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October 2014
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Myriad Mysteries

Posted: October 23rd, 2014

 

48-hour rainfall according to RainWatch

48-hour rainfall according to RainWatch

 

While the rain that fell over the past 36 hours was never that intense, SPU’s drainage system performed pretty well, thanks in part field crews who worked hard through yesterday’s rush hour. Between 10 AM yesterday and 10 AM today, there were 63 drainage calls from customers, none of which represented emergencies.

 

The wettest area of the City during the past day-and-a-half ended up being around Magnuson Park, which had a maximum 24-hour total of 1.78″. The next wettest spots, according to preliminary SPU data, were Admiral (1.56″) and Woodland Park (1.51″). The driest location (1.26″) appears to have been just on the Ballard side of the locks, which if the data are accurate, may have benefited from Magnolia-induced rain shadowing—one of Seattle’s myriad mysterious microclimates!

 

At 1.26″, Seattle (Seatac) ended up breaking yesterday’s daily rainfall record. That’s good for both the 10th wettest October day on record, and the 10th time this year we’ve broken a daily rainfall record. The latter point has got to represent some kind of statistical significance and be relevant to climate change—SPU plans to answer that question soon.

 

Showery weather will gradually decrease overnight, and tomorrow will thankfully be mostly dry. The next round of showers, associated with a warm front, are expected to roll through late Friday/early Saturday. Depending on the track of the then approaching storm to which the aforementioned front was attached, midday Saturday has a decent chance at being dry; however by Saturday evening at the latest widespread rain should return.

 

But the most concerning issue for the weekend will be winds. Again, the forecast is tricky but developing pressure fields could send strong easterly winds down through Cascade gaps overnight Friday into Saturday morning, after which point they should shift to southerly then westerly before dying down Sunday evening. Beyond Sunday, mostly nuisance level shower and wind activity is expected to continue.

 

Today’s 3 PM citywide NWS forecast predicts 0.29″ tomorrow, 0.82″ Sat, 0.23″ Sun, and 0.20″ on Mon. The UW WRF-GFS has 0.19″ tomorrow, 0.65″ on Saturday, and 0.26″  on Sunday. The ECMWF still continues to be drier overall. The NWS also has sustained winds over 20mph for the 12-hour period ending at 5 AM on Sunday.

 

Over SPU’s mountain reservoirs, the NWS currently predicts similar precipitation and wind levels to the lowlands, save for Saturday evening which should be significantly wetter and Friday night which has a chance at being significantly windier—the National Weather Service just issued a Special Weather Statement for the Cascades indicating gusts to 45mph.

 

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

 

A STRONG WSW 140-150 KT JET AT 300 MB AND AN ASSOCIATED UPPER LEVEL TROUGH IS MOVING IN ACROSS NW OREGON AND SW WASHINGTON TODAY. THESE FEATURES HAVE GIVEN SHOWERS AND THUNDERSTORMS TO THE AREA THIS AFTERNOON…AND A FEW OF THE THUNDERSTORMS  HAVE BEEN SHOWING WEAK ROTATION ON OUR RADARS. ONE LIKELY SEVERE STORM WAS LOCATED BY RADAR EARLIER THIS AFTERNOON IN NORTHERN GRAYS HARBOR COUNTY. SHOWERS WILL GRADUALLY DIMINISH TONIGHT AND COME TO AN END EARLY FRIDAY MORNING AS THE UPPER TROUGH MOVES OFF TO THE NORTHEAST OF THE AREA AND SHORT WAVE RIDGING AND OFFSHORE FLOW DEVELOPS DOWNSTREAM OF A DEVELOPING LOW NOW SEEN NEAR 38N 140W.

THE LOW NOW STARTING TO SHOW ITSELF WELL OFF THE NORTHERN CALIFORNIA COAST IS DEVELOPING IN RESPONSE TO A COLD TROUGH THAT DROPPED DOWN OUT OF THE BERING SEA AS IT PHASED WITH WARMTH AND MOISTURE STREAMING EASTWARD AHEAD OF A SUBTROPICAL FEATURE OUT OVER THE WEST CENTRAL PACIFIC. 18Z AND 12Z MODELS ARE A LITTLE WEAKER THAN
SOLUTIONS FROM LAST NIGHT…BUT THEY REMAIN CONSISTENT IN SHOWING THE LOW BOTTOMING OUT AT AROUND 990 MB OFF THE OREGON COAST THEN MOVING INLAND OVER SW WASHINGTON THEN INTO THE SEATTLE AREA SATURDAY EVENING. THE LOW IS EXPECTED TO BE WEAKENING AS IT MOVES INLAND.

AHEAD OF THE LOW…OFFSHORE FLOW WILL BRING A DRY PERIOD ON FRIDAY. THEN FRIDAY NIGHT AS LOW LEVEL FLOW REMAINS OFFSHORE…A FRONTAL INVERSION ALOFT LOWERS…AND FLOW BECOMES SOUTHERLY AROUND 700 MB…WINDY CONDITIONS FROM AN EASTERLY DIRECTION ARE EXPECTED TO DEVELOP FOR A FEW HOURS ALONG THE WEST SLOPES OF THE CASCADES. THIS IS BEST SHOWN BY THE NAM12.

WIND SPEEDS WITH AND BEHIND THE LOW LATE SATURDAY AFTERNOON AND SATURDAY NIGHT WILL REALLY DEPEND ON HOW DEEP THE LOW GETS…HOW QUICKLY IT FILLS AS IT MOVES INLAND…AND ITS WIND FIELD STRUCTURE AS IT MOVES INLAND. THE NAM12 AND ECMWF SHOW A SAGGY TROUGH TO THE SOUTH OF THE LOW THAT WOULD INHIBIT A PUNCH OF STRONG PRESSURE RISES BEHIND THE LOW. AT THIS POINT…FORECASTS INDICATE SOUTH TO SOUTHWEST WINDS 20 TO 30 MPH WITH GUSTS TO 45 MPH BEHIND THE LOW. A SPECIAL WEATHER STATEMENT WAS ISSUED JUST AS A HEADS UP. IF LATER SOLUTIONS BECOME DEEPER WITH THE LOW AGAIN…A HIGH WIND WATCH MAY BE NEEDED FOR PORTIONS OF THE FORECAST AREA FOR SATURDAY AFTERNOON AND SATURDAY NIGHT. POST FRONTAL SHOWERS IN ONSHORE FLOW WILL CONTINUE ON SUNDAY AND WILL SLOWLY DECREASE SUNDAY NIGHT.

AFTER A BRIEF LULL IN PRECIPITATION ON MONDAY…ANOTHER LOW WILL MOVE INLAND OVER VANCOUVER ISLAND ON TUESDAY BRINGING WET AND POSSIBLY BREEZY WEATHER TO THE AREA. WET AND LOCALLY BREEZY CONDITIONS IS EXPECTED TO CONTINUE WEDNESDAY AND THURSDAY AS YET MORE FEATURES MOVE INTO THE AREA.

 

– JRH

 

References:

 

NWS Area Forecast Discussion
NWS Forecast Table Interface