textproduct: Seattle/Tacoma

SYNOPSIS

Cooler temperatures are expected through the weekend as low pressure lingers over the area. Isolated showers are expected through Friday, with chances for thunderstorms in the Cascades. A more widespread thunderstorm threat exists on Saturday as the low pressure system parks overhead. Unsettled conditions linger into early next week.

SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/

The main line of showers moved through Puget Sound this afternoon, with weak convergence zone activity expected this evening. Rainfall has not amounted to much if anything in this band of activity, as the lower atmosphere is still relatively dry following yesterday's heat. The threat for thunderstorms has greatly diminished this evening along the Cascades, with current chances at 10% or less.

Weak high pressure will build in on Thursday, but will ultimately behave more like zonal flow. Expect cloudy conditions in the morning, breaking out to partly cloudy in the afternoon with highs in the 60s to low 70s. Deep low pressure in the Gulf of Alaska will drift south on Friday, ejecting a shortwave off the parent low and bringing in enough moisture for another round of scattered showers and isolated thunderstorms (10-20% chance) to the Olympics and portions of the central and northern Cascades. Highs on Friday dip back into the low 60s.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

The aforementioned low pressure system will traverse directly overhead on Saturday, increasing the favorability of more widespread area thunderstorms in the afternoon. Current guidance suggests a 20-30% chance of thunderstorms on Saturday across western Washington. Right now, the primary hazards will be lightning and gusty/erratic winds. The low moves east on Sunday, clearing things up a bit but warming temperatures up around 4 to 6 degrees into the upper 60s.

Unsettled weather persists into early next week as more low pressure systems drop in from the Gulf of Alaska. While there are some positioning discrepancies within the ensembles as it pertains to the low pressure, general consensus is reasonably consistent with it lingering through the first half of next week. Beyond then, potential scenarios start to diverge.

AVIATION

Flow aloft becoming westerly this evening as an upper level trough traverses the area. VFR conditions continue with upper level clouds as a frontal system approaches offshore. As the system moves through this evening, MVFR to IFR conditions have develop along the coast. In the interior, most places will be low-end VFR to higher-end MVFR, except over northern King/southern Snohomish counties where a convergence zone is set to develop late this evening and persist into early Thursday morning, where MVFR cigs will be more likely. Ceilings will slowly lift and scatter during the morning on Thursday. S/SW winds 8-15 kt with gusts to 20 kt will ease to around 5-10 kt overnight and remain like so into Thursday. Winds switch to N at PAE, but the convergence zone looks to remain north of the Seattle terminals.

KSEA...VFR conditions continue through tonight, then lowering to low end VFR/high end MVFR after around 06-09Z Thu. Ceilings look to start to lift and scatter around 16-19Z Thur. SW winds prevail through the TAF period, generally around 8-12 kt.

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MARINE

A frontal system will continue to traverse the area waters this into tonight. A surface ridge will build in quickly behind the front offshore. Winds will increase through the Strait of Juan de Fuca this evening, allowing for the Small Craft Advisory to continue. Winds will ease slightly Thursday morning but remain above thresholds through Thursday evening, when the surface ridge offshore begins to ease. A broad low pressure system will move in from the Gulf of Alaska Friday into the weekend, which will allow for onshore flow to continue. Weak pushes will continue down the Strait of Juan de Fuca each afternoon/evening, with the push on Saturday evening being the most likely to warrant additional headlines. Another frontal system looks to approach and move through the area waters early next week.

Seas remain 6-8 ft or less through the next seven days.

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FIRE WEATHER

Rain from the showers progressing through western Washington has struggled to culminate in anything measurable, largely evaporated by a drier atmosphere from yesterday's heat. Relative humidity this afternoon is generally between 30 to 45 percent, despite the clouds and cooler temperatures. The next period to watch will be Saturday as an expanded threat of thunderstorms is forecast for the area. Lightning will be a primary hazard, and depending on the moisture accompanying each thunderstorm, may pose a risk to drier areas of grasses/cured fuels. Preceding cool conditions (and live fuels still growing and in the green up phase) will limit holdover risk as a rapid warmup is not expected.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 11 PM PDT Thursday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.