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December 2014
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Evolution of an Atmospheric River Forecast

Posted: December 16th, 2014

 

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Possible AR this weekend

 

Clouds, cool-but-still-warmer-than-normal temperatures and occasional light rain will continue this evening all the way through Friday. But the jet stream, which split up into a few branches after last week’s onslaught, is showing signs of reorganizing and migrating toward us this weekend.

 

We may get a preview of the shift late on Thursday or early on Friday when a front that otherwise would have shredded apart offshore is forecast to make its way inland and provide us with 12-or-so hours of light-to-moderate rainfall. With that door knocked down, an atmospheric river will then take aim at the west coast. When and where it will strike is highly uncertain at the moment, but a few recent models runs suggested we’d have one of our more significant events of the season on Sunday.

 


 

To get a sense of how the forecast is evolving, I’d like to post shots of successive UW GFS-WRF (column-integrated) water vapor forecasts to this blog (see bottom). I’ll also caption images with NWS discussion of said event. It should be noted that there are many more forecasting tools under consideration than what I’ve selected, especially to the highly skilled meteorologists at the NWS, but I’m hoping to provide a little insight into operational forecasting and uncertainty with this exercise. 

 


 

The latest National Weather Service rainfall forecast for Seattle calls for 0.08″ tomorrow, 0.30″ Thursday, 0.37″ Friday, 0.60″ Saturday, 1.22″ Sunday, and 0.06″ Monday. Those weekend numbers would certainly result in higher-than-normal SPU customers calls related to drainage; if forecast amounts increase a little more, then storm preparations will also soon be considered.

 

Over SPU’s mountain reservoirs, the NWS has about 1″ tomorrow through Friday, then over 7″ inches Saturday through Monday. Fortunately, some of that precipitation—if it even verifies—looks to fall in the form of much needed snow, as freezing levels are expected to bounce between 3000-5000 feet.

 

– JRH

 

NWS Area Forecast Discussion
NWS Forecast Table Interface—Seattle
NWS Forecast Table Interface—Tolt Reservoir
NWS Forecast Table Interface—Chester Morse Lake

 


 

The sequence starts at the bottom with Sunday evening’s 162-hour forecast, which at the time is about when the event was predicted to begin. I’ll update this post with subsequent UW WRF-GFS 36km imagery and NWS AFD text as the week goes along…

 

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Sunday morning: STRONG WESTERLY WINDS ALOFT WILL CONTINUE TO BRING MOISTURE TO WESTERN WA THIS AFTERNOON AND EVENING. SHOWERS ARE MAINLY ALONG THE COAST AND IN THE MOUNTAINS. SEATTLE IS IN THE RAIN SHADOW WITH LITTLE TO NO PRECIP.

 

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Saturday evening: STRONG WEST FLOW ALOFT WILL CONTINUE OVER W WA TODAY…THEN EASE A LITTLE TONIGHT. LOW LEVEL MOISTURE AND LARGE SCALE LIFT HAS DIMINISHED AS WELL…SO MUCH OF THE LOWLANDS IS CURRENTLY FREE OF RAIN. GIVEN THE STRONG W FLOW THE MAIN FORCING TODAY WILL BE OROGRAPHIC IN NATURE…WITH PRECIPITATION MAINLY OVER THE MOUNTAINS. THE STRONGEST OROGRAPHIC FLOW WILL BE FROM ABOUT SNOHOMISH COUNTY SOUTHWARD. LOW LEVEL ONSHORE FLOW IS ALSO PRESENT SO COMBINED WITH THE STRONG W FLOW ALOFT THIS MAY CREATE SOME NARROW CONVERGENCE LIKE RAIN BANDS OVER PARTS OF THE PUGET SOUND REGION. OTHER THAN THESE RAIN BANDS THE PUGET SOUND REGION WILL BE IN THE EAST RAIN SHADOW OF THE OLYMPICS SO OVERALL NOT MUCH RAIN IS EXPECTED. THE ONSHORE FLOW IS STRONG ENOUGH TO KEEP WINDS BREEZY IN MANY AREAS.

