textproduct: Seattle/Tacoma

SYNOPSIS

Upper level ridge offshore nosing into Northern British Columbia combined with upper level low well to the east giving Western Washington northerly flow aloft through the weekend. Air mass unstable enough for a chance of late day showers over the mountains Sunday and Monday. The upper level ridge will move over Western Washington in the middle of next week.

SHORT TERM /TODAY THROUGH SATURDAY/

Satellite imagery shows cloudy skies over most of Western Washington this morning. Doppler radar still has a few showers over the Cascades at 2 am/09z. With the cloud cover temperatures were pretty uniform, mostly in the mid 40s to lower 50s.

Upper level ridge building into Northern British Columbia today while an upper level low remains well to the east. End result northerly flow aloft over the area. The forecast problem of the day is when the moisture over Western Washington will dissipate. While the northerly flow aloft is dry the flow in the lower levels remains weakly onshore this morning. Not getting any upper level ridging to provide some subsidence to help out with the clearing. The layer is about 2500 feet thick with tops 5000-6000 feet. The low level flow does turn northerly this afternoon. This plus the late April sunshine will slowly erode the moisture layer over Western Washington this afternoon into the evening hours. Cloudy skies for the most part this morning. Highs in the mid 50s to lower 60s.

Skies clearing tonight. Lows in the mid 30s to lower 40s.

Dry northerly flow aloft with northerly flow in the lower levels will give Western Washington plenty of sunshine Friday and Saturday. Highs in the upper 50s to mid 60s. It will be a cool night Friday night/Saturday morning. The potential is there for another frost advisory for the Southwest Interior with lows near freezing. For the remainder of the area lows in the mid and upper 30s. Felton

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

Extended models in pretty good agreement with the upper level low well to the east digging back to the west a little Sunday and Monday pinching off the upper level ridge. North northeasterly flow aloft still dry. Late day instablilty along with weak upslope flow could produce a shower over the Cascades in the late afternoon and evening hours both days. Upper level low weakens and moves east Tuesday and Wednesday allowing the upper level ridge offshore to move into the area. Low level flow never turns offshore through the entire period. This will keep highs mostly in the 60s with lower 70s possible as the ridge axis moves overhead Wednesday. Lows upper 30s to mid 40s.

AVIATION

Ceilings are slowly improving across the area early this morning, with most locations now at low-end VFR, through areas of MVFR persist across the northern interior terminals. MVFR will likely persist through much of the day and into the early afternoon in this area as weak low-level convergence continues to persist. However, all clouds should begin to clear out between 22Z Thu - 03Z Fri. Areas of fog may then begin to develop around/after 12Z Fri in the valleys. Southerly winds around 5 to 10 kt will slowly ease this morning, becoming mostly light westerly after 21Z, then northwest to north after 00Z Mon.

KSEA...Ceilings have lifted to VFR and have begun to break apart early this morning. Low-end VFR to upper-end MVFR cigs will continue to persist over the terminal through around 00Z Fri when clouds will begin to scatter. Best chances for additional lower cigs will be 15- 20Z this morning (25-35% chance MVFR cigs). South winds 8 to 12 kt will decrease after 18Z as winds become more westerly around 21Z then north after 00Z Friday. There will be a 15-20% of low stratus or fog development after 12Z Friday.

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MARINE

The overall weather pattern looks to remain fairly static over the next week. Broad high pressure over the northeast Pacific will remain offshore and get pushed out as lower pressure remains in place over the inland Pacific Northwest. Winds may be a bit breezy, mainly over the interior waters and the gaps in the coastal terrain, on Friday as winds turn north to northeasterly. Lighter onshore flow resumes Sunday into early next week. Seas currently 10 to 12 ft will continue to ease this morning to below 10 ft by this afternoon, with seas remaining 5 to 8 ft through the remainder of the forecast period.

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HYDROLOGY

No river flooding the next 7 days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 11 AM PDT this morning 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-Grays Harbor Bar.