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September 2014
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Monday Wx Outlook

Posted: September 29th, 2014

 

Modeled Puget Sound Convergence Zone passing through the City tonight (via NWS EAFD)

Modeled Puget Sound Convergence Zone passing through the City tonight (via NWS EAFD)

 

A frontal system will pass through today and provide us with a an hour or two of moderate rainfall followed occasional light showers through tomorrow. Forecast models also indicate that a healthy Puget Sound Convergence Zone will move south through the city and watersheds after the sun goes down tonight. Brief, perhaps 30-minute moderate-to-heavy showers are likely.

 

The second half of the week looks to be dry and partly- to mostly-sunny due to high pressure building offshore. Said ridge should push high temperatures back into the 70s by the weekend as well as attempt to fend off frontal systems attacking from the Gulf of Alaska. We’ll see over the course of the week how strong the ridge becomes—this is similar to the scenario that has kept us warm and sunny pretty much all summer.

 

The NWS currently predicts 0.30″ inches of rain to fall citywide tomorrow, again with perhaps double that in a short period of time as the convergence zone passes through this evening. The ECMWF has about 0.35″ citywide, and the UW WRF-GFS is currently driest with only about 0.12″. Over SPU’s mountain reservoirs, 0.75-1.00″ plus convergence zone enhancment is expected.

 

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

 

LATEST RADAR LOOP INDICATES RAIN WITH THE APPROACHING FRONT IS STARTING TO PUSH INLAND FROM THE COAST. DRIZZLE IS ALSO FALLING OVER PARTS OF THE INTERIOR AS INCREASING LIFT AHEAD OF THE FRONT ACTS ON THE LOW LEVEL MOISTURE IN PLACE. THIS WILL BE A FAST MOVING SYSTEM DRIVEN BY A 130 KT JET ON THE BACK SIDE OF THE UPPER TROUGH. ALTHOUGH THE FRONT APPEARS WEAK AND DIFFUSE ON IR IMAGERY… WITH THAT IN MIND…THE 00Z WRF-GFS/12Z NAM QPF AMOUNTS OF ONE TENTH TO A QUARTER INCH OF RAIN ACROSS WRN WA BY THE END OF THE DAY IS REASONABLE. MODELS ALSO AGREE ON TIMING WITH STEADY RAIN REACHING THE REST OF THE INTERIOR BY MIDDAY.

POST FRONTAL FLOW BECOMES FAVORABLE FOR A PUGET SOUND CONVERGENCE ZONE. BOTH THE WRF/NAM ADVERTISE THIS DEVELOPING NEAR THE KING/SNOHOMISH COUNTY LINE EARLY THIS EVENING…THEN DRIFTING FURTHER SOUTH INTO THE REST OF THE SEATTLE AREA DURING THE EVENING. THIS MAKES SENSE WITH THE 850 MB FLOW TURNING MORE NWLY. MESO MODELS INDICATE THE CONVERGENCE ZONE DISSIPATES LATE TONIGHT AS IT REACHES THE PIERCE/KING COUNTY LINE NEAR TACOMA. LOCALLY HIGHER RAIN AMOUNTS SHOULD BE EXPECTED WITHIN THE FAVORED CONVERGENCE CORRIDOR. ELSEWHERE…THERE WILL BE SHOWERS AROUND BUT ADDITIONAL AMOUNTS WILL BE LIGHT AND VARIABLE.

THE COLD POOL ALOFT SEEN ON IR IMAGERY OFF THE CENTRAL B.C. COAST WILL CLIP THE AREA LATE TONIGHT AND TUESDAY UNDER NW FLOW ALOFT. CYCLONIC FLOW AND COLDER AIR ALOFT WILL CONTINUE TO FAVOR INSTABILITY AND SHOWERS ACROSS THE AREA. SHOWERS WILL BEGIN TO TAPER OFF TUESDAY NIGHT SO THE LOWER CHANCE POPS IN THE FORECAST LOOK FINE. EXPECT GENERALLY DRY WEATHER BY WEDNESDAY EXCEPT FOR A FEW LINGERING SHOWERS IN THE MOUNTAINS. HIGH TEMPERATURES WILL GENERALLY RUN IN THE LOW 60S THROUGH MID WEEK WHICH IS A FEW DEGREES BELOW AVERAGE FOR THE PUGET SOUND REGION. MERCER

A BROAD UPPER RIDGE WILL BE OVER THE AREA THURSDAY THROUGH SUNDAY. MOISTURE COMING OVER THE TOP OF THE RIDGE WILL GIVE A THREAT OF RAIN AT TIMES BUT MODELS ARE HAVING A TOUGH TIME WITH THE DETAILS AND ARE NOT SHOWING MUCH RUN TO RUN CONSISTENCY. FOR NOW…CONFIDENCE IS REASONABLY HIGH THAT THURSDAY WILL BE DRY. A LITTLE RAIN COULD AFFECT THE NORTHERN ZONES FRIDAY. THE GFS AND ECMWF HAVE FLIP-FLOPPED WITH THE GFS NOW WETTER AND THE ECMWF MOSTLY DRY. THERE IS PROBABLY A BETTER CHANCE OF RAIN SATURDAY FOR MOST AREAS AS AN UPPER SHORT WAVE PUSHES THE MOISTURE A BIT FURTHER SOUTH.

 

Climate Notes:
 

  • Seasonal outlooks continue to 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