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October 2014
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Where’d the Atmospheric River Go?

Posted: October 10th, 2014

 

Last week, the story was about the not-so-dirty ridge and record warm temperatures. This week, we’re left asking what happened to the forecasted rain? In each case, stronger than expected high pressure is to blame.

 

Blocking high pressure, or ridging, has been a recurring theme over Western North America for an entire year now. Recall that last October’s “fogmageddon” left us dry and eventually set up the infamous blob of warm water in the NE Pacific Ocean. While this week’s ridge is more transient in nature, it still seems as if there’s some sort of feedback loop in process. This summer was simply hot for the northwest, and now fall is having trouble gaining momentum. Many in the scientific community are looking into the relationship between warming water, jet stream blocking, ridging, and warming water again. For the time being, we should just imagine that Richard Sherman himself is covering the skies offshore. (M. Crabtree is obviously the atmospheric river.)

 

Onto the latest forecast… Citywide, the NWS currently predicts 0.21″ tonight through Saturday, followed by no rain on Sunday, and then around 0.25″ per day through Wednesday. The latest ECMWF and UW WRF-GFS runs generally agree. Over SPU’s mountain reservoirs, about 50% more rainfall is expected during the same period.

 

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

 

A WEAK SHORTWAVE ALOFT WILL FOLLOW THE FRONT ON SATURDAY…KEEPING SCATTERED SHOWERS OVER THE AREA THROUGH SATURDAY NIGHT. ONSHORE FLOW

IN THE POST FRONTAL ENVIRONMENT MAY SUPPORT A WEAK PUGET SOUND CONVERGENCE ZONE OVER THE NORTH PUGET SOUND REGION ENHANCING SHOWERS OVER SNOHOMISH AND NORTH KING COUNTIES. EXPECT THE HEAVIEST ACCUMULATIONS TO RANGE FROM 0.25 TO 0.50 INCHES…MAINLY OVER THE CASCADES NORTH OF SNOQUALMIE PASS. THE SNOW LEVEL SHOULD BE AROUND
6000 FT.

AN UPPER RIDGE WILL BEGIN TO BUILD SATURDAY NIGHT OVER THE OFFSHORE WATERS AND SHIFT INLAND SUNDAY AND SUNDAY NIGHT. A WET FRONTAL SYSTEM WILL BRING RAIN TO BRITISH COLUMBIA SUNDAY AFTERNOON…AND TO
THE COAST BEGINNING LATE IN THE AFTERNOON FOLLOWED BY THE NORTH INTERIOR AND MOUNTAINS IN THE EVENING. THE REMAINDER OF THE FORECAST AREA WILL MOST LIKELY REMAIN DRY OR NEARLY DRY THROUGH SUNDAY NIGHT. RAINFALL WILL GENERALLY REMAIN LIGHT.

THE UPPER RIDGE WILL PROGRESS FURTHER INLAND ON COLUMBUS DAY. THE UPSTREAM FRONT WILL PROBABLY BRING SOME RAIN TO THE COAST BY THE END OF THE DAY…AND RAIN SHOULD SPREAD INLAND ACROSS THE FORECAST AREA MONDAY NIGHT AND MORE OR LESS CONTINUE TUESDAY.

MODELS DISAGREE ON DETAILS BEYOND THAT…BUT THEY ALL INDICATE A CONTINUING WET PATTERN WITH THE POST-FRONTAL TROUGH AND/OR ADDITIONAL FRONTAL WAVES THROUGH THURSDAY. THE SNOW LEVEL WILL FALL
INTO THE 5000 TO 6000 FT RANGE BY TUESDAY AND HIGHER MOUNTAIN SPOTS — LIKE THE ROAD TO MT BAKER…WASHINGTON PASS…AND PARADISE AND
SUNRISE AT MT RAINIER — COULD RECEIVE THEIR FIRST SIGNIFICANT SNOWFALL OF THE SEASON. THE THREAT OF RIVER FLOODING — EVEN FOR THE EXCEPTIONALLY FLOOD-PRONE SKOKOMISH RIVER — LOOKS MINIMAL.

 

– JRH

 

References:

 

NWS Area Forecast Discussion
NWS Forecast Table Interface