144  
FXUS02 KWBC 230653  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
252 AM EDT MON OCT 23 2017  
 
VALID 12Z THU OCT 26 2017 - 12Z MON OCT 30 2017  
   
..OVERVIEW/GUIDANCE EVALUATION/PREFERENCES
 
 
A GOOD CONSENSUS OF GUIDANCE CONTINUES TO SHOW AMPLIFIED EASTERN  
U.S. TROUGHING ALOFT IN THE MEAN, AS SUPPORTED BY STRONG POSITIVE  
HEIGHT ANOMALY CENTERS OFF THE BC COAST AND NORTHERN ATLANTIC PER  
MULTI-DAY MEAN CHARTS. WITHIN THIS AGREEABLE PATTERN THERE HAS  
BEEN MEANINGFUL SPREAD AND VARIABILITY AMONG INDIVIDUAL MODELS AND  
ENSEMBLE MEMBERS FOR SOME OF THE IMPORTANT DETAILS.  
 
ONE SUCH DETAIL INVOLVES THE SHORT RANGE UPPER TROUGH LIFTING INTO  
THE NORTHEAST BY THU. AS IN PAST DAYS THERE IS STILL A DECENT  
AMOUNT OF TIMING SPREAD, WHICH IMPACTS THE DURATION OF HEAVY  
RAINFALL OVER NEW ENGLAND. TRENDS OVER THE PAST 24 HOURS HAVE  
GENERALLY BEEN SOMEWHAT FASTER WHILE THE UPPER SHORTWAVE TAKES  
LONGER TO CLOSE OFF A LOW IF IT OCCURS. AN AVERAGE OF THE 12Z  
GFS/ECMWF AND 18Z GEFS/12Z ECMWF ENSEMBLE MEANS PROVIDED A  
REASONABLE STARTING POINT TO REFLECT THE TRENDS WHILE WAITING FOR  
FURTHER CONFIRMATION BEFORE ADJUSTING EVEN FASTER SUCH AS  
SUGGESTED BY THE CMC/UKMET/18Z GFS. THUS FAR NEW 00Z RUNS STILL  
FAVOR FASTER PROGRESSION.  
 
UPSTREAM THE GUIDANCE HAS DIFFERED WITH THE TIMING/ORIENTATION OF  
TROUGH ENERGY AMPLIFYING INTO THE CENTRAL-EASTERN U.S., IN  
PARTICULAR WITH SOME ECMWF RUNS TENDING TO HOLD ENERGY BACK  
FARTHER WEST THAN THE MAJORITY OF MODELS/MEANS. PRIOR 00Z CYCLE  
HAD NUDGED SOMEWHAT SLOWER VERSUS CONTINUITY ONLY TO REVERT BACK  
TO TIMING AT LEAST AS FAST OR EVEN FASTER IN THE NEWER 12Z/18Z  
CYCLE. THIS TREND CORRESPONDS TO A SLIGHT EASTWARD SHIFT OF THE  
STRONG RIDGE THAT BUILDS OFF THE WEST COAST BY THU. IN  
ASSOCIATION WITH THIS TROUGH EXPECT A FAIRLY STRONG STORM SYSTEM  
TO TRACK ACROSS THE NORTHERN TIER/UPPER GREAT LAKES AND THEN  
PROGRESS/ELONGATE INTO CANADA WHILE THE TRAILING COLD FRONT PUSHES  
INTO THE EAST FROM THE PLAINS THU-SAT. AS WITH THE PRECEDING  
TROUGH THERE IS A DECENT SIGNAL THAT THIS TROUGH MAY TAKE ON A  
NEGATIVE TILT LATE IN THE PERIOD AND SUPPORT EAST COAST/NEW  
ENGLAND INTO EASTERN CANADA WAVE DEVELOPMENT. OVERALL A  
MODEL/ENSEMBLE MEAN BLEND (SAME COMPONENTS AS FOR THE LEADING  
TROUGH), WITH MORE ENSEMBLE MEAN WEIGHT LATER IN THE FORECAST,  
REPRESENTS THE IDEAS OF LATEST GUIDANCE WELL. AHEAD OF THE  
SOUTHERN PART OF THE COLD FRONT THAT APPROACHES FLORIDA, GUIDANCE  
SEEMS TO BE NAVIGATING TOWARD A COMPROMISE BETWEEN PRIOR WET  
GFS/DRY ECMWF RUNS IN THE LATE WEEK/WEEKEND TIME FRAME.  
 
