326  
FXUS02 KWBC 220649  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
249 AM EDT SUN OCT 22 2017  
 
VALID 12Z WED OCT 25 2017 - 12Z SUN OCT 29 2017  
   
..OVERVIEW/GUIDANCE EVALUATION/PREFERENCES
 
 
GUIDANCE PERSISTS WITH THE AGREEABLE IDEA OF AMPLIFIED TROUGHING  
OVER THE LOWER 48 BETWEEN STRONG RIDGES OVER THE EASTERN  
PACIFIC/WEST COAST AND WEST-CENTRAL ATLANTIC. TWO UNCERTAINTIES  
WITHIN THIS PATTERN WILL INVOLVE A LEADING EASTERN TROUGH LIFTING  
NORTHEASTWARD MID-LATE WEEK, FOLLOWED BY UPSTREAM TROUGH ENERGY  
AMPLIFYING INTO THE CENTRAL U.S. AND ASSOC SURFACE EVOLUTION.  
THEN BY LATER IN THE PERIOD FURTHER UNCERTAINTIES ORIGINATING OVER  
THE PACIFIC HAVE THE POTENTIAL TO DISRUPT MEDIUM-SCALE DETAILS  
EVEN IF THE RIDGE-TROUGH-RIDGE LONGWAVE PATTERN ENDS UP PREVAILING  
FROM THE MULTI-DAY MEAN PERSPECTIVE.  
 
THERE IS REASONABLE AGREEMENT THAT THE INITIAL EASTERN TROUGH WILL  
TAKE ON A NEGATIVE TILT AS IT PROGRESSES NORTHEASTWARD, POSSIBLY  
CONTAINING AN EMBEDDED CLOSED LOW FOR A TIME, LEADING TO POTENTIAL  
SURFACE WAVE DEVELOPMENT THAT WOULD AFFECT NEW ENGLAND PRIMARILY  
WED-THU. THE MOST SIGNIFICANT DIFFERENCES WITH THIS FEATURE  
CURRENTLY INVOLVE TIMING AS THE 12Z ECMWF AND RECENT CMC RUNS  
STRAY FASTER THAN THE MAJORITY CLUSTER CONSISTING OF THE GFS/UKMET  
RUNS AND GEFS/ECMWF MEANS. THE 12Z ECMWF IS QUESTIONABLE ALOFT AS  
IT PULLS OFF ENERGY FROM THE SOUTHERN PART OF THE TROUGH TO FORM A  
WESTERN ATLANTIC SYSTEM THAT ENDS UP WHERE ALL OTHER GUIDANCE HAS  
HIGH PRESSURE BY FRI. IN ADDITION TO REPRESENTING CONSENSUS, THE  
GFS CLUSTER MAINTAINS REASONABLE CONTINUITY FROM PRIOR CYCLE.  
 
ADJUSTMENTS IN THE 12Z ECMWF FROM THE MORE EXTREME 00Z/21 RUN HAVE  
YIELDED SOMEWHAT LESS SPREAD IN GUIDANCE FOR THE TRAILING CENTRAL  
U.S. TROUGH AMPLIFICATION. CURRENTLY THERE IS A DECENT SIGNAL FOR  
THE CORE OF THE TROUGH TO TRACK OVER THE NORTHERN PLAINS/UPPER MS  
VALLEY REGION, WITH SOUTHWESTERN CANADA LOW PRESSURE GENERALLY  
EXPECTED TO TRACK OVER OR NEAR A NORTHERN TIER-UPPER GREAT LAKES  
PATH WHILE THE TRAILING COLD FRONT DROPS THROUGH THE WEST/PLAINS  
AND EVENTUALLY PUSHES INTO THE EAST. WITH THE 18Z GFS (AS WELL AS  
NEW 00Z RUN) LEANING ON THE SOUTHERN SIDE OF THE SPREAD FOR THE  
SURFACE LOW, AN AVERAGE OF THE 12Z GFS/ECMWF AND 18Z GEFS/ECMWF  
MEANS PROVIDE A GOOD INTERMEDIATE SOLUTION MID-LATE WEEK.  
 
WELL AHEAD OF THE FRONT THERE CONTINUES TO BE A LOT OF SPREAD  
REGARDING POTENTIAL MOISTURE FEED OR EVEN TROPICAL ACTIVITY (IN  
THE CASE OF THE CMC RUNS AND 00Z UKMET) EMANATING FROM THE  
NORTHWEST CARIBBEAN AND AFFECTING PARTS OF FLORIDA. THE ENVELOPE  
REMAINS FAIRLY SIMILAR TO 24 HOURS AGO WITH GFS RUNS SHOWING THE  
MOST PRONOUNCED INCREASE OF MOISTURE OVER FL WHILE ECMWF-BASED  
GUIDANCE THE LEAST. GEFS MEANS ARE STILL BETWEEN THE TWO  
EXTREMES. SOME DEGREE OF COMPROMISE LOOKS BEST GIVEN THAT  
EVENTUAL APPROACH OF THE SURFACE FRONT MAY HELP TO PULL UP  
SOMEWHAT MORE MOISTURE THAN FORECAST BY ECMWF RUNS.  
 
