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September 2014
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Weekend Wx Outlook

Posted: September 25th, 2014

 

Satellite imagery showing thunderstorms offshore (NWS EAFD)

Satellite imagery showing thunderstorms offshore (NWS EAFD)

 

The large, wet system that kicked-off fall yesterday is slowly exiting the region. Post-frontal shower activity, mostly light, will continue today and into tomorrow. As the lowering, but still strong sun tries to poke through the clouds this afternoon, there’s a chance that thunderstorms will develop around us. There are currently no NWS watches or warnings for Seattle, but short-duration, high-intensity rainfall remains possible into this evening.

 

Drier weather will take over later tomorrow, and Saturday and most of Sunday look to be partly sunny, though still cool. While timing may change, showery weather is expected to approach the region early next week.

 

Excerpts from the 9:00 AM NWS Area Forecast Discussion (emphasis added):

 

RADAR IMAGERY SHOWS AN AREA OF SHOWERS FROM PUGET SOUND EAST TO THE CASCADE CREST AND FROM SEATTLE NORTHWARD AT 15Z/8 AM…HEADED NORTH TOWARD THE BORDER. THIS AREA LOOKS LIKE IT IS RELATED TO THE RIGHT ENTRANCE REGION OF A 125 KT 300 MB JET STREAK THAT IS CROSSING OVERHEAD THIS MORNING. BY 18Z/11 AM THE JET STREAK SHOULD BE OVER E WA…SO THE SHOWERS SHOULD BE DIMINISHING RAPIDLY OVER THE NEXT COUPLE OF HOURS.

THE UPPER LEVEL LOW CENTERED OFFSHORE ALONG ROUGHLY 135W WILL BE THE MAIN WEATHER PRODUCER THROUGH FRIDAY. AN UPPER LEVEL SHORTWAVE TROUGH SWINGING AROUND THE MAIN LOW WAS ON THE SE SIDE OF THE LOW THIS MORNING AND HEADED SLOWLY TOWARD THE WA COAST. IR SATELLITE IMAGERY SHOWS AN UNSTABLE AIR MASS OFFSHORE WITH THE TROUGH…WHICH CONTAINS THE REMNANTS OF AN OLD BENT-BACK OCCLUSION. THERE HAVE ALSO BEEN A COUPLE OF LIGHTNING STRIKES OFFSHORE THIS MORNING. THE MOST RECENT PROGS BRING THIS AREA TO THE COAST LATER THIS AFTERNOON AND THEN INLAND OVER PUGET SOUND THIS EVENING. DESPITE MODEL AGREEMENT CONFIDENCE IN THIS TIMING IS ONLY MARGINAL. THE CONVECTIVE ELEMENTS SEEM TO BE JUST CREEPING EASTWARD AND THE MODEL TREND SEEMS TO BE A LITTLE SLOWER THAN YESTERDAYS RUNS. HOWEVER I WILL LEAVE THE FORECAST AS IS…WITH SHOWERS LIKELY AND A CHANCE OF THUNDERSTORMS THIS AFTERNOON AND EVENING. LIFTED INDICES ARE IN THE ZERO TO -2 RANGE WITH CAPE AROUND 500 JOULES/KG. CONVECTIVE TEMPERATURES SHOULD BE AROUND 66 EARLY THIS AFTERNOON WHICH MAY BE A STRETCH IF NO BREAKS IN THE CLOUD COVER DEVELOP.

THE UPPER LEVEL LOW WILL WEAKEN INTO A TROUGH ON FRIDAY…WITH THE AXIS JUST OFFSHORE ALONG 126W AT 00Z/5 PM FRIDAY. THE TROUGH WILL CONTINUE WEAKENING FRIDAY NIGHT WITH THE AXIS MOVING INLAND OVER W WA BY 12Z/5 AM SATURDAY. SHOWER ACTIVITY SHOULD DIMINISH RAPIDLY OVERNIGHT…ALTHOUGH I MAY HAVE TO PUT A CHANCE OF SHOWERS BACK IN FOR LATE FRIDAY NIGHT.

MODELS HAVE BEEN CONSISTENT IN BUILDING A WEAK POSITIVE TILT UPPER LEVEL RIDGE INLAND OVER W WA ON SATURDAY…ALONG WITH A WEAK SURFACE RIDGE. THE DRY PARTLY SUNNY FORECAST STILL LOOKS ON TRACK. LATEST GFS/NAM TIME-HEIGHT SECTION FOR KSEA SHOWS THE LOW LEVEL MOISTURE DISSIPATING DURING THE EARLY AFTERNOON ON SATURDAY…SO IT MIGHT ACTUALLY WORK OUT FOR MOSTLY SUNNY SKIES BY LATE AFTERNOON. THE NAM SUGGESTS AREAS OF FOG SUNDAY MORNING.

EXTENDED MODELS IN GOOD AGREEMENT THIS MORNING. ON SUNDAY UPPER LEVEL RIDGE OVER THE AREA BUT 500 MB HEIGHTS ONLY IN THE MID AND UPPER 570 DECAMETERS AND SURFACE GRADIENTS REMAIN LIGHT TO SLIGHTLY ONSHORE. WILL GO WITH PARTLY SUNNY WORDING ON SUNDAY WITH HIGHS A COUPLE OF DEGREES MORE THAN SATURDAY. RIDGE SHIFTS OFF TO THE EAST SUNDAY NIGHT WITH BOTH THE ECMWF AND GFS AGREEING IN SLOWING DOWN THE APPROACH OF THE INCOMING FRONT. HAVE DROPPED THE POPS FROM THE SUNDAY EVENING PERIOD. IF THE MODEL TRENDS CONTINUE LOOK FOR THE POPS TO BE TAKEN OUT OF THE  MONDAY MORNING TIME FRAME AS WELL. WEAKENING FRONT MOVING THROUGH THE AREA LATER MONDAY FOR A CHANCE OF RAIN AT THAT TIME. UPPER LEVEL TROUGH BEHIND THE FRONT KEEPING A CHANCE OF SHOWERS IN THE FORECAST MONDAY NIGHT AND TUESDAY. SLIGHT DISAGREEMENT IN THE SOLUTION FOR NEXT WEDNESDAY WITH THE GFS BRINGING A STRONGER RIDGE INTO THE AREA WHILE THE WEAKER ECMWF SHOWS SOME LIGHT PRECIPITATION IN THE AREA. WITH THE LACK OF CONSENSUS WILL STAY WITH THE CHANCE POPS FOR WEDNESDAY.

 

References:
NWS Area Forecast Discussion
NWS Forecast Table Interface