BY NEXT WEEKEND CMC/UKMET RUNS AND CMC ENSEMBLES THROUGH THE 12Z  
CYCLE HAVE MAINTAINED THEIR IDEA OF DAMPENING THE STRONG WESTERN  
RIDGE MORE AGGRESSIVELY WITH INCOMING NORTHERN PACIFIC ENERGY  
VERSUS THE GFS/ECMWF AND THEIR MEANS. THE AMPLIFIED PATTERN  
LEADING UP TO THAT POINT IN TIME HAS FAVORED THE GFS/ECMWF  
CLUSTER. THE NEW 00Z RUNS ARE FINALLY ADDING MORE SUPPORT FOR  
THAT SCENARIO WITH THE UKMET NOW MUCH CLOSER TO THE GFS/ECMWF AND  
THE CMC NUDGING ITS FORECAST A LITTLE IN THAT DIRECTION. WHAT  
SHORTWAVE ENERGY DOES ROUND THE RIDGE WILL LIKELY SUPPORT A SYSTEM  
DROPPING INTO THE NORTHERN TIER LATE IN THE PERIOD. HOWEVER THERE  
IS MINIMAL CLUSTERING OR CONTINUITY AMONG MODELS/ENSEMBLE MEMBERS  
AT THIS TIME SO CONFIDENCE IN THE SPECIFICS IS RATHER LOW.  
 
THE FORECAST IS STILL FAIRLY AMBIGUOUS REGARDING WHAT MAY HAPPEN  
WITH EASTERN PACIFIC TROUGH ENERGY REACHING NEAR 140W BY SAT, WITH  
SOLUTIONS RANGING FROM KEEPING IT ENTIRELY OFFSHORE TO HAVING AT  
LEAST SOME OF THE ENERGY REACH THE COAST. THIS FEATURE WILL BE  
NEAR OR WITHIN THE NEXT WEAKENING/REBUILDING CYCLE OF THE OVERALL  
WESTERN U.S./EASTERN PACIFIC RIDGE SO IT MAY TAKE A WHILE FOR THIS  
ASPECT OF THE FORECAST TO BE RESOLVED.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
POTENTIALLY HEAVY RAINFALL OVER NEW ENGLAND SHOULD TREND LIGHTER  
EARLY IN THE PERIOD AS DYNAMICS ALOFT/WAVY FRONT LIFT INTO CANADA.  
UPSTREAM SYSTEM TRACKING ACROSS THE UPPER MS VALLEY/UPPER GREAT  
LAKES LATE THIS WEEK MAY PRODUCE FOCUSED PRECIP AND STRONG WINDS  
IN ITS VICINITY, WITH TEMPS AT SOME LOCATIONS POTENTIALLY COLD  
ENOUGH TO PRODUCE SNOW. BEHIND THE TRAILING FRONT PORTIONS OF THE  
ROCKIES/HIGH PLAINS MAY SEE A PERIOD OF RAIN/SNOW DEPENDING ON  
ELEVATION. FROM LATE THIS WEEK ONWARD THE COLD FRONT SHOULD BE  
ACCOMPANIED BY SOME AREAS OF RAIN FROM THE MS VALLEY INTO THE  
EAST. RECENT FASTER TRENDS IN GUIDANCE ARE REDUCING THE  
PROBABILITY OF HEAVY AMOUNTS WEST OF THE APPALACHIANS BUT SOME  
LOCALIZED HEAVY ACTIVITY MAY STILL BE POSSIBLE. AS THE SUPPORTING  
UPPER TROUGH TAKES ON A NEGATIVE TILT LATE IN THE PERIOD,  
POTENTIAL REMAINS FOR ATLANTIC INFLOW TO GENERATE ENHANCED  
ACTIVITY NEAR THE EAST COAST BY SUN-MON. THERE MAY ALSO BE A  
WINDOW OF TIME IN WHICH THE FRONT COULD TAP SOME OF THE NORTHWEST  
CARIBBEAN MOISTURE THUS FURTHER INCREASING RAINFALL.  
 
UNDER THE AMPLIFYING CENTRAL-EASTERN U.S. UPPER TROUGH EXPECT  
HIGHS 10-20F BELOW NORMAL TO SPREAD FROM THE NORTHERN PLAINS THU  
INTO MUCH OF THE PLAINS/EAST-CENTRAL U.S. BY FRI-SAT WITH SOME  
POCKETS OF SUCH ANOMALIES PERSISTING OVER THE EAST INTO NEXT MON.  
WESTERN STATES WILL GENERALLY BE WARMER THAN NORMAL, BY 5-15F IN  
MOST CASES, THROUGH THE PERIOD WITH GRADUAL MODERATION OVER THE  
SOUTHWEST/CALIFORNIA.  
 
RAUSCH  
 

 
 
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