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September 2014
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Weekend Wx Outlook

Posted: September 4th, 2014

 

00z GFS 500mb 96hr forecast via UW

^ 00z GFS 500mb 96hr forecast via UW.

 

Dry and relatively warm weather will continue through at least Saturday. The high pressure responsible for the summer reprise is then expected to weaken on Sunday. Forecast models disagree slightly on the extent to which troughing, i.e. decreasing pressure aloft, will occur on Monday (see above).

 

The National Weather Service currently has a chance of showers (Seattle: 0.05″/30hrs ending Mon 11PM; Watersheds: 0.09″ – 0.12″/42hrs ending Tue 11AM) with drying and warming again early next week. The latest UW WRF-GFS predicts slightly less rainfall during the period, and the ECMWF currently has none through 10 days.

 

Excerpts from this morning’s NWS Area Forecast Discussion:

 

OFFSHORE FLOW WILL CONTINUE FRI AND SAT AS THE INVERTED SURFACE
TROUGH EXTENDS NORTH INTO THE WA COAST. THE BROAD UPPER LEVEL RIDGE
WILL REMAIN IN CONTROL ACROSS THE PAC NW FOR MORE DRY WEATHER. TEMPS
WILL BE WARM BOTH DAYS AND SHOULD SEE PLENTY OF 80S…INCLUDING THE
COAST. A FEW LOW 90S ARE POSSIBLE TOO. IT WILL BE SLIGHTLY COOLER
NEAR THE STRAIT AND NORTH INTERIOR WATERS WITH HIGHS IN THE 70S.

LOW PRES WILL START TO SHIFT EAST OF THE CASCADES ON SUNDAY. THIS
WILL BRING NORTHWESTERLY WINDS TO THE COAST FOR COOLER AND CLOUDIER
WEATHER – HIGHS WILL DROP BACK DOWN INTO THE 60S. THE INTERIOR WILL
REMAIN CLEAR WITH TEMPS STILL IN THE MID 70S TO LOWER 80S. A WEAK
ONSHORE PUSH WILL BRING STRATUS CLOUDS FARTHER INLAND SUN NIGHT.

MODELS SHOW A WEAK COLD FRONT CROSSING WESTERN WA ON MONDAY WHICH WILL INCREASE ONSHORE FLOW…THUS BRINGING MORE CLOUDS AND A FEW SHOWERS. TEMPERATURES WILL REMAIN CLOSE TO NORMAL WITH HIGHS IN THE LOW TO MID 70S. ONSHORE FLOW WILL CONTINUE ON TUE FOR PARTLY SUNNY SKIES. A WEAK UPPER LEVEL TROUGH MAY GENERATE ISOLATED SHOWERS. BUT DRIER AND WARMER WEATHER MIGHT BE IN STORE WED ONWARDS AS ANOTHER UPPER LEVEL RIDGE STARTS TO BUILD IN FROM THE WEST.

 

References:
GFS 500mb Model Loop (UW)
NWS Area Forecast Discussion
NWS Forecast Table Interface
SPU Climate Change program