905  
FXUS02 KWNH 260635  
PREEPD  
 
PRELIMINARY EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
235 AM EDT TUE JUL 26 2016  
 
VALID 12Z FRI JUL 29 2016 - 12Z TUE AUG 02 2016  
   
..OVERVIEW
 
 
INITIAL NERN PAC TROUGH ENERGY WILL AMPLIFY AS IT HEADS TOWARD WRN  
CANADA AND THE PAC NW DURING FRI-SUN. THE CORE OF THIS TROUGH  
SHOULD THEN CONTINUE ACROSS SRN CANADA THROUGH TUE...HELPING TO  
PUSH A COLD FRONT INTO THE NRN PLAINS. SOME DEGREE OF RESIDUAL  
TROUGHING SHOULD PERSIST ALONG THE WEST COAST. AHEAD OF THIS  
TROUGH EXPECT THE RIDGE OVER THE GREAT BASIN/SOUTHWEST AS OF EARLY  
FRI TO MOVE STEADILY EWD INTO THE PLAINS. DOWNSTREAM A TROUGH  
ALIGNED OVER THE MIDWEST WILL PROGRESS INTO THE NORTHEAST.  
 
   
..GUIDANCE EVALUATION/UNCERTAINTY ASSESSMENT
 
 
ACROSS THE ERN PAC AND WRN NOAM A MODEL/ENSEMBLE MEAN BLEND LOOKS  
REASONABLE INTO DAY 5 SUN WITH SOME LINGERING DETAIL OR TIMING  
DIFFS AS TROUGHING AMPLIFIES INTO THE WEST COAST. IN SOME  
RESPECTS GUIDANCE ACTUALLY LOOKS A LITTLE MORE SIMILAR BY EARLY  
SUN THAN IN THE TWO DAYS LEADING UP TO THAT TIME. AFTER EARLY SUN  
GUIDANCE BEGINS DIVERGING IN RESPONSE TO ISSUES THAT DEVELOP WITH  
UPSTREAM BERING SEA/ALASKA/NERN PAC FLOW. TO VARYING DEGREES  
THERE IS DECENT AGREEMENT THAT THIS UPSTREAM FLOW SHOULD BE MORE  
PROGRESSIVE THAN DEPICTED IN 12Z/18Z GFS AND GEFS MEAN RUNS...  
RESULTING IN MORE SRN CANADA PROGRESSION OF THE LEADING SYSTEM  
THAN DEPICTED BY THE 18Z GFS/GEFS MEAN IN PARTICULAR. THE  
GFS/GEFS SCENARIO ACROSS SRN CANADA AND NRN TIER CONUS LATE IN THE  
PERIOD REPRESENTS A SLOWER TREND FROM 24 HRS AGO WHEN THOSE SOLNS  
WERE MORE SIMILAR TO OTHER GUIDANCE. BASED ON MODEL/ENSEMBLE  
SPREAD THROUGH THE 12Z/18Z CYCLE AND DIFFS IN NEW 00Z SOLNS THUS  
FAR IT APPEARS THE BERING SEA/NERN PAC WILL BE A SIGNIFICANT  
SOURCE OF UNCERTAINTY FOR THE WEST COAST FCST IN COMING DAYS.  
 
