586  
FXUS02 KWBC 251543  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1143 AM EDT WED OCT 25 2017  
 
VALID 12Z SAT OCT 28 2017 - 12Z WED NOV 01 2017  
 
...POTENTIAL HEAVY RAINFALL EVENT FOR THE NORTHEASTERN U.S. SUNDAY  
AND MONDAY...  
 
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
CONFIDENCE REMAINS GENERALLY ABOVE AVERAGE WITH RESPECT TO THE  
LARGE SCALE FLOW PATTERN ACROSS THE CONUS THROUGH THE MEDIUM  
RANGE. UPPER RIDGING IS EXPECTED TO PERSIST OVER THE NORTHEAST  
PACIFIC, WITH GROWING SUPPORT FOR FURTHER STRENGTHENING/EXPANSION  
NORTH ACROSS ALASKA BY EARLY TO MID NEXT WEEK. SHORTWAVE ENERGY  
MOVING SOUTHEASTWARD ON THE EASTERN SIDE OF THE RIDGE WILL SUPPORT  
PERSISTENT UPPER TROUGHING ACROSS THE CENTRAL/EASTERN STATES. AS  
THE RIDGE EXPANDS POLEWARD EARLY NEXT WEEK, THE FLOW PATTERN  
ACROSS NORTH AMERICA WILL SUPPORT INCURSION OF POLAR/ARCTIC AIR  
MASSES SOUTHWARD ACROSS CENTRAL CANADA AND PERHAPS EVEN PORTIONS  
OF THE U.S. NORTHERN TIER, ALTHOUGH CONFIDENCE IN THE EXACT  
DETAILS IS LOW AT THIS POINT.  
 
TROUGH/UPPER LOW INITIALLY IN PLACE ON DAYS 3-4 (SAT-SUN) ACROSS  
THE MIDWEST/GREAT LAKES SHOWS SOME STRENGTH/TIMING DIFFERENCES  
AMONG THE GUIDANCE, WITH THE 00Z ECMWF/CMC ON THE SLOWER/DEEPER  
SIDE, THE 06Z GFS ON THE WEAK/FAST SIDE, AND THE 00Z UKMET IN THE  
MIDDLE. NO SIGNIFICANT TRENDS ARE NOTED AMONG THE GUIDANCE WITH  
RESPECT TO THIS SYSTEM, SO A MULTI-MODEL BLEND IS THE PREFERRED  
APPROACH (MORE WEIGHT INITIALLY PLACED ON THE ECMWF/CMC GIVEN  
BETTER RUN-TO-RUN CONTINUITY). AS ADDITIONAL SHORTWAVE ENERGY  
CROSSES THE CANADIAN PRAIRIES DAYS 4-5 (SUN-MON), GUIDANCE HAS  
TRENDED A BIT FASTER/STRONGER. THIS RESULTS IN A HIGHER DEGREE OF  
PHASING WITH THE LEADING SHORTWAVE WHICH WILL BE CROSSING THE  
NORTHEAST BY MON. THE RESULT IS AN INCREASED LIKELIHOOD FOR A  
DEEPER SURFACE CYCLONE ACROSS THE NORTHEAST ON MON, AND THIS TREND  
IS REFLECTED IN THE UPDATED WPC FORECAST PROGS. A TROPICAL  
DISTURBANCE FORECAST BY MOST GUIDANCE TO BE PULLED NORTHWARD AND  
ABSORBED INTO THE FRONTAL SYSTEM WILL PROVIDE SIGNIFICANT AMOUNTS  
OF MOISTURE AS THE CYCLONE ACROSS THE NORTHEAST DEEPENS, SETTING  
THE STAGE FOR A POTENTIAL HEAVY RAINFALL EVENT (MORE DETAILS  
BELOW).  
 
ADDITIONAL SHORTWAVE ENERGY MOVES AROUND THE PACIFIC RIDGE BY DAYS  
5-7 (TUE-WED), AND THIS IS WHERE FORECAST CONFIDENCE BEGINS TO  
DECREASE FAIRLY SUBSTANTIALLY. DESPITE GENERAL AGREEMENT ON THE  
PACIFIC RIDGE EXPANDING NORTH, MODELS DISAGREE ON THE EXACT  
CHARACTER OF THE RIDGE, WHICH IN TURN INFLUENCES THE EVOLUTION OF  
THE SHORTWAVES ON THE EASTERN SIDE. THE DEGREE THAT SHORTWAVES  
UNDERCUT THE EASTERN/SOUTHERN SIDE OF THE RIDGE IS ONE SIGNIFICANT  
QUESTION, WITH THE ECMWF PERSISTENTLY SHOWING THE GREATEST DEGREE  
OF UNDERCUTTING (AN ECMWF BIAS, BASED ON INTERNAL FORECASTER  
DISCUSSIONS THIS MORNING). AN ADDITIONAL SHORTWAVE IS SHOWN BY  
MOST GUIDANCE MOVING SOUTH INTO THE NORTHERN TIER BY TUE-WED, BUT  
MODEL SOLUTIONS ALSO RANGE WIDELY. IF HEIGHTS CAN FALL  
SUFFICIENTLY ACROSS THE NORTHERN TIER, THIS WOULD OPEN THE  
NORTHERN PLAINS/MIDWEST UP MORE TO ARCTIC FLOW (AS SHOWN BY MUCH  
OF THE 00Z GUIDANCE) AND RESULTANT COLDER TEMPERATURES. RUN-TO-RUN  
INCONSISTENCIES CAST SOME DOUBT/UNCERTAINTY ON THIS SCENARIO,  
HOWEVER, WITH PREVIOUS RUNS OF THE ECMWF/GFS/CMC SHOWING WEAKER  
HEIGHT FALLS ACROSS THE NORTHERN TIER BY EARLY NEXT WEEK, AND THIS  
SCENARIO IS NOT REFLECTED IN THE WPC FORECAST AT THIS TIME. GIVEN  
THE INCREASED UNCERTAINTY BY LATE IN THE PERIOD, THE WPC FORECAST  
WAS TRENDED HEAVILY TOWARD A BLEND OF THE 00Z ECENS/00Z NAEFS/06Z  
GEFS BY DAYS 6-7.  
 
   
..WEATHER/THREATS HIGHLIGHTS
 
 
STRONG DYNAMICS ALOFT, COMPLEX SURFACE EVOLUTION NEAR THE EAST  
COAST, AND LIKELY INCORPORATION OF TROPICAL MOISTURE WILL LEAD TO  
A THREAT OF HEAVY RAINFALL AND STRONG WINDS FOR PORTIONS OF THE  
MID-ATLANTIC/NORTHEAST, INCLUDING OVER SOME AREAS THAT ALSO  
EXPERIENCED SIGNIFICANT RAINFALL IN THE PRECEDING EVENT.  
ADDITIONAL HEAVY RAINS MAY FOCUS OVER THE FLORIDA PENINSULA EARLY  
IN THE PERIOD, WITH THE PASSAGE OF NORTHWEST CARIBBEAN  
MOISTURE/LOW PRESSURE. PERSISTENT TROUGHING WILL KEEP TEMPERATURES  
GENERALLY BELOW AVERAGE ACROSS MUCH OF THE CENTRAL/EASTERN U.S.  
THROUGH THE MEDIUM RANGE, WITH ABOVE AVERAGE TEMPS FOCUSED ACROSS  
THE WESTERN STATES DOWNSTREAM OF THE RIDGE.  
 
RYAN  
 

 
 
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