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October 2014
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Lighter than Expected

Posted: October 28th, 2014

 

NASA MODIS image of ex-Hurricane Ana

NASA MODIS image of ex-Hurricane Ana yesterday

 

As expected, the remnants of Hurricane Ana passed through quietly, without causing problems overnight. SPU’s Operations Response Center received 30 drainage calls, a third of them for street ponding since midnight. SPU field crews have responded to at least 20 of those calls, one unrelated to the weather but still high priority, in the past 12 hours.

 

Last night’s This morning’s rainfall started a little later than expected and never got that intense, peaking between 10 AM and noon today. Since midnight, which is about when the event started, preliminary records show that Magnuson Park once again received the most rainfall at 0.72″. Crown Hill and Haller Lake tied for second place with 0.69″. The driest location appears to have been SODO with 0.36″.

 

Transient high pressure is building in the wake of today’s frontal passage. Light shower activity will continue to decrease tonight, and tomorrow should turn out to be a nice day. The next front will pass through on Thursday. Models indicate that the system is wet, though southwest facing slopes of the Olympics and North Cascades are once again expected to receive more rainfall that the lowlands or Central Cascades. Post-frontal shower activity is expected to decrease in time for trick-or-treating, though truly dry weather doesn’t look likely until late Saturday.

 

The latest citywide NWS forecast has 0.04″ tomorrow, 1.01″ Thursday, 0.11″ Friday, and 0.06″ Saturday, and 0.11″ Sunday. The UW WRF-GFS and ECMWF are in general agreement with the above. Winds will continue to diminish and almost disappear toward the weekend, and temperatures will remain slightly above normal. One developing impact of note: Seatac has surpassed the USGS landslide threshold; Seattle is not far behind.

 

Over SPU’s mountain reservoirs, 20-25mph sustained SE-lies are expected on Thursday morning, followed by around 1.50″ ending later that night. Snow levels should drop to under 5000′ Friday through Sunday.

 

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

 

AN UPPER RIDGE WILL PREVAIL OVER THE PACIFIC NORTHWEST ON WED. THE AIR MASS WILL BE SLIGHTLY UNSTABLE FOR A CONTINUED RISK OF SHOWERS…ESPECIALLY OVER THE MOUNTAINS. THERE WILL LIKELY BE A BRIEF DRY PERIOD LATE WED AFTERNOON THROUGH WED EVENING.

LOOK FOR A MOIST LOW PRESSURE SYSTEM TO BEGIN IMPACTING THE REGION WED NIGHT. THE ASSOCIATED OCCLUDED FRONT WILL BE ORIENTED NEARLY PARALLEL TO THE FRONT…THUS IT WILL BE SLOW TO MOVE EWD. THIS ALONG WITH PLENTY OF MOISTURE…STRONG SOUTHERLY FLOW IN THE 800 TO 700 MB LAYER…AND RISING SNOW LEVELS WILL RESULT IN THE POTENTIAL FOR HEAVY RAIN…ESPECIALLY ON THE COAST…OLYMPICS…AND NORTH CASCADES THU AND THU NIGHT. RAINFALL AMOUNTS OF 2.5 TO 4.5 INCHES ARE POSSIBLE IN THE 24-HOUR PERIOD ENDING AT 5 AM PDT FRI. THIS AMOUNT OF RAIN COULD BE ENOUGH TO DRIVE SOME RIVERS ABOVE FLOOD STAGE THU NIGHT AND/OR FRI.

THE STEADY RAIN IS EXPECTED TO BECOME SHOWERY EARLY FRI AS THE FRONT EXITS THE CWA. THERE IS A CHANCE THAT THE FRONT MIGHT BE A LITTLE SLOWER TO EXIT THE AREA…WHICH WOULD KEEP THE HEAVY RAIN THREAT GOING THROUGH AT LEAST FRI MORNING. FOR NOW…WENT WITH THE IDEA OF JUST SHOWERY WEATHER ON FRI.

A COOL UPPER TROF WILL MOVE OVER THE REGION FRI NIGHT. SATURDAY WILL BE A COOL DAY ACROSS THE CWA…WITH THE THREAT OF SHOWERS BEING CONFINED TO MAINLY THE MOUNTAINS AND SOUTHWEST THIRD OF THE CWA.

BEYOND SATURDAY…CONFIDENCE IN THE FORECAST WAS NOT HIGH DUE TO DISAGREEMENT IN THE MEDIUM RANGE SOLUTIONS. FOR THE MOST PART…THE SUNDAY THRU TUE FORECAST WAS BROAD-BRUSHED TO SHOW ABOVE CLIMO POPS. A MORE SIGNIFICANT SYSTEM MAY IMPACT THE PACIFIC NORTHWEST ON MON BUT THE MODELS HAVE NOT BEEN CONSISTENT WITH THE TIMING OF
THIS FEATURE.

 

– JRH

 

References:

 

NWS Area Forecast Discussion
NWS Forecast Table Interface