159  
FXUS02 KWBC 160627  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
127 AM EST TUE JAN 16 2018  
 
VALID 12Z FRI JAN 19 2018 - 12Z TUE JAN 23 2018  
 
...ACTIVE PATTERN FOR MUCH OF THE WEST WHILE HEAVY SNOW LOOMS FOR  
THE UPPER MIDWEST/GREAT LAKES BY SUNDAY/MONDAY...  
 
   
..PATTERN OVERVIEW
 
 
THE MID/UPPER-LEVEL PATTERN WILL FEATURE A SPLIT FLOW REGIME WITH  
A SOUTHERN STREAM SHORTWAVE TRACKING FROM EXTREME SOUTH TX OUT  
TOWARD THE NORTHERN GULF OF MX OVER THE WEEKEND. LARGE-SCALE  
AMPLIFICATION BEGINS TO TAKE SHAPE WITHIN THE MID-LATITUDES AS  
SHARPENING HEIGHT FALLS DIG INTO THE FOUR CORNERS REGION ON  
SATURDAY. ADDITIONAL STRENGTHENING IS EXPECTED AS THE SYSTEM  
ATTAINS A NEGATIVE TILT OVER THE MO RIVER VALLEY ON SUNDAY  
EVENING. CONSEQUENTLY...A SIGNIFICANT WAVE OF LOW PRESSURE SWINGS  
UP TOWARD THE UPPER GREAT LAKES ON MONDAY WITH A TRACK INTO  
SOUTHERN QUEBEC BY THE FOLLOWING DAY. MEANWHILE...AS THE INITIAL  
SYSTEM MOVES FROM THE WEST COAST TOWARD THE SOUTHERN HIGH PLAINS  
THIS WEEKEND...ADDITIONAL UPPER TROUGHING IS LIKELY TO MOVE INTO  
THE PACIFIC NORTHWEST/NORTHERN CA BY SUNDAY WITH EASTWARD  
PROGRESSION THEREAFTER. BY DAY 7/JANUARY 23...A PAIR OF TROUGHS  
SHOULD INHABIT THE COUNTRY...ONE ACROSS THE WESTERN/CENTRAL STATES  
WHILE THE OTHER EXITS NEW ENGLAND.  
 
   
..GUIDANCE ASSESSMENT/UNCERTAINTIES/PREFERENCES
 
 
TO BEGIN THE PERIOD ON FRIDAY...LOW-AMPLITUDE SHORTWAVES WILL BE  
IN THE PROCESS OF EXITING LOWER NEW ENGLAND WHICH GIVES WAY TO  
MARKED HEIGHT RISES ACROSS THE EASTERN U.S. THIS WEEKEND. WITHIN  
THE SOUTHERN STREAM...THERE HAVE BEEN TIMING DIFFERENCES EJECTING  
A COMPACT CLOSED LOW FROM SOUTH TX EASTWARD INTO THE ADJACENT GULF  
WATERS. THE GFS/ECMWF HAVE GENERALLY REMAINED ON THE SLOWER SIDE  
OF THE ENVELOPE WITH THE EXCEPTION BEING THE 12Z GFS WHICH APPEARS  
TO BE AN OUTLIER. CONSIDERING THE 570-DM ENSEMBLE SPAGHETTI  
PLOTS...A MAJORITY OF THE 12Z ECMWF ENSEMBLE MEMBERS SUPPORT THIS  
SLOWER PROGRESSION. EVENTUALLY THIS SYSTEM SHOULD FIZZLE OUT WHILE  
MOVING INTO THE NORTHEASTERN GULF OF MX LATER IN THE WEEKEND.  
 
ON THE HEELS OF THIS FEATURE...LARGE-SCALE HEIGHT FALLS TAKE SHAPE  
ACROSS THE SOUTHWESTERN U.S. WITH EVENTUALLY EJECTION INTO THE  
SOUTHERN HIGH PLAINS BY SUNDAY MORNING. MULTI-DAY ENSEMBLE  
SPAGHETTI TRENDS SHOW ONLY MINIMAL CHANGES WITH AGREEMENT IN  
DEVELOPING A STRONG AREA OF LOW PRESSURE INTO THE MIDDLE MS VALLEY  
BY 22/0000Z. A SUB-990 MB CYCLONE SHOULD BE CROSSING INTO THE  
UPPER GREAT LAKES ON MONDAY MORNING WITH THE 12Z ECMWF ENSEMBLE  
MEAN BEING THE STRONGEST AMONG THE ENSEMBLE SOLUTIONS. EVENTUALLY  
A COLD FRONTAL PASSAGE WILL OCCUR ALONG THE I-95 CORRIDOR BY LATE  
MONDAY WITH LOWER HEIGHTS LINGERING ACROSS THE NEW ENGLAND INTO  
THE END OF THE FORECAST PERIOD.  
 
