410  
FXUS02 KWBC 250647  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
247 AM EDT WED OCT 25 2017  
 
VALID 12Z SAT OCT 28 2017 - 12Z WED NOV 01 2017  
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
MOST GUIDANCE IS SHOWING AN EVENTUAL BROADENING TREND FOR CONUS  
MEAN TROUGHING AFTER A TENDENCY TOWARD FAIRLY SHARP/AMPLIFIED  
CENTRAL-EASTERN U.S. TROUGHS BETWEEN NOW AND THE FIRST PART OF THE  
MEDIUM RANGE PERIOD. THERE IS DECENT AGREEMENT IN PRINCIPLE FOR  
THE SIGNIFICANT SYSTEM AFFECTING PORTIONS OF THE EAST FROM THE  
WEEKEND INTO EARLY NEXT WEEK, FOLLOWED BY INCREASING  
SPREAD/DECREASING CONFIDENCE FOR SUCCESSIVE UPSTREAM FEATURES.  
 
FOR THE DEEP MS VALLEY UPPER TROUGH LIFTING NORTHEAST SAT ONWARD  
AND ASSOCIATED SURFACE DEVELOPMENT, GUIDANCE HAS STABILIZED TO A  
DECENT DEGREE COMPARED TO PAST DAYS BUT THERE IS STILL SOME TIMING  
SPREAD. IN ADDITION A NUMBER OF SOLUTIONS CONTINUE INTO INDICATE  
THAT THIS SYSTEM WILL INCORPORATE NORTHWEST CARIBBEAN MOISTURE AND  
POSSIBLY LOW PRESSURE (WITH CONSIDERABLE SPREAD FOR SUCH A LOW),  
ENHANCING SURFACE DEVELOPMENT FROM THE MID ATLANTIC COAST THROUGH  
THE NORTHEAST INTO WESTERN CANADA. BASED ON YESTERDAY'S NHC/WPC  
COORDINATION EXPECT ANY TROPICAL LOW PRESSURE TO BE ON THE WEAK  
SIDE. A MODEL/ENSEMBLE MEAN BLEND LEANING SOMEWHAT MORE TOWARD  
THE OPERATIONAL RUNS (12Z-18Z GFS/12Z ECMWF) YIELDS FAIRLY GOOD  
CONTINUITY WHILE WAITING FOR POTENTIAL DETAIL/TIMING CHANGES.  
 
UPSTREAM THE GUIDANCE HAS BEEN SHOWING ANOTHER SHORTWAVE ROUNDING  
THE WESTERN RIDGE AND DROPPING INTO THE NORTHERN TIER BY EARLY IN  
THE WEEK, PUSHING A COLD FRONT SOUTHWARD OVER THE WESTERN/CENTRAL  
STATES. THERE HAS BEEN A FAIR AMOUNT OF SPREAD FOR  
TIMING/AMPLITUDE OF THE SHORTWAVE THOUGH. AMONG GUIDANCE THROUGH  
THE 12Z/18Z CYCLES THE GFS/GEFS MEMBERS LEAN TOWARD THE  
SLOWER/LESS AMPLIFIED SIDE OF THE SPREAD, WITH A FASTER ECMWF  
TREND AND 12Z UKMET/CMC RUNS SUGGESTING AT LEAST INTERMEDIATE OR  
FASTER TIMING. THE NEW 00Z GFS HAS TRENDED FASTER THAN PREVIOUS  
RUNS AND THE UKMET/CMC REMAIN PROGRESSIVE SO THE FASTER SCENARIO  
IS LOOKING MORE LIKELY.  
 
