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September 2014
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Tuesday Wx Update

Posted: September 23rd, 2014

 

2014-09-23 12km wrf48hr rainfall according to the latest (12km) UW WRF-GFS

 

No major changes to overall rainfall amounts this week, however forecast models continue to adjust the timing of frontal passages—moderate-to-heavy rainfall is presently expected around midnight tonight. Also, this weekend is now looking generally dry but showers cannot be ruled out.

 

The latest NWS predictions show 0.80″ today through Friday citywide with a 6-hour maximum of 0.23″ ending at 11 PM tonight. Last night’s UW WRF-GFS had 1.55″ through Friday with a 6-hour max of 0.43″ ending Wed 5 AM. But this morning’s UW WRF-GFS (pictured above) is going for 0.94″ in the 24-hours ending 11 AM tomorrow, with a 6-hour max of 0.56″ ending 5 AM tomorrow. The ECMWF continues to predict lighter rainfall amounts throughout the period; its current 6-hour max is 0.45″ ending 1 AM tomorrow. Predicted rainfall amounts for SPU’s mountain reservoirs have also decreased slightly to under 1.5 inches through Friday.

 

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

 

THE DOMINANT WEATHER FEATURE THIS WEEK WILL BE THE LARGE VERTICALLY STACKED UPPER LEVEL/SURFACE LOW CURRENTLY CENTERED NEAR 45N/140W AT 15Z/8 AM. THE FIRST SHOT OF SHOWERS WILL BE TODAY AS AN UPPER LEVEL SHORTWAVE TROUGH EMBEDDED IN THE FLOW AROUND THE MAIN LOW MOVES NE ACROSS W WA. THIS SHORTWAVE IS A LITTLE HARD TO PICK OUT FROM THE MESS OF CLOUD COVER OFFSHORE…BUT RADAR SHOWS AN ASSOCIATED WELL DEVELOPED LINE OF SHOWERS ALONG THE WA AND N OREGON COAST THAT WILL MOVE OVER THE W WA INTERIOR BEFORE NOON. A 110 KT 300 MB JET STREAK NOSING INLAND OVER W WA BY 18Z WILL STRENGTHEN THE SHORTWAVE TROUGH AS IT MOVES ACROSS THE AREA.

THE MAIN RAIN EVENT WILL BE TONIGHT THROUGH WEDNESDAY MORNING AS A COLD FRONT MOVES ACROSS THE AREA. MODELS ARE IN FAIRLY GOOD AGREEMENT WITH THE TIMING…RAIN WILL BECOME MORE STEADY ON THE COAST EARLY THIS EVENING THEN SPREADING OVER THE INTERIOR LATER THIS EVENING. FROPA ON THE COAST WILL OCCUR AFTER MIDNIGHT AND OVER PUGET SOUND MID TO LATE WEDNESDAY MORNING. COMBINED QPF FOR BOTH TODAYS SHORTWAVE TROUGH AND COLD FRONT SHOULD BE HEALTHY WITH 1-2
INCHES ON THE COAST AND ROUGHLY 0.50-1.00 INCHES OVER THE INTERIORWIND SHOULD NOT BE A BIG DEAL THIS TIME AS PRESSURE GRADIENTS ARE TOO EASTERLY.

THE MAIN OFFSHORE LOW WILL HAVE DRIFTED NE TO AROUND 49N/135W BY 18Z/11 AM WEDNESDAY AND IT WILL REMAIN AROUND THAT AREA THROUGH THURSDAY. THIS WILL KEEP W WA UNDER GENERALLY MOIST SW FLOW ALOFT. WEAK EMBEDDED SHORTWAVE TROUGHS WILL PROBABLY GENERATE PERIODS OF SHOWERS WEDNESDAY AFTERNOON THROUGH THURSDAY. CHANCE POPS WILL PROBABLY WORK OUT BEST FOR THIS PERIOD. MAX TEMPS WILL REMAIN CLOSE TO NORMAL WITH HIGHS IN THE MID 60S TO NEAR 70.

AN UPPER TROUGH OVER THE PACIFIC NORTHWEST WILL WEAKEN ON FRIDAY FOR A CONTINUED CHANCE OF SHOWERS. FOR THE WEEKEND…THERE WILL BE AN UPPER LOW WELL TO THE SOUTH OF THE AREA OVER THE SOUTHWEST U.S. AND A SECOND UPPER LOW OVER THE GULF OF ALASKA. THIS PUTS WESTERN WASHINGTON IN BETWEEN MOST OF THE ACTIVE WEATHER WITH WEAK HIGH PRESSURE ALOFT FOR GENERALLY DRY WEATHER. THE FLOW ALOFT WILL LIKELY BECOME MORE CONSOLIDATED ON MONDAY WITH AN UPPER TROUGH CARVING OUT OVER THE AREA FOR A BETTER CHANCE OR RAIN.

 

References:

 

NWS Area Forecast Discussion
NWS Forecast Table Interface