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September 2014
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Monday Wx Outlook

Posted: September 8th, 2014

 

2014-09-08 4km wrf

 

A broad area of low pressure will pass through the region today. Some light rain is possible overnight tonight, but by later tomorrow high pressure and dry, sunny weather will return. Another run at 80 degrees is likely next weekend.

 

The NWS currently predicts 0.07 inches of rain to fall overnight. The HRRR and ECMWF models each show no rain at all. The above-pictured WRF-GFS, however, indicates that a Puget Sound Convergence Zone (PSCZ) will form after 5 PM and provide parts of the City with up to 0.40 inches over 6 hours. While the atmosphere is not primed like it was before last week’s under-predicted PSCZ event, it’s worth watching this afternoon.

 

Over SPU’s mountain reservoirs, between 0.04 and 0.11 inches of rainfall (north to south) is expected overnight, again, with perhaps double that where convergence occurs.

 

Excerpts from this morning’s NWS Area Forecast Discussion:

 

MARINE AIR WILL BRING A DRAMATIC COOL DOWN TO THE REGION TODAY… A WEAK COLD FRONT WILL CROSS THE AREA FROM NW TO SE THIS AFTERNOON WHICH MAY TRIGGER ISOLATED SHOWERS. THE MESOSCALE MODELS SHOW A PUGET SOUND CONVERGENCE ZONE FORMING OVER KING COUNTY LATE THIS AFTERNOON AND EVENING FOR ADDITIONAL SHOWERS. SHOWERS ASSOCIATED WITH THE PSCZ MAY LINGER THROUGH LATE TONIGHT. REMOVED THE MENTION OF MORNING DRIZZLE TODAY… OTHERWISE THE REST OF THE FORECAST IS ON TRACK.

AN UPPER LEVEL RIDGE WILL BUILD OVER THE NE PAC ON TUE WITH NORTHERLY FLOW ALOFT OVER WESTERN WA. PRESSURE GRADIENTS ARE STILL ONSHORE SO EXPECT ANOTHER MOSTLY CLOUDY DAY WITH TEMPS AGAIN IN THE MID 60S TO LOWER 70S. A FEW SHOWERS MAY DEVELOP IN THE NORTH CASCADES AS A WEAK UPPER LEVEL DISTURBANCE TRACKS THROUGH B.C.

THE UPPER LEVEL RIDGE WILL NUDGE INLAND ON WED WITH THE LOW LEVEL FLOW STARTING TO FLIP BACK TO OFFSHORE. THIS WILL RESULT IN LESS CLOUDS AND WARMER TEMPS…CLOSER TO NORMAL. THIS DRY AND WARMING TREND WILL CONTINUE INTO THU.

MODELS ARE IN GOOD AGREEMENT THAT HIGH PRESSURE GRADUALLY STRENGTHENS OVER THE REGION THROUGH THE END OF THE WEEK INTO NEXT WEEKEND. GRADUALLY RISING HEIGHTS AND LOW LEVEL OFFSHORE FLOW SHOULD ALLOW TEMPERATURES TO SLOWLY WARM EACH DAY. HIGHS AROUND PUGET SOUND WILL RANGE FROM THE MID 70S ON THURSDAY TO THE LOW 80S OVER THE WEEKEND. SOME MODELS IMPLY EVEN WARMER HIGHS THIS WEEKEND BUT IT IS PROBABLY UNWISE TO DEVIATE ANY FURTHER FROM CLIMATOLOGY ON DAY 6/7.

 

Climate Notes:

 

  • After Saturday’s record high of 90 degrees, Seattle’s count of days above 80 degrees this year stands at 41, good for 4th place all-time. The record is 47 set in 1958.
  • El Niño chances continue to drop and are now at 60-65%. That said, Climate Prediction Center outlooks continue to show below normal precipitation and above normal temperatures through November.