AFTER NORTHEAST PACIFIC RIDGING REBUILDS DURING THE WEEKEND INTO  
EARLY NEXT WEEK, EASTERN PACIFIC/WESTERN NORTH AMERICA DETAILS  
BECOME INCREASINGLY UNCERTAIN TOWARD MIDWEEK. THIS IS DUE TO  
LIKELY SEPARATION OF FLOW WITHIN THE BERING SEA/ALASKA INTO  
CENTRAL PACIFIC TROUGH AFTER MON WITH DIFFERENT PROPORTIONS OF  
SEPARATION LEADING TO VERY DIFFERENT FORECASTS DOWNSTREAM.  
UNCERTAINTY OVER WHETHER UPSTREAM RIDGING REMAINS OPEN OR CLOSES  
OFF A HIGH IS A FURTHER COMPLICATION. BY DAY 7 WED SOLUTIONS  
RANGE FROM A PERSISTENT NORTHEAST PACIFIC AND/OR WEST COAST RIDGE  
IN 12Z-18Z GFS/GEFS AND 12Z CMC/CMC MEAN WITH INTERIOR WEST  
TROUGHING (WHICH HAS SOME SUPPORT FROM TELECONNECTIONS RELATIVE TO  
THE NORTHEAST PACIFIC POSITIVE HEIGHT ANOMALY CENTER) TO EAST-WEST  
ELONGATED NORTHEAST PACIFIC TROUGHING AND NEARLY ZONAL PACIFIC  
NORTHWEST FLOW IN THE 12Z ECMWF. THE ECMWF MEAN OFFERS SOMEWHAT  
OF A COMPROMISE BETWEEN THE TWO EXTREMES. CONTINUITY AND  
TELECONNECTIONS SEEM TO FAVOR LEANING AWAY FROM THE 12Z ECMWF LATE  
IN THE PERIOD, HOWEVER THE 00Z GFS EXHIBITS A TREND IN THE ECMWF  
DIRECTION. THIS IS PARTICULARLY TRUE FOR THE SYSTEM WHICH THOSE  
RUNS BRING INTO SOUTHERN CANADA BY NEXT WED. BOTH THE ECMWF AND  
GFS HAVE VARIED CONSIDERABLY IN RECENT RUNS SO PREFER A  
CONSERVATIVE ENSEMBLE MEAN APPROACH LATE IN THE PERIOD.  
 
BASED ON DATA THROUGH 18Z, A BLEND OF THE 12Z-18Z GFS/12Z ECMWF  
AND 18Z GEFS/12Z ECMWF MEANS WITH SOMEWHAT MORE WEIGHT TO THE  
OPERATIONAL RUNS PROVIDED A REASONABLE STARTING POINT AND GOOD  
OVERALL CONTINUITY EARLY-MID PERIOD. FOR THE REST OF THE FORECAST  
THE BLEND PHASED OUT THE 12Z ECMWF AND PLACED INCREASING EMPHASIS  
ON THE 18Z GEFS/12Z ECMWF MEANS AS THE BEST STARTING POINT GIVEN  
THE RAPIDLY INCREASING SOLUTION ENVELOPE TOWARD MIDWEEK.  
 
   
..WEATHER/THREATS HIGHLIGHTS
 
 
STRONG DYNAMICS ALOFT, COMPLEX SURFACE EVOLUTION NEAR THE EAST  
COAST, AND POSSIBILITY OF INCORPORATION OF TROPICAL MOISTURE/LOW  
PRESSURE WILL LEAD TO A THREAT OF HEAVY RAINFALL AND STRONG WINDS,  
INCLUDING OVER SOME AREAS THAT ALSO EXPERIENCED SIGNIFICANT  
RAINFALL IN THE PRECEDING EVENT. CURRENTLY EXPECT HIGHEST TOTALS  
TO BE FOCUSED OVER PARTS OF THE MID ATLANTIC AND NORTHEAST AS WELL  
AS EARLY IN THE PERIOD OVER THE FLORIDA PENINSULA WITH THE PASSAGE  
OF NORTHWEST CARIBBEAN MOISTURE/LOW PRESSURE. THE GREAT LAKES AND  
VICINITY WILL BE UNSETTLED FOR MOST OF THE PERIOD WITH AREAS OF  
RAIN AND SNOW AND SOME LAKE ENHANCEMENT, SUPPORTED BY CHILLY  
CYCLONIC MEAN FLOW. THE FRONT DROPPING SOUTH FROM CANADA MID-LATE  
PERIOD WILL PROVIDE A GRADUALLY INCREASING FOCUS FOR PRECIP OVER  
SOME LOCATIONS FROM THE ROCKIES EASTWARD. BEHIND THE EAST COAST  
SYSTEM EXPECT HIGHS 10-20F BELOW NORMAL TO PROGRESS FROM THE  
PLAINS/MS VALLEY INTO THE EAST SAT-MON WITH A TREND CLOSER TO  
NORMAL THEREAFTER. ON THE OTHER HAND THE WEST WILL LIKELY SEE  
DECENT COVERAGE OF PLUS 10F OR GREATER ANOMALIES DURING THE  
WEEKEND, FOLLOWED BY A MODERATING TREND TOWARD NEAR NORMAL TEMPS  
BY MIDWEEK. COOLEST AIR RELATIVE TO NORMAL TUE-WED SHOULD EXTEND  
ACROSS THE NORTHERN TIER INTO THE CENTRAL PLAINS/MID MS VALLEY.  
 
RAUSCH  
 

 
 
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