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October 2014
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One Down, More to Go

Posted: October 22nd, 2014

 

2 PM Update: The latest 14-hour storm tally gives Magnuson Park the lead with 1.15″ total, followed by Maple Leaf at 1.09″, then Woodland Park with 0.97″. (For reference, 1.56″ in 12 hours would be a 2-year event.) SPU also received 27 more customer calls between 11 AM and 2 PM, most for ponding in streets; none for flooding or backups. Also as of 2 PM, the National Weather Service reports that we are within 0.05″ of setting a new daily rainfall record. Those additional hundredths of an inch are certain to fall before midnight.

 

Seattle RainWatch Image showing over an inch in green.

Seattle RainWatch image showing over an inch of rainfall in green (selected rain gauges in red).

 

 

The first in a series of storms is exiting the region without causing too many problems for urban drainage. SPU has received 20 calls to its Operations Response Center since midnight, 4 of which were for roadway ponding, and 1 for a residential sewer backup. SPU maintenance crews have been monitoring the weather and have responded in high priority to at least 3 of those customer calls during the past 12 hours.

 

Storm totals cannot be tallied until the rainfall has completely ended, but preliminary data show that over 0.50″ fell over a 12 hour period this morning. As of this post, SPU’s Woodland Park rain gauge is leading the pack at 0.67″, 0.01″ ahead of Maple Leaf and Admiral weather stations. SPU monitors maximum rainfall intensity in real-time using a sophisticated, soon to be public interface.

 

While occasionally heavy showers will continue into this evening, the back edge of the system, clearly visible on the radar, has crossed the Olympics and is marching in evergreen waves across the lowlands.

 

Air in the wake of this storm will be unstable tomorrow, meaning thunderstorms and brief downpours will be possible. Once that regime passes, the next storm will approach from off California. The latest guidance suggests that Friday and early Saturday will be mostly dry, and that a deep but weakening low pressure system will develop and move toward Vancouver Island. Since models suggest weakening, the threat of a windstorm late Saturday into Sunday is diminishing. That said, the storm is still expected to provide a reprise of the same atmospheric river were experiencing today, so rainfall forecasts will be monitored closely.

 

The latest citywide NWS forecast has 0.56″ this afternoon, 0.44″ tomorrow, 0.33″ Fri, 0.66″ Sat, and 0.14″ Sun. The UW WRF-GFS has 0.57″ this afternoon, 0.42″ tomorrow, and 0.10″ on Friday. The ECMWF, which by the way seems to have outperformed other models for the second, maybe third storm of this season, continues to be drier overall.

 

Over SPU’s mountain reservoirs, the NWS currently predicts about 50% more precipitation and periods of gusty E-SE-ly winds late Friday and SW-lies late Saturday.

 

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

 

SATELLITE IMAGERY AND RADARS SHOW THE FRONT THAT BROUGHT LOCALLY HEAVY RAIN AND WINDY CONDITIONS TO THE AREA DRAPED FROM THE NORTHERN CASCADES INTO NORTHWEST OREGON THIS MORNING. THE FRONT IS MOVING OFF TO THE SE. WIND ALONG THE COAST AND THROUGH THE NORTHWEST INTERIOR DROPPED OFF RAPIDLY BEHIND THE FRONT…SO ALL OF THE WIND ADVISORIES AND WARNINGS HAVE BEEN DROPPED. THE NATURE OF THE PRECIPITATION HAS BECOME SHOWERY BEHIND THE FRONT…AND RADAR AND LIGHTNING DATA SHOW THUNDERSTORMS OVER THE COASTAL WATERS. AS A RESULT…A CHANCE OF THUNDERSTORMS WAS ADDED TO THE COAST AND COASTAL WATERS FOR TODAY.

THE 12Z NAM12 AND GFS SOLUTIONS SHOW THE COOL AND UNSTABLE AIR MASS OFFSHORE GRADUALLY MOVING INLAND TONIGHT INTO THURSDAY. GFS BUFR SOUNDINGS SHOW CAPE VALUES RISING TO ABOUT 500 J/KG OVER THE INTERIOR ON THU AND GFS MOS SHOWS THE CONDITIONAL PROBABILITY OF TSTMS RISING TO OVER 30 PERCENT ACROSS THE AREA ON THU. SO A CHANCE OF TSTMS WAS ADDED TO THE OLYMPIC PENINSULA FOR TONIGHT AND THE ENTIRE AREA ON THU AS DISTURBANCES IN THE COOL AIR ROTATE INTO THE AREA FROM THE SW.

SHOWERS DIMINISH THU NIGHT AND COME TO AN END FRI MORNING AS A DEEP LOW DEVELOPS OFF THE NORTHERN CALIFORNIA OFFSHORE WATERS. THIS LOW APPEARS TO BE DEVELOPING IN RESPONSE TO A SYSTEM DROPPING SE FROM THE BERING SEA INTERACTS WITH THE REMAINS OF ANA NW OF HAWAII. THE LATEST NAM12 AND GFS SOLUTIONS SHOW THE LOW REACHING ITS MAXIMUM DEPTH AT AROUND 985-987 MB OFF THE CENTRAL OREGON COAST…WITH A VERTICALLY STACKED MATURE LOW IN ITS DECAYING STAGE AS IT MOVES NE ACROSS WESTERN WASHINGTON SAT NIGHT. OFFSHORE FLOW AND SHORT WAVE RIDGING IN ADVANCE OF THIS LOW WILL LIKELY GIVE A DRY PERIOD FRI AFTN THROUGH EARLY SAT WITH RAIN MOVING IN LATER SAT AND SAT NIGHT. WHILE IT APPEARS THAT SUNDAY WILL BE WINDY…THE THREAT OF A WIND STORM OVER WESTERN WASHINGTON APPEARS TO BE RELATIVELY LOW AT THIS TIME DUE TO THE DECAYING NATURE OF THE STORM AND THE WIND FIELD SPREADING OUT FROM ITS CENTER.

THE AIR MASS WILL REMAIN COOL AND UNSTABLE ON SUNDAY AS AN UPPER LEVEL TROUGH MOVES INLAND. A RIDGE WILL THEN PASS OVERHEAD ON MON FOR A BREAK IN THE WEATHER…WILL LIKELY SEE POPS DECREASED OVER THIS PERIOD. THEN MORE RAIN AND BLUSTERY WEATHER BY TUE AS ANOTHER WET PAC FRONTAL SYSTEM MOVES IN.

 

– JRH

 

References:

 

NWS Area Forecast Discussion
NWS Forecast Table Interface