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September 2014
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Monday Wx Outlook

Posted: September 22nd, 2014

 

Pre-frontal showers this morning

 

Meteorological fall began three weeks ago, but the atmosphere decided to wait until astronomical fall to deliver quintessentially seasonal weather.

 

A weak frontal system will pass through later today and provide the region with a few more light showers. A stronger system will then arrive tomorrow. Forecast models currently show a period of moderate-to-heavy rainfall on Tuesday night, followed by periods of mostly light rain and possibly isolated thunderstorms through at least Friday. At the moment, models suggest that the weekend could be dry and partly sunny.

 

The NWS currently predicts around 1.58 inches of rain to fall citywide Tuesday through Friday, with peak intensity occurring in the 6-hours ending Tuesday 11 PM. The UW WRF-GFS currently has 0.72 inches falling in the 24-hours ending Wednesday 11 AM, with 0.41 inches of that occurring in the 6-hours ending Wed 5 AM. The ECMWF has lighter amounts throughout the period. Over SPU’s mountain reservoirs, up to 2 inches of rainfall is expected Tuesday through Friday.

 

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

 

THE 12Z NAM AND A QUICK LOOK AT THE INCOMING 12Z GFS SHOWS A SMALL SHORT WAVE TROUGH LIFTING NE ACROSS THE AREA FROM ABOUT MIDNIGHT TONIGHT TIL ABOUT 7 AM TUE. THE MID LEVEL AIR MASS WILL REMAIN SOMEWHAT UNSTABLE OVERNIGHT AND THE NAM SHOWS RATHER IMPRESSIVE LIFT AS THIS FEATURE MOVES THROUGH. WILL UPDATE THE FORECAST FOR LATER TONIGHT TO INCLUDE A CHANCE OF SHOWERS THROUGHOUT WESTERN WASHINGTON. AT THIS TIME…MID LEVEL INSTABILITY APPEARS MARGINAL ENOUGH TO KEEP TSTMS OUT OF THE FORECAST LATE TONIGHT.

YET ANOTHER BATCH OF SHOWERS BLOSSOM OVER THE AREA TUESDAY AFTERNOON AS ANOTHER SHORT WAVE TROUGH WITH ITS UPPER LEVEL DIFFLUENCE AND SOME INSTABILITY SHOOTS OUT AHEAD OF A DEVELOPING WARM FRONT FROM THE CENTRAL OREGON COAST INTO AN OCCLUDING 972 MB LOW THAT WILL BE MOVING TO 46N 138W. WILL ALSO UPDATE THE FORECAST FOR TUE TO INCLUDE SHOWERS IN THE AFTERNOON HOURS. THERE IS AGAIN A MARGINAL CHANCE OF TSTM ACTIVITY WITH THIS SHORT WAVE TROUGH.

THE MAIN COLD FRONT ASSOCIATED WITH THE OFFSHORE LOW WILL MOVE ACROSS THE AREA TUE NIGHT. THE 12Z GFS SHOWS NEARLY AN INCH OF RAIN WITH THIS FRONT TUE NIGHT…AND THE FRONT WILL BE PRECEDED BY WINDY CONDITIONS ALONG THE COAST. SHOWERS WILL TAPER OFF FROM THE WEST BEHIND THE FRONT ON WEDNESDAY. WITH THE INCREASE IN CLOUDS AND THE PRECIPITATION…THE AROUND 80 DEGREE TEMPERATURES THAT WE EXPERIENCED ON SUNDAY WILL BECOME AN INCREASINGLY DISTANT MEMORY AS WE MOVE INTO THE FALL SEASON.

AN UPPER TROUGH OVER THE NORTHEAST PACIFIC ON THURSDAY WILL SHIFT ONSHORE AND WEAKEN ON FRIDAY. THERE SHOULD BE A GRADUALLY DECREASING CHANCE OF SHOWERS DURING THIS PERIOD. FOR THE WEEKEND…A WEAK UPPER RIDGE WILL MOVE OVER THE AREA FOR DRY AND SUNNIER WEATHER WITH A SLIGHT WARMING TREND.

 

Climate Notes:

 

  • We only need 0.79 inches of rainfall to make September the seventh wetter-than-normal month of 2014.
  • Seasonal outlooks show below normal precipitation and above normal temperatures through November, and a very weak El Niño past December.

 

References:

 

NWS Area Forecast Discussion
NWS Forecast Table Interface
Climate Prediction Center El Niño Status