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Wet and Windy Week

Posted: October 13th, 2014

 

drain

 

A wet and windy week lies ahead. Later this afternoon, a cold front will pass through and provide moderate rainfall and gusty winds. Post-frontal showers are then expected to be plentiful and occasionally heavy late tomorrow through early Wednesday. Later in the week, high pressure will start to build back over the region, however at this time forecast models are allowing periods of rain to continue through Saturday.

 

Citywide, the NWS currently predicts 0.46″ today, followed by 0.45″ tomorrow—essentially 0.25-0.50″ each day through Saturday. The UW WRF-GFS leans a little wetter throughout the forecast period. 10-15mph southwesterly winds with gusts to 35mph are expected later tonight and again on Wednesday afternoon, which should rapidly accelerate leaf fall and begin to impact drainage inlet performance.

 

Over SPU’s mountain reservoirs, up to an inch is expected today, followed by around 0.50″ tomorrow and Wednesday—essentially 2.50-3.00″ total through Saturday. Again, the UW WRF-GRF leans a little wetter, notably tonight over the Tolt. 15-20mph southwestely winds with gusts to 40 mph are expected tonight, and then southeasterly and steady Wednesday through Friday. Snow levels will also drop to around 5000′ Wednesday.

 

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

 

AN UPPER RIDGE IS OVER THE PACIFIC NORTHWEST EARLY THIS MORNING. A FRONTAL SYSTEM CARRYING LOTS OF MOISTURE IS JUST OFFSHORE…AND A DEEP UPPER TROUGH IS WEST OF THAT CENTERED ALONG 140W. SKIES ARE GENERALLY CLOUDY ACROSS WESTERN WASHINGTON. THERE HAS BEEN LITTLE TO NO PRECIPITATION SINCE MIDNIGHT…BUT RADAR SHOWS SOME FRONTAL RAINS OVER THE OFFSHORE WATERS. TEMPERATURES WERE STILL IN THE 50S AT 2 AM.

THE UPPER RIDGE WILL MOVE EAST TO THE NORTHERN ROCKY MOUNTAINS TODAY AND THE FRONT WILL START SPREADING RAIN INTO WESTERN WASHINGTON. RAIN SHOULD BEGIN ON THE COAST BY MIDDAY…WITH HEAVY RAIN AT TIMES ON THE NORTH COAST THIS AFTERNOON. THE FRONT WILL ALSO PRODUCE WINDY CONDITIONS AT THE COAST — 20 TO 30 MPH WITH GUSTS TO 45 MPH — AND WE WILL ISSUE A LOW-END WIND ADVISORY. RAIN SHOULD REACH THE NORTH INTERIOR OF WESTERN WASHINGTON THIS AFTERNOON AND THE PUGET SOUND REGION AND SOUTH INTERIOR BY LATE AFTERNOON OR EVENING. RAIN WILL CONTINUE WITH THE COLD FRONT CROSSING THE CASCADES LATE TONIGHT.

MODELS HAVE BEEN CONSISTENT WITH REGARD TO PRECIPITATION AMOUNTS. WE CAN EXPECT A HALF INCH TO AN INCH OVER MOST OF THE INTERIOR LOWLANDS…WITH 1 TO 1.5 INCHES AT THE COAST…AND 1 TO 2 INCHES
OVER THE MOUNTAINS WITH LOCALLY 2+ INCHES OVER THE OLYMPICS AND AROUND MT BAKER. THE SNOW LEVEL WILL BE HIGH — ROUGHLY 9000 TO 10000 FT WHEN MOST OF THE PRECIPITATION FALLS.

THE COOL SHOWERY AIR MASS BEHIND THE FRONT WILL START MOVING INTO THE FORECAST AREA TUESDAY. SHOWERS ARE LIKELY TUESDAY…BUT PRECIPITATION AMOUNTS SHOULD BE RELATIVELY LIGHT. A SHORTWAVE UPPER TROUGH EXTENDING SOUTHEAST FROM THE PARENT OFFSHORE UPPER LOW WILL SWING THROUGH WESTERN WASHINGTON TUESDAY NIGHT AND WEDNESDAY. THIS FEATURE SHOULD PRODUCE ANOTHER ROUND OF HEAVIER SHOWERS…WITH A CHANCE OF THUNDERSTORMS AT LEAST ALONG THE COAST.

IN ADDITION THE SNOW LEVEL WILL FALL TO 5000 TO 6000 FT…AND WE COULD GET THE FIRST REAL SNOW OF THE SEASON AT SOME OF OUR HIGHER ELEVATION SPOTS LIKE MOUNT BAKER HIGHWAY…WASHINGTON PASS…SUNRISE…PARADISE…OR HURRICANE RIDGE.

AN UPPER RIDGE SHOULD START BUILDING OVER THE PACIFIC NORTHWEST AGAIN ON THURSDAY…AS ANOTHER DEVELOPING FRONTAL SYSTEM APPROACHES THE REGION FROM THE SOUTHWEST. IT SHOULD BRING MORE RAIN
THURSDAY NIGHT THROUGH FRIDAY NIGHT WITH A RETURN TO HIGHER SNOW LEVELS. THE STEADY RAIN SHOULD TAPER OFF TO SHOWERS ON SATURDAY AS ANOTHER RIDGE STARTS BUILDING. BASED ON THE MOST RECENT MODELS RUNS…WHICH SHOW A GOOD CHANCE OF DRY WEATHER ON SUNDAY…WE HAVE
REDUCED THE POPS INTO THE CHANCE CATEGORY.

 

– JRH

 

References:

 

NWS Area Forecast Discussion
NWS Forecast Table Interface