textproduct: Seattle/Tacoma

SYNOPSIS

Very cool upper level trough over Western Washington today will move south tonight and Sunday. Upper level ridge building Sunday with the ridge axis offshore extending into Southern British Columbia. The ridge will remain through the week with weak systems passing by to the north at times.

SHORT TERM /TODAY THROUGH MONDAY/

The latest forecast remains on track this morning with no updates. Upper level trough remaining over Western Washington today. Very cold for May temperatures aloft with 500 mb temperatures around -30C. Air mass slightly unstable this morning will become more unstable this afternoon. Convective indices by 21z include lifted index of 0 to -2, lapse rates plus 7 to 8C/km, convective temperatures in the lower 50s and even 500 J/kg of CAPE. Scattered showers this morning with the rain shadow continuing over the Central Puget Sound. With the convective temperatures in the lower 50s will take a little daytime heating to fire things up with thunderstorm chances increasing around midday in addition to increasing showers. Convergence zone developing late in the afternoon. Flow aloft northwesterly which will push the convergence zone development location a little further south than usual, mostly likely over Seattle. The very cold air aloft means heavier showers at any point during day could produce small hail. Snow levels remaining near 3500 feet with the possibility of briefly lower snow levels in the stronger showers. Winter weather advisory for the mountains will remain in effect. Another cool day with highs only in the lower to mid 50s. Olympia was the only location Friday to tie a record low maximum temperature. Like Friday highs mostly only a couple of degrees above the record low maximums.

Upper level trough digging south tonight with shower activity decreasing in the evening. The exception to this will be the convergence zone which will remain between Seattle and Olympia extending southeast into the Cascades. The zone will dissipate after midnight. Snow levels remaining near 3500 feet bringing a few inches of snow to the Cascades south of Snoqualmie Pass. The winter weather advisory will continue overnight. Lows in the mid 30s to lower 40s.

Upper level ridge building offshore Sunday with the ridge axis nosing into Southern British Columbia. Low level flow going light. This combined with plenty of low level moisture will produce a mostly cloudy morning, especially over the interior. More sunshine in the afternoon. Highs warming into the mid 50s to lower 60s. Still about 5 degrees below normal.

Little change in the pattern Sunday night and Monday. Low clouds forming Sunday night with the light flow in the lower levels. The marine layer at this point does not look deep enough to continue much past midday Monday. Lows in the upper 30s to mid 40s. Highs getting back to normal, in the 60s.

Felton

LONG TERM /MONDAY NIGHT THROUGH FRIDAY/

Models in good agreement with the upper level ridge remaining over the area into Friday although the ridge does flatten out a bit beginning Wednesday. Weak systems moving over the top of the ridge will keep the low level low weakly onshore through the period. This will prevent temperatures from warming very much. Highs in the 60s Tuesday, 60s and lower 70s Wednesday through Friday. Lows mostly in the 40s and lower 50s.

AVIATION

Westerly flow aloft will become more northwesterly this morning and then more northerly by late tonight as an upper level trough moves across the area. Onshore flow continues in the low levels, with breezy west/northwesterly winds persisting at coastal terminals, with gusts to 20-25 kt. Winds across the interior remain southerly, primarily between 7-12 kt. Couple areas of showers continue on radar this morning - along the coast and in the north interior. These have resulted in MVFR flight conditions with ceilings/visibilities dipping. Satellite is showing some clear pockets forming in the central interior, which will help destabilize the atmosphere for a 30% chance of isolated thunderstorms this afternoon (via a PSCZ). The interior terminals were updated to have a PROB30 of thunder potential from 21Z-00Z (with most ensembles showing a PSCZ complex developing over the Kitsap and moving southeast into Seattle). Lightning, small hail and gusty variable winds will be possible with any convection that takes place. Tops of thunderstorm clouds will likely not exceed 20,000 ft. The shower/thunder threat will diminish into Sunday. Broken/overcast ceilings will remain into Sunday, and likely lower to MVFR in the morning for most.

KSEA...Mix of VFR to MVFR conditions expected throughout the day with shower activity. Hi-res guidance continues to indicate a convergence zone moving into the terminal between 21-00Z. Southerly winds will persist through the morning hours, primarily between 7-12 kt, but will become variable in the vicinity of the PSCZ and then shift to the north for a few hours this afternoon and evening. There is a 30% chance of thunder within the 21-00Z window this afternoon, so a TEMPO group was included with this morning's TAF issuance.

14/HPR

MARINE

A system moving through the region today will enhance winds and seas over most of the area waters with small craft advisory conditions expected for the coastal waters and along the Strait of Juan de Fuca. Also added the admiralty inlet with the morning update to the advisory area. Couple gale gusts cab't be ruled out briefly for the central and eastern Strait, with the latest HREF probabilistic guidance indicating roughly a 50-60 percent chance of gusts briefly exceeding 34 kt this afternoon. Showers and a slight chance of thunderstorms will be possible across the area waters as well today, which could bring brief drops in visibilities in any heavier shower activity. Seas will build to above 10 ft across the coastal waters today and could become steep at times.

High pressure will build back into the area waters on Sunday, before a weak frontal system approaches and moves into the area waters Monday into Tuesday. Onshore flow will persist and will result in daily pushes of westerly winds through the Strait of Juan de Fuca through the first half of next week. High pressure will then build back into the waters again by Wednesday. Seas will generally persist between 6-9 ft Sunday into early next week, though could build back to near 10 ft around Monday into Tuesday.

14/HPR

HYDROLOGY

The daily hydrology discussion has ended until the start of the next rainy season; it will only be updated during this time as needed.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory until 8 AM PDT Sunday for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and Northern King Counties-Cascades of Southern King County- Cascades of Whatcom and Skagit Counties-Olympics.

PZ...Small Craft Advisory until 3 AM PDT Sunday for Central U.S. Waters Strait Of Juan De Fuca-Grays Harbor Bar.

Small Craft Advisory until midnight PDT tonight for West Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 3 AM PDT Sunday for Admiralty Inlet- East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 2 AM PDT Sunday 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.