WHILE THIS LATTER SYSTEM IS OF HIGHER CONFIDENCE...THE TRAILING  
EVOLUTION OF THE PATTERN IS LESS CERTAIN. THE GFS/CMC SOLUTIONS  
APPEAR TO BE ON ONE SIDE WHICH FAVORS A FLATTER/QUICKER  
PROGRESSION FROM THE PACIFIC NORTHWEST TOWARD THE NORTH-CENTRAL  
U.S. ON THE OTHER END...THE 12Z ECMWF AND A MAJORITY OF ITS  
ENSEMBLE MEMBERS FAVOR A SOLUTION THAT DIGS A TROUGH BACK INTO THE  
SOUTHWESTERN U.S. WHICH APPEARS TO BE ITS THEME THE PAST COUPLE OF  
RUNS.  
 
GIVEN SOLID MODEL AGREEMENT BETWEEN THE 18Z GFS/12Z  
ECMWF...FAVORED AN EQUAL BLEND OF THESE SOLUTIONS THROUGH DAY  
4/SATURDAY BEFORE INCORPORATING SOME ENSEMBLE INFLUENCES FROM DAY  
5/SUNDAY ONWARD. DECIDED TO FAVOR THE 12Z ECMWF ENSEMBLE MEAN MORE  
STRONGLY DEEPER INTO THE PERIOD AS IT WAS THE ONLY ENSEMBLE  
SOLUTION WITH A DEEP GREAT LAKES LOW. THE 18Z/12Z GEFS MEAN WERE  
SEVERAL MB WEAKER WHICH IS NOT REFLECTIVE OF THE STRONG NATURE THE  
OPERATIONAL GUIDANCE HAS BEEN SHOWING.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
FOR THE MOST PART...ABOVE AVERAGE TEMPERATURES SHOULD DOMINATE A  
MAJORITY OF THE FORECAST AS STRONG WARM ADVECTION ENSUES IN  
ADVANCE OF THE DEEP UPPER TROUGH. ON FRIDAY...READINGS  
APPROXIMATELY 15 TO 20 DEGREES ABOVE CLIMATOLOGY SHOULD BE  
COMMONPLACE FROM THE SOUTHERN/CENTRAL HIGH PLAINS UP INTO THE  
UPPER MIDWEST. SUCH DEPARTURES FROM AVERAGE WILL GRADUALLY SLIDE  
EASTWARD REACHING THE EASTERN U.S. ON SUNDAY/MONDAY. A FAIRLY  
SHARP TEMPERATURE GRADIENT WILL SET UP ALONG THE BOUNDARY WITH  
HIGHS ON SUNDAY IN THE 50S ACROSS CENTRAL IL/IN WHILE TEENS LOOM  
BACK ACROSS THE UPPER MIDWEST. ACROSS THE WESTERN U.S...OVERALL  
TEMPERATURES SHOULD BE SLIGHTLY BELOW NORMAL FOR MUCH OF THE  
PERIOD GIVEN ACTIVE PRECIPITATION.  
 
REGARDING PRECIPITATION...SOME CONVECTION COULD HUG THE TX COAST  
ON FRIDAY IN ADVANCE OF THE SOUTHERN STREAM CLOSED LOW. ACROSS THE  
WESTERN U.S...WIDESPREAD LOWER ELEVATION RAIN/MOUNTAIN SNOW IS  
LIKELY THROUGH SATURDAY WITH A MUCH HEAVIER BATCH OF RAIN/SNOW  
HITTING NORTHERN CA UP INTO THE PACIFIC NORTHWEST ON  
SUNDAY/MONDAY. SOME OF THE GUIDANCE HAS 24-HOUR AMOUNTS  
APPROACHING THE 2 TO 3 INCH RANGE AS AN ATMOSPHERIC RIVER EVENT  
SETS UP. CONSIDERING THE IMPACTS FROM THE DEEP UPPER TROUGH  
LIFTING TOWARD THE UPPER MIDWEST...HEAVY SNOW WILL INITIALLY BE  
POSSIBLE OVER THE CENTRAL ROCKIES. AS THE SURFACE LOW BEGINS TO  
WIND UP ACROSS THE MIDDLE MS VALLEY/UPPER GREAT LAKES...THE  
POTENTIAL FOR MODERATE/HEAVY SNOW IS LIKELY ANYWHERE FROM THE  
CENTRAL PLAINS INTO THE UPPER GREAT LAKES. EVEN SOME BLIZZARD  
CONDITIONS MAY BE A POSSIBILITY IF THE SURFACE LOW DEEPENS AS MUCH  
AS SUGGESTED. FARTHER SOUTH ALONG THE FRONT...LIGHT/MODERATE  
RAINFALL SHOULD SPREAD ALONG AND AHEAD OF THE BOUNDARY.  
 
RUBIN-OSTER  
 

 
 
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