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October 2014
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Landslide Season has Arrived

Posted: October 30th, 2014

 

USGS 2014-10-30

Approaching the red line

 

After today and tomorrow’s predicted rainfall, the likelihood of landslides in the Seattle area will increase. That’s based on a methodology developed by the USGS with the help of Seattle Public Utilities in which rainfall data prior to past slides were closely examined. Despite the fact that we have yet to experience widespread flooding or soil saturation this season, October is already much wetter than normal, so it shouldn’t be much of a surprise that landslide season is here.

 

“Historically, the end of October is the start of landslide season,” notes Jim Lee, SPU Senior Civil Engineer. In the event that Seattle residents see uncontrolled street water in a steep slope area, hey are encouraged to call SPU’s Operations Response Center. If a landslide damages your property and you have an immediate concern for your safety, leave the premises and call 9-1-1. For more information on how to protect you home from landslides, please visit the Department of Planning and Development’s Emergency Management site.

 

The forecast… Rainfall, mostly light-to-moderate, will continue throughout today and into tomorrow. Southerly flow aloft is once again expected to spare the City and Watersheds from the heaviest rainfall, but it’ll still be wet. A few hour-or-two-long periods of moderate-to-heavy rainfall are likely overnight, and then again during tomorrow’s post-frontal environment. At the moment Saturday looks to be dry; Sunday and much of next week do not.

 

The latest citywide NWS precipitation forecast indicates 0.86″ today, 0.41″ tomorrow, and 0.00″ Saturday, and 0.80″ Sunday. The UW WRF-GFS and ECMWF models are in general agreement with amounts above. Winds should remain light, and temperatures still slightly above normal.

 

Over SPU’s mountain reservoirs, 20-25mph sustained SE-lies with higher gusts will die down this morning, and around 1.50″ of rain is forecast to end midday Friday. After the front passes, snow levels will drop to under 5000′ tomorrow afternoon into Sunday.

 

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

 

RAIN RATES ARE EXPECTED TO PICK UP THIS AFTERNOON IN RESPONSE TO STRONG UPWARD FORCING. THE SOUTH FACING SLOPES OF THE OLYMPICS WILL LIKELY SEE RAIN RATES OF A 0.10 TO 0.2 INCH PER HOUR THIS AFTERNOON THROUGH TONIGHT. IN FACT…A COUPLE OF GAGES ON THE OLYMPIC RANGE RECORDED RAIN RATES OF NEAR 0.2 PER HOUR THIS MORNING BUT THIS WILL BE SHORT LIVED. NWS DOPPLER RADARS SHOWED A RELATIVE DRY SLOT MOVING INTO SOUTHWEST WA AT THIS TIME.

EXPECT LOCALLY HEAVY RAIN TO DEVELOP ON THE SOUTH FACING SLOPES OF THE CASCADES TONIGHT…CONTINUING INTO EARLY FRI MORNING. THE STEADY PRECIP WILL BECOME SHOWERY FRI AFTERNOON BEHIND THE COLD FRONT. THE UPPER TROF WILL MOVE OVER THE REGION FRI NIGHT AND IS EXPECTED TO BE EAST OF THE CWA SATURDAY EVENING. SATELLITE IMAGERY INDICATED THAT THE AIR MASS BEHIND THE FRONT WAS QUITE UNSTABLE. THUS…STARTING TO WONDER WHETHER THE FORECAST FOR FRI NIGHT AND SATURDAY IS TOO OPTIMISTIC.

CONFIDENCE NOT VERY HIGH IN THE EXTENDED PERIOD EARLY ON WITH THE MODELS LOSING THEIR CONTINUITY BOTH RUN TO RUN AND AMONG THEMSELVES ON THE 00Z RUN. THE ECMWF WAS SLOWER IN EXITING THE TROUGH ON SATURDAY…WHICH SLOWED DOWN THE APPROACH OF THE NEXT SYSTEM THAT WAS TIMED INTO THE AREA ON SUNDAY. NOW IT IS SUNDAY NIGHT INTO MONDAY. THE GFS WAS A LITTLE FASTER WITH THIS SYSTEM BUT STILL SLOWER THAN PREVIOUS MODELS. WITH THE GFS SOLUTION SHOWING THE MOST CONTINUITY AT THIS POINT…WILL STAY WITH THE LIKELY POPS ON SUNDAY. MODELS WERE IN BETTER AGREEMENT WITH A WET DAY ON MONDAY…WITH A SYSTEM MOVING THROUGH THE AREA MONDAY NIGHT/TUESDAY MORNING. BOTH THE GFS AND THE ECMWF WERE SHOWING AN ATMOSPHERIC RIVER SCENARIO SETTING UP FOR THE MIDDLE OF NEXT WEEK…WITH THE ECMWF BEING THE STRONGER AND WETTER OF THE TWO MODELS. KEPT RAIN IN THE FORECAST THROUGH AT LEAST THE MIDDLE OF NEXT WEEK.

 

– JRH

 

References:

 

USGS Seattle Landslide Monitoring
NWS Area Forecast Discussion
NWS Forecast Table Interface