ACROSS THE ERN HALF OF THE COUNTRY THE PERIOD STARTS WITH  
UNUSUALLY LOW CONFIDENCE FOR A DAY 3 FCST AS SUBTLE DIFFS WITHIN  
THE E-CNTRL U.S. TROUGH LEAD TO SIGNIFICANT SPREAD AT THE SFC.  
THESE DIFFS ARISE IN THE LATTER HALF OF THE SHORT RANGE PERIOD.  
IT APPEARS THERE IS SOME MID LVL SUPPORT FOR A LEADING WAVE NEAR  
THE EAST COAST IN RESPONSE TO SHORT RANGE ENERGY LIFTING OUT OF  
THE LWR MS VLY AND CAPTURED BY FLOW AHEAD OF THE MEAN TROUGH.  
UPSTREAM ENERGY ORIGINATING FROM THE NORTHWEST MAY SUPPORT ANOTHER  
WAVE FARTHER WWD. AT THE VERY LEAST PREFER TO LEAN AWAY FROM  
SPECIFICS OF RECENT ECMWF RUNS. IN SPITE OF RELATIVE CONSISTENCY  
OF THE PAST THREE RUNS THROUGH 12Z/25... DEVELOPMENT SFC/ALOFT  
ACROSS THE GRTLKS LOOKS OVERDONE. HOWEVER ECMWF MEAN RUNS HAVE  
BEEN CONSISTENT IN DEPICTING A MORE MODEST WAVE CROSSING THE LWR  
GRTLKS. IN THE INTEREST OF MAINTAINING CONTINUITY WHILE WAITING  
FOR STRONGER CONFIRMATION OF A CONFIDENT ALTERNATE SCENARIO...  
PREFER TO MAINTAIN A FCST THAT LEANS CLOSEST TO THE ECMWF MEAN FOR  
THE TRAILING WAVE. A MORE GENERAL COMPROMISE AMONG GUIDANCE  
APPEARS REASONABLE FOR THE LEADING EAST COAST WAVE DURING THE  
FIRST HALF OF THE PERIOD. CONSULT THE MODEL DIAGNOSTIC DISCUSSION  
PMDHMD FOR INFO/PREFS REGARDING NEW 00Z GUIDANCE UP TO 12Z FRI.  
 
IN ORDER TO MESH THE DIFFERING PREFS OVER EACH AREA OF INTEREST...  
THE FCST BLEND INCLUDED HIGHEST WEIGHTING TOWARD THE 12Z ECMWF  
MEAN WITH SMALLER 12Z ECMWF AND 18Z GFS/GEFS MEAN COMPONENTS  
THROUGH DAY 5 SUN. LATE PERIOD PREFS FAVORED ADJUSTING THE BLEND  
TOWARD A 50/30/20 ECMWF MEAN/NAEFS MEAN/ECMWF SOLN.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS
 
 
UNCERTAINTY IN SPECIFICS OVER THE EAST FROM THE START OF THE FCST  
PERIOD ONWARD CONTINUES TO CAUSE DIFFICULTY IN DETERMINING  
COVERAGE/INTENSITY OF RNFL OVER SOME AREAS... ESPECIALLY OVER THE  
NRN THIRD OF THE EAST. THERE APPEARS TO BE SOMEWHAT BETTER  
CONSISTENCY IN SIGNALS FOR POTENTIAL OF LOCALLY HEAVIER ACTIVITY  
FROM THE LWR MS VLY/CNTRL GULF COAST IN ASSOC WITH A MID LVL  
WEAKNESS AND EXTENDING NEWD TOWARD THE WAVY FRONTAL BNDRY  
AFFECTING AREAS FARTHER NWD. IMPULSES CARRIED WITHIN NWLY FLOW  
ALOFT MAY SUPPORT ONE OR MORE ORGANIZED AREAS OF CONVECTION OVER  
THE PLAINS DURING THE FIRST HALF OF THE PERIOD. COLD FRONT  
REACHING THE NRN PLAINS EARLY NEXT WEEK COULD SUPPORT A PERIOD OF  
SHWRS/TSTMS. MEANWHILE EXPECT DIURNAL CONVECTION OVER THE  
SOUTHWEST/FOUR CORNERS STATES TO INCREASE IN COVERAGE/INTENSITY AS  
THE INITIAL WRN U.S. RIDGE WEAKENS/MOVES EWD.  
 
THE WEST WILL BE ON THE HOT SIDE FRI WITH FAIRLY BROAD COVERAGE OF  
HIGHS 10-15F ABOVE NORMAL. A FEW DAILY RECORDS FOR HIGHS OR WARM  
LOWS MAY BE PSBL. ARRIVAL OF THE WEST COAST TROUGH ALOFT WILL  
BRING A RAPID COOLING TREND WITH MOST HIGHS A FEW DEGS ON EITHER  
SIDE OF NORMAL SUN-TUE. SOME OF THE WRN HEAT WILL SPREAD INTO THE  
NRN PLAINS AHEAD OF THE FRONT REACHING THE AREA BY MON-TUE.  
EXPECT THE CAROLINAS AND GA/NRN FL TO BE PERSISTENTLY WARM DURING  
THE PERIOD WITH TEMPS AVERAGING SEVERAL DEGS ABOVE NORMAL. ISOLD  
RECORD WARM LOWS ARE PSBL ON ONE OR MORE DAYS.  
 
RAUSCH  
 

 
 
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