textproduct: Seattle/Tacoma

SYNOPSIS

Cool and unsettled conditions will continue across Western Washington for the first half of the week as a pair of systems move through the region. A pattern shift toward dry and considerably warmer conditions is expected late in week as strong high pressure aloft builds into the region.

SHORT TERM /TODAY THROUGH WEDNESDAY/

Latest IR satellite imagery shows leading edge of the next incoming system still out over the coastal waters with just the slightest hints at associated precip showing up on current radar. This reflects the consistent model data suggesting a very slow approach, making it to the coastline by late this morning and beginning its inland intrusion this afternoon. Widespread rainfall still on tap by this evening and into the overnight hours with QPF values suggesting periods of moderate rainfall rates over the southern half of the CWA, stretching as far north as Everett.

The parent upper low will be the main headline for Tuesday as rains over W WA persist. The placement and track of the main low remains a point of contention amongst model data and while this will not specifically impact the amount of chances of precip, it does play a role in any potential thunderstorm development. Levels of CAPE available has seen some variation, but LIs remain on the positive side of things which would not be helpful to any development. Persistent cloudy conditions and cooler daytime temps will certainly be another factor to overcome convection-wise. This leaves a lot of the work to be done by the upper low and the uncertainty regarding its path does not foster confidence in potential thunderstorm development. NBM remains consistent in its advertising and given associated messaging, felt prudent to leave the prospect in the forecast at this time...although given the factors listed above, an emphasis on isolated development is certainly warranted.

Wednesday will see the upper level trough pull away from W WA and thus will see precip chances whittle away through the day. Latest data suggests this to be a bit faster than previously advertised, with the majority of the CWA dry by afternoon /with only some low end PoPs to contend with/.

Daytime highs for the near term do not see much in the way of variation with upper 50s to lower 60s expected today and Tuesday. As the system exits the area Wednesday, that should allow some breaks in the clouds, letting peeks of sunshine to nudge temps upward, but not by too much...ranging in the lower to mid 60s.

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LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/

Dry conditions areawide by Wednesday night as the influence from an upper level ridge building over the eastern Pacific begins to nudge its way into W WA. With this feature being very slow to move eastward throughout the remainder of the forecast period, the song will remain the same for the latter half of the week as conditions will remain dry and temperatures begin a steep warming trend. Daytime highs Thursday jump up into the upper 60s to lower 70s. Friday sees further upward motion into the mid to upper 70s. Saturday gets many interior locations into the upper 70s to mid 80s. Sunday finally sees some locations break 90 as temps range from the mid 80s to lower 90s.

Temps of this caliber will induce thermal troughing over the area, allowing for offshore flow to develop Saturday night into Sunday. This easterly flow importing more hot and dry air into the area. As this heat persists, HeatRisk becomes an increasing concern. The start of this warming trend, Thursday and Friday, continue to look to be Minor in these terms. Moderate HeatRisk areas begin to pop up, mainly in the SW interior, Saturday while widespread Moderate conditions expected for Sunday. While having any Major HeatRisk emerging beyond the forecast period seems to have lower probabilities at this time, persisting Moderate conditions appear to be more favorable.

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AVIATION

South to southwesterly flow aloft will become westerly tonight as a surface low moves into the area and weakens. Stratiform rain along the Olympic Peninsula spreads inland later today. MVFR cigs developing along the coast this afternoon, with MVFR cigs by this evening into early tonight for the interior. Periods of IFR are also likely tonight, particularly along the Olympic Peninsula. Showers lingering on Tuesday, with a convergence zone developing around Snohomish County in the afternoon. Light, variable winds mainly under 7 kts for the interior into this afternoon. South-southwest winds increase tonight into Tuesday.

KSEA...VFR cigs into this afternoon. Cigs gradually lowering into this evening with MVFR cigs developing between 02-04z. MVFR cigs continue through Tuesday AM. Cigs may fluctuate to IFR at times tonight, particularly between 07-12z. Winds are expected to remain mainly W/SW into this afternoon, although periods of VRB are likely into this evening. Brief N/NW winds may occur, mainly between 23-00z, but confidence remains low. JD

MARINE

A surface low and its associated frontal system will move across the coastal waters this morning, gradually push onshore, and weaken this afternoon and evening. This system will bring gusty southeasterly winds to the coastal waters through this afternoon hours today. Strong onshore flow is likely to develop in the wake of the system, with the latest HREF probabilities indicating a 50-70% chance of gales developing along the central and eastern Strait of Juan de Fuca on Tuesday. Southerly winds will increase and become breezy for portions of the Puget Sound waters on Tuesday morning as well, before easing later Tuesday night.

High pressure will rebuild back into the coastal waters on Wednesday and Thursday, while a thermal trough gradually expands northward along the Oregon coast. Northwesterly winds over the outer coastal waters could become breezy at times the second half of the week as a result. The thermal trough will then expand northward along the Washington coast by the weekend, allowing for flow to turn offshore.

Seas will generally hover between 6-9 ft through the first part of the week, before subsiding towards 5-7 ft near midweek and remaining at this range heading into the weekend.

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FIRE WEATHER

As upper level ridging builds into the area starting Thursday, conditions, including fuels, will need to be monitored as temperatures are expected to climb to above normal values by the end of the extended forecast period. Given the steep climb in temperatures, extra caution would be advised in any planning during this time. Current NBM minimum relatively humidity values are running in the 25-35 percent range by day 7 across portions of the area, but lower values are entirely within the realm of possibility. If the cross Cascade gradient goes negative by Sunday as advertised in a number of models, humidity values dropping into the teens in the Cascades valleys and portions of the Southwest Interior become a distinct possibility.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Gale Watch from Tuesday morning through Tuesday evening for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 5 PM PDT this afternoon 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.