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August 2014
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Weekend Wx Outlook

Posted: August 27th, 2014

 

Even though summer still has one more month to go, this weekend’s weather will make us all start to think about fall.

 

A large trough of low pressure, which appears be heading southward because of an active “polar vortex” over Alaska, will pass by the Pacific Northwest on Friday and Saturday. Swirling around the edges of the unequivocally polar system will be a few shortwaves; each disturbance bringing either cooler, cloudier, or rainy weather starting tomorrow.

 

Forecast models tend to have a difficult time resolving shortwaves and this weekend is no exception. Current rainfall predictions are varied and uncertain, but the chance for some light rain will remain high throughout the holiday weekend. The NWS is wettest overall with up to 0.75″ over 60 hours, with the north end of Seattle likely being wetter than south Seattle. There is little clarity beyond the weekend, but a more active, fall-like pattern appears likely.

 

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

A MORE DRAMATIC CHANGE WILL OCCUR TONIGHT AND THURSDAY. ONSHORE FLOW WILL INCREASE AND THE MARINE LAYER WILL DEEPEN TONIGHT…WITH STRATUS PUSHING WELL INLAND OVERNIGHT. THEN A SMALL UPPER SHORTWAVE WILL MOVE THROUGH THE AREA THURSDAY. THERE COULD BE SOME DRIZZLE ALONG THE COAST IN THE MORNING…AND SOME SHOWERS IN THE NORTH CASCADES IN THE AFTERNOON. FOR WESTERN WASHINGTON ON THE WHOLE…IT WILL JUST BE A MOSTLY CLOUDY AND COOLER DAY.

THIS TREND WILL CONTINUE THURSDAY NIGHT AND FRIDAY…WITH THE FLOW ALOFT BACKING TO SOUTHWESTERLY AS A LARGER UPPER TROUGH MOVES INTO THE OFFSHORE WATERS. LOW LEVEL FLOW WILL REMAIN ONSHORE…WITH A
FAIRLY DEEP MARINE LAYER OVER WESTERN WASHINGTON. FRIDAY WILL BE ANOTHER MOSTLY CLOUDY DAY AND EVEN COOLER THAN THURSDAY. SHOWERS ARE POSSIBLE LATE FRIDAY AND FRIDAY NIGHT…ESPECIALLY AT THE COAST AND
IN THE MOUNTAINS.

THE UPPER TROUGH WILL MOVE INLAND SATURDAY…AND THIS DAY HAS THE BEST CHANCE FOR WIDESPREAD SHOWERS. THE NORTH COAST…OLYMPICS…AND NORTH CASCADES IN PARTICULAR SHOULD HAVE PRECIPITATION…WITH A CHANCE EVERYWHERE ELSE. THE LONG WAVE UPPER TROUGH AXIS WILL MOVE WELL EAST OF THE CASCADES SATURDAY NIGHT…BUT IT LOOKS INCREASINGLY LIKELY THAT SEVERAL SHORTWAVES WILL AFFECT WESTERN WASHINGTON SUNDAY THROUGH TUESDAY. THE MODELS DIFFER ON THE TIMING AND STRENGTH OF THESE FEATURES THOUGH…SO FOR NOW WE CAN REALISTICALLY ONLY FORECAST A BROAD BRUSH CHANCE OF SHOWERS FOR THAT PERIOD. THE GFS SHOWS AN ESPECIALLY COOL AND WET SOLUTION FOR TUESDAY…BUT THE EUROPEAN AND CANADIAN MODELS WAIT TILL WEDNESDAY TO BRING IN A WEAKER UPPER TROUGH.

 

References:

SPU Climate Change program