast than what are likely to occur for both Monday and Tuesday. As for the interior, it'll warm well into the 80s away from the water both Monday and Tuesday with some locations in the Southwest Interior getting into the lower 90s. Moderate HeatRisk is expected during this time for most of the interior lowlands and a Heat Advisory remains in effect beginning midday Monday.

LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/

The thermal trough shifts east of the Cascades by Tuesday night inducing a pretty good onshore push. As it often does, the NBM does not adequately weigh the cooling effect this will have and is likely around 10 degrees too warm for Seattle metro and other interior locations on Wednesday afternoon.

The upper ridge flattens and weakens Wednesday into Thursday setting the stage for the previously advertised arrival of some precipitation at the conclusion of the week. Ensembles are in good agreement that most of the area will see some measurable precipitation by later Friday into Saturday as an upper trough and associated front reach the area. We're likely to see several consecutive days of below normal temperatures as well...something we haven't experienced in a couple weeks.

27

AVIATION

VFR across the majority of terminals, with the exception of the coast, where it is MVFR due to marine stratus. Improvement for the coastal terminals is expected after 18z-19z. Another round of MVFR cigs possible tonight/Monday morning. Latest guidance suggests a 60-80% chance for the coastline after 03z and a 30-50% chance of IFR cigs after 07z. As for the interior terminals, current probabilities are 20% or less for MVFR cigs. Southerly surface winds this morning will then transition to the north after 17z-20z at 5-10 kt.

KSEA...VFR generally expected through the TAF period, with only a 20% chance of MVFR in the morning (less likely to form with high pressure building). Southwesterly winds 4-7 kt this morning, then northwesterly after 18z-20z at 7-10 kt. Northeasterly winds this evening 5-8 kt.

29/HPR

MARINE

High pressure will strengthen over area waters through the middle of the week. This will result in diurnal pushes through the Strait of Juan de Fuca. Latest guidance suggests the strongest push will be on Wednesday. There is increasing confidence (75-95%) for high-end small craft winds through the Strait. Additionally, probabilities for gales slightly increased to 25-40%. A weaker push of westerlies is expected on Thursday, with a 55-70% chance of small craft winds and 15-25% chance for gales. Unsettled conditions return late in the week and high pressure will weaken Thursday/Friday as a frontal system moves over area waters. High pressure looks to quickly rebuild behind the front.

Combined seas below 10 feet through early next week. Seas will begin to build late Tuesday into Wednesday to 7-10 ft, decreasing below 10 ft by Thursday. In addition, seas may get steep at times late Sunday through Wednesday over some portions of the coastal waters with seas hovering around 7-9 ft and a dominant period around 7-8 seconds.

29

FIRE WEATHER

Hot and dry weather returns in earnest for the first half of the week, with minimum relative humidity values dropping into the 20 percent range or less through Tuesday in the south interior and Cascade valleys. A thermal trough developing slightly inland from the immediate coast on Monday and Tuesday lends itself to a conditionally unstable atmosphere early in the week. Area fuels are getting much closer to critical levels as additional waves of hot and dry weather continues. Grasses are curing expeditiously and have already contributed to a number of roadside brush fires in recent days. Please continue to exercise caution with ignition sources.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Heat Advisory from 11 AM Monday to 11 PM PDT Tuesday for Eastern Strait of Juan de Fuca-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-Lower Chehalis River Valley-Lowlands of Lewis and Southern Thurston 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- Southern Hood Canal-Willapa and Black Hills.

Heat Advisory from 11 AM Monday to 11 PM PDT Wednesday for City of Seattle-Downtown Everett / Marysville Area- Eastern Kitsap County-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-Lowlands of Pierce and Southern King Counties-Shoreline / Lynnwood / South Everett Area.

PZ...None.