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/

Satellite imagery shows mostly cloudy skies over Western Washington this morning with an upper level trough over the area. Plenty of showers on the Doppler radar with a rain shadow evident over the Central Puget Sound. Temperatures at 2 am/09z were in the mid 40s.

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. Radar shows widespread scattered shower activity across western Washington this morning, which is resulting in a mixed bag of VFR to MVFR conditions across the area terminals. Shower activity will continue throughout the day for a continued mixed bag of conditions, with guidance showing a strong signal for a PSCZ developing across the central Sound this afternoon and evening. Isolated thunderstorm activity will be possible throughout the day, but confidence in occurrence is too low for inclusion in any single TAF site at this time. Southerly winds will persist between 7-12 kt through the day for interior terminals, but could briefly shift northerly this afternoon within the vicinity of the convergence zone. Showers will taper area-wide heading into Sunday morning.

KSEA...A 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. Isolated thunderstorm activity will be possible, with latest guidance highlighting a 15-20 percent chance of lightning within the convergence zone, but confidence is too low to include in the TAF at this time.

14

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. The increase in onshore flow could even bring a few gale gusts to portions of 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

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 5 PM PDT this afternoon for Grays Harbor Bar.

Small Craft Advisory from 6 AM this morning to midnight PDT tonight for Central U.S. Waters Strait Of Juan De Fuca- West Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory from noon today to midnight PDT tonight for East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 4 AM PDT early this morning for Puget Sound and Hood Canal.

Small Craft Advisory until 11 PM PDT this evening 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.