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September 2014
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Monday Wx Outlook

Posted: September 15th, 2014

 

Hot and smoky

 

After one more day of heat, considerably cooler temperatures will arrive tomorrow, as will cloudiness and a few chances for light rain Wednesday through Saturday. Forecast models are struggling to resolve details toward the weekend, however; the jet stream is actively trying to break down the persistent high pressure that may still give us another run at heat next week.

 

The NWS (as well as the UW WRF-GFS) currently predict around 0.15 inches of rain to fall Wednesday through Friday. The ECMWF model shows only a tiny bit of rain on Wednesday, followed by stronger frontal passages and more significant rainfall amounts next week.

 

Over SPU’s mountain reservoirs, around 0.25 inches of rainfall is expected Wednesday through Saturday. The WRF-GFS hints at a weak Puget Sound Convergence Zone event later in the weekend, which could increase rainfall amounts.

 

Excerpts from this morning’s NWS Area Forecast Discussion (emphasis added):

 

SURFACE FLOW TURNS WEAKLY ONSHORE TONIGHT AND BECOMES STRONGER TUESDAY AFTERNOON. HARD TO SEE STRATUS MOVING FAR INLAND TUESDAY MORNING BUT THERE COULD BE SOME HIGHER CLOUDS FROM A WEAK SHORTWAVE PASSING FROM SOUTHWEST TO NORTHEAST. MESOSCALE MODELS ARE IN REASONABLY GOOD AGREEMENT…BRINGING SOME CLOUDS BY 12Z. WITH UPPER RIDGING STILL EVIDENT AND ONSHORE FLOW UNIMPRESSIVE IT SEEMS LIKELY

THE CLOUDS WILL MOSTLY CLEAR TUESDAY AFTERNOON EXCEPT FOR THE COAST. HIGHS ON TUESDAY WILL CERTAINLY BE LOWER THAN TODAY…MOSTLY 70S.

STRONGER ONSHORE FLOW WILL GIVE MORE CLOUD COVER WEDNESDAY MORNING. ANOTHER SHORTWAVE SPINNING OUT OF AN UPPER LEVEL LOW OFF THE NORTHERN CALIFORNIA COAST WILL MOVE TOWARD THE AREA DURING THE DAY ON WEDNESDAY. PLENTY OF MID AND HIGH LEVEL CLOUD COVER IN ADDITION TO THE LOW STRATUS WEDNESDAY WILL KEEP HIGHS IN THE MID 60S TO MID 70S. THERE IS A CHANCE OF SHOWERS WEDNESDAY AFTERNOON BUT LATEST MODEL RUNS TEND TO CONFINE SHOWERS TO THE COAST AND OLYMPICS.

EXTENDED MODELS STILL HAVING TROUBLE WITH THE TIMING AND STRENGTH OF THE UPPER LEVEL TROUGH MOVING INTO WESTERN WASHINGTON WEDNESDAY NIGHT AND THURSDAY. THE GENERAL TREND OF THE MODELS IS TO BE SLOWER AND WEAKER WITH THIS FEATURE. CURRENT FORECAST OF A CHANCE OF SHOWERS FOR THE MOST PART LOOKS GOOD AT THIS POINT. FRIDAY WILL BE A RACE BETWEEN HOW FAST THE NEXT UPPER LEVEL RIDGE CAN BUILD VERSUS A WEAK FRONT MOVING INTO BRITISH COLUMBIA. THE ECMWF IS A LITTLE STRONGER WITH THE RIDGE PUSHING THE SYSTEM WELL NORTH OF THE AREA. THE GFS STILL KEEPS SOME LIGHT PRECIPITATION OVER THE AREA WITH A WEAKER RIDGE. WITH THE LACK OF CONSENSUS WILL STAY WITH THE CHANCE OF SHOWERS FORECAST. UPPER LEVEL RIDGE CONTINUING TO BUILD OVER THE AREA ON SATURDAY FOR A DRY DAY. LOW LEVEL FLOW DOES NOT TURN OFFSHORE KEEPING TEMPERATURES JUST A FEW DEGREES ABOVE NORMAL. SOME DIFFERENCES IN THE MODELS ON SUNDAY WITH THE ECMWF SHOWING A STRONGER RIDGE VERSUS THE GFS SOLUTION. STILL A DRY DAY EITHER WAY. BOTH OF THE MODELS SHOW THE LOW LEVEL FLOW GOING ONSHORE IN THE AFTERNOON SO WILL KEEP HIGHS IN THE 70S EVEN WITH THE STRONG RIDGING.

 

Climate Notes:

 

  • As we flirt with 90 degrees, today marks Seattle’s 45th day above 80 degrees this year, two from 1958’s record of 47. If we use 85 degrees as a threshold, today will tie the record of 22 set in 1967.
  • El Niño chances still continue to drop. Climate Prediction Center outlooks also continue to show below normal precipitation and above normal temperatures through November, however.