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September 2014
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Monday Wx Outlook

Posted: September 2nd, 2014

 

12z UW WRF-GFS 24-hour precipitation.

12z UW WRF-GFS 24-hour precipitation.

Now that we’ve been reminded of fall’s impending return, summer will return for at least one more week, but not before a fast moving low pressure system clips the region today.

 

Up to 0.15 inches of rain is expected to fall across the City, mostly between 4 and 10 PM. Southwest winds off the Sound will briefly gust to 30 mph at about the same time. Forecast models also have Puget Sound Convergence Zone passing through overnight, which may slightly enhance rainfall amounts.

 

In the mountains, between 0.25 and 0.50 inches (south to north) of rainfall is expected later today, again, with perhaps double that where convergence occurs late. A few inches of snow is also likely overnight above 6000 feet.  

 

Skies will start to clear on Wednesday, and by the weekend temperatures are expected to be above normal as they have been all summer. This upcoming bout of warmth will be due to a thermally-induced trough that will visit from California. Curiously, our historically warm summer has not seen many of these offshore outbreaks, which are what typically provide heat.

 

Excerpts from this morning’s NWS Area Forecast Discussion:

 

A RELATIVELY VIGOROUS AND FAST MOVING UPPER TROUGH AND COLD FRONT WILL BRING A LITTLE RAIN TO THE AREA LATER TODAY. RAIN SHOULD REACH THE NORTH COAST LATE THIS MORNING AND SPREAD INTO THE INTERIOR DURING THE AFTERNOON. SHOWERS WILL BECOME MOSTLY CONFINED TO THE CASCADES AND A PUGET SOUND CONVERGENCE ZONE BY EARLY EVENING AS THE SYSTEM QUICKLY MOVES INLAND. A FEW SHOWERS WILL LINGER OVER THE CASCADES INTO WEDNESDAY MORNING. OTHERWISE…WEDNESDAY WILL BE DRY WITH DECREASING CLOUDS AS DRIER NORTHERLY FLOW ALOFT DEVELOPS.

 

HIGH PRESSURE ALOFT WILL BE IN CONTROL THURSDAY AND FRIDAY WITH LOW LEVEL OFFSHORE FLOW DEVELOPING. THIS WILL GIVE SUNNY WEATHER WITH HIGHS WARMING 3-5 DEGREES EACH DAY. HIGHS IN THE 70S WILL BE COMMON WEDNESDAY AND PARTS OF THE SOUTH SOUND SHOULD SEE HIGHS NEAR 80 BY THURSDAY.

 

HIGH PRESSURE ALOFT OVER THE NE PACIFIC WILL PERSIST FRIDAY AND SATURDAY BEFORE POSSIBLE CHANGES IN THE PATTERN. THERMALLY INDUCED LOW PRESSURE WILL EXPAND NWD ALONG THE WA COAST DURING THIS PERIOD WITH DRY OFFSHORE FLOW. TEMPERATURES WILL WARM IN THIS PATTERN…WITH LOW 80S FROM THE CENTRAL SOUND TO THE SW INTERIOR.

 

500 MB HEIGHTS FALL SATURDAY NIGHT…THEN A WEAK TROUGH ARRIVES SUNDAY. THIS SHOULD INDUCE AN ONSHORE PUSH BRINGING MARINE AIR INLAND AND RESULTING IN MODERATING DAYTIME TEMPERATURES. THIS SYSTEM WILL LIKELY BRING LITTLE IF ANY PRECIPITATION SO THE MAIN AFFECT WILL BE THE CHANGE ONSHORE FLOW CONDITIONS. HIGHS WILL ONLY REACH THE 70S AND POSSIBLY STRUGGLE OUT OF THE UPPER 60S NEAR THE COAST.

 

WEAK ONSHORE FLOW BUT MOSTLY DRY CONDITIONS SHOULD PREVAIL EARLY NEXT WEEK. THE MAJORITY OF MODELS SHOW A TROUGH CLIPPING THE REGION BUT AT THIS TIME DOES NOT APPEAR TO BE A RAIN PRODUCER.

Climate Notes:

  • With an average temperature of 69.2, Seattle just had its warmest July through August on record. (Last Jul-Aug is now at number 3.) Myriad high temperature records have been broken throughout the Northwest this summer.
  • El Niño continues to fade. While chances officially remain at 65%, models continue to trend towards a very weak event at best. Climate Prediction Center outlooks continue to show below normal precipitation and above normal temperatures through November.

References:
SPU Climate Change program

Climate Prediction Center