 

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Saturday morning: WARMER AIR WILL SURGE INLAND THIS AFTERNOON AND EVENING WITH SNOW LEVELS RISING TO 5000-7000 FT. SNOW WILL CHANGE OVER TO RAIN IN THE PASSES. STRONG WESTERLY FLOW WILL PREVAIL TONIGHT WITH HEAVY RAIN CONTINUING IN THE MOUNTAINS. THIS WILL LEAD TO POSSIBLE RIVER FLOODING AND A FLOOD WATCH REMAINS IN EFFECT. WHATCOM AND SKAGIT COUNTIES WERE DROPPED FROM THE WATCH AS PRECIP
AMOUNTS ARE NOT HIGH ENOUGH TO FORCE FLOODING. MODELS SHOW SEATTLE IN THE RAIN SHADOW TONIGHT DUE TO WESTERLY DOWNSLOPE FLOW OFF THE OLYMPICS…RAINFALL TOTALS WILL BE LIGHT IF ANYTHING. STRONG WESTERLY UPSLOPE FLOW WILL CONTINUE ON SUN WITH MORE HEAVY RAIN ACROSS THE WEST SLOPES OF THE MOUNTAINS…THE THREAT OF RIVER FLOODING WILL CONTINUE.

 

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Friday evening: A STRONG WARM FRONT WILL MOVE INLAND TODAY FOR MORE RAIN AND HEAVY MOUNTAIN PRECIP. HIGH WIND IS MARGINAL OVER THE NORTH INTERIOR AND WILL LET THE WARNING CONTINUE THROUGH NOON TODAY. WA DOT SHOWS SNOW IN THE PASSES THIS MORNING AND A WINTER WEATHER ADVISORY REMAINS IN EFFECT. THE SNOW LEVELS WILL RISE TO 5000-6000 FT THIS AFTERNOON AS WARMER AIR MOVES INLAND…WITH SNOW CHANGING OVER TO RAIN IN THE PASSES. STRONG PERSISTENT WESTERLY FLOW WILL PREVAIL TONIGHT WITH HEAVY RAIN CONTINUING IN THE MOUNTAINS. THIS WILL LEAD TO POSSIBLE RIVER FLOODING AND A FLOOD WATCH REMAINS IN EFFECT. MODELS SHOW SEATTLE IN THE RAIN SHADOW TONIGHT DUE TO WESTERLY DOWNSLOPE FLOW OFF THE OLYMPICS…RAINFALL TOTALS WILL BE LIGHT IF ANYTHING.

 

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Friday morning: A PLUME OF MOISTURE IS BEING TRANSPORTED EAST ALONG A STRONG 175 KT JET AIMED AT WASHINGTON…WHICH WILL LIKELY RESULT IN HEAVY PRECIP ALONG THE COAST AND IN THE MOUNTAINS … THE SNOW LEVELS WILL REMAIN HIGH SAT NIGHT THROUGH SUN WITH MAINLY RAIN IN THE MOUNTAINS. THIS MAY RESULT IN RIVER FLOODING AND A HYDROLOGIC OUTLOOK HAS BEEN ISSUED..

 

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Thursday evening: A STRONG ZONAL JETSTREAM HAS FORMED OVER THE NE PACIFIC AND WILL DRIVE A COUPLE OF STRONG WET WARM FRONTS ACROSS W WA TONIGHT THROUGH THIS WEEKEND. MODEL SOLUTIONS ARE IN FAIRLY GOOD AGREEMENT THROUGH SATURDAY…BUT CONTINUE TO EVOLVE REGARDING SOME DETAILS AND ESPECIALLY WITH REGARD TO PRECIP AMOUNTS. [much, much more said…]

 

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Thursday morning: STRONG AND MOIST WLY FLOW DRIVEN BY A STRONG 180+ KT JET WILL STEER A WARM FRONT INTO WRN WA ON SATURDAY. TIMING IS A LITTLE FASTER WITH MOST MODELS DEVELOPING WIDESPREAD RAIN ACROSS WRN WA BY LATE FRIDAY NIGHT OR EARLY SATURDAY MORNING. INITIALLY THE FLOW WILL BE OUT OF THE S/SW AND STRONG AT 40 TO 50 KT. THIS WILL PRODUCE HEFTY OROGRAPHICALLY ENHANCED RAINFALL AMOUNTS ALONG S/SW FACING SLOPES OF THE OLYMPICS AND CASCADES. THIS WILL ALMOST CERTAINLY CAUSE FLOODING ON THE SKOKOMISH RIVER BY LATER SATURDAY NIGHT. THE FLOW TURNS MORE WLY SATURDAY NIGHT AND SUNDAY WHICH IS A FAVORABLE DIRECTION FOR HEAVY RAIN IN MORE BASINS ACROSS THE W SLOPES OF THE CASCADES. THIS WILL CAUSE ADDITIONAL FLOOD CONCERNS.

