660  
FXUS02 KWBC 220637  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
137 AM EST MON JAN 22 2018  
 
VALID 12Z THU JAN 25 2018 - 12Z MON JAN 29 2018  
   
..OVERVIEW
 
 
PROGRESSIVE FLOW ACROSS THE CONUS WILL GRADUALLY BECOME MORE  
AMPLIFIED BY EARLY NEXT WEEK. A BLOCKING UPPER HIGH EXPECTED TO  
ESTABLISH ITSELF FROM THE NORTH CENTRAL PACIFIC TO THE BERING SEA  
WILL PROMOTE DOWNSTREAM TROUGHING ACROSS THE GULF OF AK, AND A  
GRADUALLY AMPLIFYING FLOW ACROSS THE CONUS, INCLUDING A WESTERN  
U.S. RIDGE AND CENTRAL/EASTERN U.S TROUGH BY EARLY NEXT WEEK. AN  
UPPER TROUGH WILL MOVE ONSHORE IN THE WEST ON DAY 3 AND SHOULD  
QUICKLY AMPLIFY AS IT CROSSES THE ROCKIES AND REACHES THE CENTRAL  
U.S., WHERE SURFACE CYCLOGENESIS IS EXPECTED TO OCCUR. THE SURFACE  
LOW WILL LIKELY DEEPEN QUICKLY AS IT MOVES NORTHEASTWARD, SIMILAR  
TO TO THE SYSTEM CROSSING THE CENTRAL U.S. THIS EVENING, ALTHOUGH  
FARTHER NORTH. UPPER TROUGH IS EXPECTED TO REACH THE EASTERN U.S.  
BY NEXT WEEKEND, WITH AN ADDITIONAL WAVE OF LOW PRESSURE EXPECTED  
TO DEVELOP SOMEWHERE NEAR THE MID-ATLANTIC REGION.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES
 
 
MAIN POINT OF CONTENTION AMONG THE GUIDANCE IN THE MEDIUM RANGE IS  
THE FORWARD SPEED OF THE UPPER TROUGH CROSSING THE CONUS. THE GFS  
HAS CONSISTENTLY BEEN ON THE FAST SIDE OF THE GUIDANCE SPREAD,  
WHILE THE ECMWF HAS ALSO TRENDED A BIT QUICKER AS WELL. THE  
ECENS/GEFS MEANS ARE BOTH SUGGESTIVE OF A SLOWER TROUGH  
PROGRESSION, AND ARE ACTUALLY QUITE SIMILAR TO EACH OTHER. THESE  
DIFFERENCES TRANSLATE TO THE SURFACE AS WELL, IN TERMS OF TIMING  
OF THE RATHER SHARP COLD FRONT MOVING FROM THE CENTRAL TO THE  
EASTERN U.S. BY THE WEEKEND. OPTED TO MAKE A SOMEWHAT MODEST  
ADJUSTMENT TOWARD A QUICKER SOLUTION IN THIS FORECAST CYCLE (GIVEN  
THE DETERMINISTIC TRENDS), BUT MAINTAINED A FORECAST SOMEWHERE  
BETWEEN THE QUICKER DETERMINISTIC SOLUTIONS AND THE SLOWER  
ENSEMBLE MEANS. TO ARRIVE AT THIS SOLUTION, AN INITIAL BLEND  
INCLUDING THE 12Z ECMWF/UKMET/18Z GFS EARLY IN THE PERIOD (DAY 3)  
WAS QUICKLY TRENDED TOWARD HEAVIER ECMWF AND GEFS/ECENS WEIGHTING  
FROM DAYS 4-5 ONWARD.  
 
MODELS ARE IN BETTER AGREEMENT RELATIVE TO LAST NIGHT ON BUILDING  
A STRONGER RIDGE ACROSS THE WEST BY THE WEEKEND. NEXT SIGNIFICANT  
SHORTWAVE/FRONTAL SYSTEM WILL ATTEMPT TO APPROACH THE NORTHWEST BY  
SUN-MON, BUT WILL STRUGGLE AS IT MOVES INTO THE RIDGE. MODEL  
AGREEMENT WITH RESPECT TO THIS SHORTWAVE AND THE ASSOCIATED  
SURFACE LOW WAS BETTER THAN AVERAGE GIVEN THE TIME SCALE, AND THE  
AFOREMENTIONED ECMWF/ECENS/GEFS BLEND BY DAYS 5-7 SHOULD HANDLE  
THIS WELL.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
WHILE THE FRONTAL SYSTEM CROSSING THE ROCKIES ON THU WILL  
INITIALLY BE RELATIVELY MOISTURE STARVED (PRODUCING MAINLY  
ELEVATION SNOWS IN THE WEST), THE SYSTEM WILL EVENTUALLY BE ABLE  
TO TAP INTO DEEP GULF OF MEXICO MOISTURE, PRODUCING WIDESPREAD  
RAINS FROM THE LOWER MS VALLEY TO THE NORTHEAST FROM FRI NIGHT  
THROUGH THE WEEKEND. MODEL SOLUTIONS VARY WITH RESPECT TO PRECIP  
AMOUNTS GIVEN VARYING SOLUTIONS FOR MOISTURE RETURN AND SYSTEM  
TIMING. AREAS ALONG THE IMMEDIATE GULF COAST APPEAR THE MOST  
LIKELY TO RECEIVE POTENTIALLY HEAVY RAINFALL AT THIS TIME, GIVEN  
THE LIKELIHOOD FOR CONVECTION AND A WEAK WEST-EAST ORIENTED  
SURFACE BOUNDARY POTENTIALLY LINGERING OVERHEAD, PROVIDING FOCUS  
FOR HEAVY RAINFALL. GIVEN THE IMPRESSIVE MOISTURE/INSTABILITY  
ADVECTION SHOWN BY SOME GUIDANCE AHEAD OF THE SYSTEM, HOWEVER,  
LOCALLY HEAVY RAINFALL WILL BE POSSIBLE ACROSS MUCH OF THE  
SOUTHEAST, TN/OH VALLEYS, MID-ATLANTIC, AND NORTHEAST.  
TEMPERATURES ARE EXPECTED TO BE WELL ABOVE AVERAGE ACROSS THE  
CENTRAL AND EASTERN U.S. AHEAD OF THE COLD FRONT, WITH HIGHS AT  
LEAST 10-2O DEG F ABOVE AVERAGE ACROSS A WIDE AREA.  
 
MEANWHILE THE WEST COAST, PARTICULARLY THE NORTHWEST, WILL REMAIN  
UNSETTLED THROUGH THE PERIOD. THE AMPLIFYING UPPER TROUGH ACROSS  
THE WEST ON THU ALONG WITH ONSHORE FLOW WILL PRODUCE RAIN AND  
MOUNTAIN SNOWS FROM CENTRAL CA NORTH ALONG THE COAST, UNTIL MUCH  
OF THIS ACTIVITY IS CUT OFF BY THE BUILDING UPPER RIDGE. MORE  
SIGNIFICANT PRECIPITATION WILL BE POSSIBLE ACROSS THE PAC NW WITH  
APPROACH OF THE NEXT SYSTEM BY EARLY NEXT WEEK, WITH HEAVY RAIN  
AND MOUNTAIN SNOWS.  
 
RYAN  
 

 
 
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