OVER THE EASTERN PACIFIC/WEST COAST, GFS/ECMWF RUNS AND THEIR  
MEANS HAVE STABILIZED OR SLIGHTLY FURTHERED PRIOR TRENDS TOWARD A  
VERY STRONG EAST PACIFIC RIDGE BUILDING BY THU-FRI AND DRIFTING  
TOWARD THE WEST COAST, DRIFTING INLAND TO ABOUT 120W BY THE END OF  
THE PERIOD. THIS IS A REASONABLE SCENARIO TO BE CONSIDERED AS  
MOST LIKELY, HOWEVER CMC RUNS/CMC ENSEMBLES ALONG WITH THE UKMET  
SUGGEST POTENTIAL FOR ANOTHER DAMPENING/REBUILDING CYCLE FOR THE  
OVERALL RIDGE IN RESPONSE TO THE NEXT NORTH PACIFIC SHORTWAVE.  
THE LESS LIKELY CMC/UKMET SCENARIO WOULD LEAD TO A SIGNIFICANT  
RESHUFFLING OF SHORTWAVE DETAILS OVER THE LOWER 48 BY NEXT  
WEEKEND. ALSO A FAIR AMOUNT OF SPREAD DEVELOPS FOR AN EASTERN  
PACIFIC TROUGH WHICH THE MEANS BRING TO ABOUT 140W BY DAY 7 SUN.  
SOME INDIVIDUAL ENSEMBLE MEMBERS BRING THE FEATURE CLOSER TO THE  
WEST COAST. SUCH TIMING MAY BE A LITTLE TOO PROGRESSIVE GIVEN THE  
STRENGTH OF THE WEST COAST RIDGE.  
 
IN ORDER TO ACCOMMODATE CONSENSUS/CONTINUITY OR INTERMEDIATE  
SOLUTION DEPENDING ON THE REGION, THE UPDATED FORECAST  
INCORPORATED COMPONENTS OF THE 12Z GFS/ECMWF, 18Z GEFS/12Z ECMWF  
MEAN, AND 12Z UKMET FOR DAYS 3-5 WED-FRI. DUE TO ISSUES OVER THE  
WESTERN ATLANTIC 12Z ECMWF WEIGHT WAS MINIMIZED FRI ONWARD WHILE  
ENSEMBLE MEANS GAINED MAJORITY WEIGHT AFTER EARLY FRI TO DOWNPLAY  
OTHER OPERATIONAL MODEL DETAIL UNCERTAINTIES.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
THE INITIAL SYSTEM AFFECTING THE EAST SHOULD BRING THE HEAVIEST  
RAINFALL POTENTIAL TO NEW ENGLAND AROUND WED-THU AS A SLOW MOVING  
WAVY FRONT ENHANCES ATLANTIC INFLOW OVER THE REGION. THERE MAY BE  
SOME LIGHTER PRECIP LINGERING OVER THE GREAT LAKES AND VICINITY ON  
WED. THE NEXT SYSTEM DROPPING INTO THE CENTRAL U.S. WILL GENERATE  
ONE AREA OF RAIN/SNOW EXTENDING SOUTHEAST ACROSS THE ROCKIES  
MID-LATE WEEK, WITH FURTHER AREAS OF PRECIP NEAR NORTHERN TIER LOW  
PRESSURE AND COLD FRONT EMERGING FROM THE PLAINS AND PUSHING INTO  
THE EAST. TEMPS OVER THE EXTREME NORTH MAY BE COLD ENOUGH FOR  
SOME PRECIP TO FALL IN THE FORM OF SNOW. DEPENDING ON EXACT  
DETAILS/TIMING OF THE OVERALL SYSTEM THERE MAY BE POCKETS OF  
MODERATE/HEAVY RAINFALL ALONG THE COLD FRONT OVER SOME LOCATIONS  
FROM THE MS VALLEY EASTWARD FRI-SUN. FARTHER SOUTHEAST THERE IS  
STILL BELOW AVERAGE CONFIDENCE REGARDING HOW MUCH MOISTURE MAY  
REACH FLORIDA LATE IN THE PERIOD.  
 
VERY WARM TEMPS OF AT LEAST 10-15F ABOVE NORMAL OVER THE  
SOUTHWEST/CALIFORNIA AND CONDITIONS THAT MAY FAVOR WILDFIRE RISK  
SHOULD MODERATE AFTER MIDWEEK. AN AREA OF HIGH TEMPS 10-20F ABOVE  
NORMAL OVER THE INTERIOR WEST/ROCKIES INTO THE NORTHERN-CENTRAL  
PLAINS ON WED WILL BE QUICKLY REPLACED BY NEGATIVE ANOMALIES OF  
SIMILAR MAGNITUDE WHICH WILL SPREAD FROM THE HIGH PLAINS INTO THE  
EAST-CENTRAL U.S.. EXCEPT FOR NEW ENGLAND WHERE THERE MAY BE SOME  
RECORD WARM LOWS ON WED, MUCH OF THE EAST WILL BE ON THE CHILLY  
SIDE TO START THE PERIOD WITH A WARMING TREND LATE IN THE WEEK.  
 
RAUSCH  
 

 
 
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