 

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Wednesday evening: VERY STRONG ZONAL FLOW DEVELOPS ACROSS THE NE PACIFIC FRIDAY NIGHT AND CONTINUES THROUGH THE WEEKEND. 300 MB ISOTACHS ARE IN THE 160-170KT RANGE. A STRONG WET WARM FRONT ASSOCIATED WITH THE 130KT NOSE OF THE JET STREAK WILL MOVE ACROSS W WA ON SATURDAY. MODELS ARE IN FAIR AGREEMENT WITH THE STRENGTH AND GENERAL TIMING OF THIS FEATURE. THIS WARM FRONT SHOULD GENERATE WIND ADVISORY LEVEL WINDS OVER THE COAST AND N INTERIOR AND HEAVY RAIN OVER THE COAST AND MOUNTAINS. ALTHOUGH THERE WILL BE 2-3 INCHES OF HEAVY RAIN DURING THE 24 HOUR PERIOD FROM 12Z SATURDAY-12Z SUNDAY…SO FAR IT LOOKS LIKE PRIMARILY THE SKOKOMISH RIVER MAY BE AT RISK FOR FLOODING. SNOW LEVELS START OUT NEAR 3000 FEET ON SATURDAY…AND WILL RISE TO 5500 TO 7000 FEET BY SATURDAY NIGHT.

 

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Wednesday morning: THE PROGS ARE STILL TRENDING A BIT DRIER WITH THIS SYSTEM GIVEN ITS MORE PROGRESSIVE NATURE. EXPECT RAIN TO INCREASE ON SATURDAY WITH THE WARM FRONT AND THE COLD FRONT MOVING ONSHORE SUNDAY NIGHT.

 

Tuesday evening: THIS SYSTEM WAS CURRENTLY MOVING ACROSS 150E THIS MORNING WITH A SUPPORTING 200KT JET. THE PROGS ARE STILL TRENDING A BIT DRIER WITH THIS SYSTEM GIVEN ITS MORE PROGRESSIVE NATURE. EXPECT RAIN TO INCREASE ON SATURDAY WITH THE WARM FRONT AND THE COLD FRONT MOVING ONSHORE SUNDAY NIGHT. RIDGING ALOFT ALONG THE WEST COAST MEANS THE JET STREAM WILL BE AT OUR LATITUDE WITH A WESTERLY COMPONENT INTO THE CASCADES AND SHADOWING OFF THE OLYMPICS THIS WEEKEND. THE COAST AND MOUNTAINS SHOULD GET THE BULK OF THE PRECIPITATION.

 

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Monday evening: GLOBAL MODELS REMAIN IN GOOD AGREEMENT THAT A MORE CONSOLIDATED WLY FLOW PATTERN DEVELOPS ACROSS THE PACIFIC OVER THE WEEKEND. THE STRONG WLY JET WILL DIRECT AN ATMOSPHERIC RIVER OF MOISTURE INTO WRN WA LATER SATURDAY THROUGH EARLY NEXT WEEK. THE INITIAL WARM FRONTAL WAVE WILL BRING STRONG SW FLOW ALOFT AND HEAVY RAIN TO THE MOUNTAINS SATURDAY AFTERNOON AND NIGHT. MODELS DIFFER ON HOW LONG THE TRAILING MOISTURE PLUME REMAINS DIRECTED INTO WRN WA. HOWEVER…MOST MODELS SHOW AT LEAST 24-36 HOURS OF HEAVY MOUNTAIN RAINFALL BEFORE TAPERING OFF WHICH COULD CAUSE SOME CONCERNS FOR FLOODING ON PRONE RIVERS.

Tuesday morning

 

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Monday evening: IT APPEARED THAT A MILD AND QUITE MOIST SYSTEM WILL IMPACT THE REGION ON SUNDAY. RELATIVELY HIGH SNOW LEVELS COMBINED WITH STRONG UPSLOPE FLOW WILL RESULT IN THE POTENTIAL FOR HEAVY RAIN ON THE COAST AND IN THE MOUNTAINS ON SUNDAY. UNLIKE THE LAST SIGNIFICANT RAIN EVENT…THE FLOW WILL BE WESTERLY AND THIS WOULD HAVE MORE OF AN IMPACT ON THE CASCADES.

 

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Monday morning: FLOW ALOFT REMAINS CONSOLIDATED INTO THE WEEKEND WITH BOTH THE GFS AND THE ECMWF SHOWING A POSSIBLE ATMOSPHERIC RIVER SCENARIO SETTING UP. THE 00Z RUN MAKES MULTIPLE RUNS IN A ROW FOR THIS SOLUTION ON SUNDAY ALTHOUGH THE HEAVIER RAIN IN THE SOLUTIONS HAS BEEN UP OVER VANCOUVER ISLAND.

 

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Sunday evening: A WETTER PAC SYSTEM WILL MOVE INTO SOUTHERN B.C. ON SUNDAY…POSSIBLY CLIPPING WESTERN WA SUN NIGHT OR MON AS IT SHIFTS SOUTH.