674  
FXUS02 KWBC 190701  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
300 AM EDT WED SEP 19 2018  
 
VALID 12Z SAT SEP 22 2018 - 12Z WED SEP 26 2018  
 
...HEAVY RAINFALL POTENTIAL ACROSS THE ARKLATEX REGION INTO THE  
OZARKS FOR SATURDAY THROUGH MONDAY...  
 
...CONSIDERABLE COOL DOWN EXPECTED IN THE MIDDLE OF THE COUNTRY  
NEXT WEEK...  
   
..PATTERN OVERVIEW/MODEL GUIDANCE/PREFERENCES
 
 
TO COMMENCE THE WEEKEND, A STRONG BAND OF HEIGHT FALLS WILL BE  
LIFTING UP THROUGH NEWFOUNDLAND DRIVING AN IMPRESSIVE AREA OF LOW  
PRESSURE TOWARD THE HIGHER LATITUDES. THE ASSOCIATED HEIGHT FALLS  
WILL GRAZE NEW ENGLAND WITH THE ATTENDANT BOUNDARY STALLING ACROSS  
THE MID-ATLANTIC. FARTHER SOUTH ACROSS THE GULF OF MEXICO, AN  
EASTERLY WAVE IS FORECAST TO SLIDE FROM CUBA BACK TOWARD THE GULF  
COAST WHILE LIKELY SHEARING IN TIME. OVER THE SOUTHERN PLAINS, A  
BROAD WEAKNESS IS EXPECTED TO SLOWLY DRAG EASTWARD TOWARD THE  
ARKLATEX REGION THIS WEEKEND. THE GUIDANCE SHOW FAIRLY IMPRESSIVE  
PRECIPITABLE WATER VALUES THE DISTURBANCES WILL HAVE ACCESS TO  
THUS AIDING IN THE HEAVY RAINFALL THREAT. ACROSS WESTERN NORTH  
AMERICA, MODELS ARE NOT AS CLEAR WITH MODEL SPREAD PLAGUING THE  
FORECAST. ULTIMATELY, A COMPLEX ARRAY OF SHORTWAVES EJECTING  
ACROSS THE WESTERN U.S. SHOULD GRADUALLY ESTABLISH A MORE DEFINED  
LONGWAVE TROUGH THAT PROGRESSES TOWARD THE NATION'S MID-SECTION.  
THIS WOULD LEAD TO A FAIRLY AMPLIFIED MID/UPPER PATTERN FOR NEXT  
WEEK WITH A TROUGH ACROSS THE CENTRAL U.S. WHILE A PAIR OF RIDGES  
INHABIT THE EASTERN SEABOARD AND EASTERN PACIFIC, RESPECTIVELY.  
 
WHILE THE GUIDANCE VARY WITH HOW QUICKLY THE EASTERLY WAVE WILL  
SHEAR ACROSS THE GULF OF MEXICO, ITS IMPACTS ARE PRIMARILY  
RESTRICTED TO THE OFFSHORE WATERS OF THE GULF. ONE KEY FEATURE  
WITH A GREAT DEAL OF PRECIPITATION UNCERTAINTY IS THE SLOW MOVING  
SHORTWAVE ACROSS THE SOUTHERN PLAINS. INCREASINGLY ROBUST QPF  
AMOUNTS ARE SUGGESTED BY THE PAST FEW RUNS OF THE ECMWF AS WELL AS  
THE 00Z CMC/UKMET SOLUTIONS. ONLY THE GFS AND ITS PARALLEL RUNS  
SEEM TO DOWNPLAY THIS SIGNAL. THE INGREDIENTS APPEAR TO BE IN  
PLACE WITH SLOW MOVING VORTICITY CENTERS TRAVERSING AN AXIS OF  
1.50 TO 2.00 INCH PRECIPITABLE WATER VALUES. BASED ON THE 500-MB  
COMPARISONS, RECENT RUNS OF THE GFS ARE FLATTER WITH THE TROUGH  
AND MORE PROGRESSIVE THAN THE ECMWF/UKMET FORECASTS. THIS AREA  
BEARS WATCHING DURING THE UPCOMING DAYS GIVEN THE POTENTIAL FOR  
FLOODING RAINS. ELSEWHERE, THE MENTIONED UNCERTAINTY WITH THE  
EVOLVING WESTERN U.S. TROUGH IS FURTHER COMPLICATED BY THE JUMPY  
NATURE OF THE ECMWF ENSEMBLES. MORE SPECIFICALLY, THE PAST FOUR  
RUNS OF THE ECMWF AND ITS ENSEMBLES HAVE SHOWN A MARKED QUICKER  
TREND, MOST NOTABLY IN THE 12Z/00Z CYCLES. OTHER GLOBAL MODELS  
HAVE TENDED TO HOLD THE TROUGH BACK MORE AND ULTIMATELY BEEN MORE  
STABLE. A QUICK LOOK AT THE MOST RECENT 00Z ECMWF RUN SHOWS IT HAS  
STUCK TO ITS GUNS WITH THE MORE PROGRESSIVE SOLUTION.  
 
REALLY FOUND IT DIFFICULT TO UTILIZE ANY SIGNIFICANT CONTRIBUTIONS  
FROM THE 12Z ECMWF/ECMWF ENSEMBLE MEAN. KEPT A 30 PERCENT FRACTION  
OF ITS FORECAST IN THE PICTURE THROUGHOUT TO ACCOUNT FOR ANY  
FUTURE 00Z GUIDANCE WHICH MOVED THAT WAY. WHILE THE 00Z GFS  
REMAINED ON THE WESTERN SIDE OF THE SPREAD, THE 00Z CMC DID MAKE A  
CONSIDERABLE JUMP EASTWARD ALTHOUGH THIS MODEL CAN EXHIBIT NOTABLE  
RUN-TO-RUN JUMPS. THE MEDIUM RANGE FORECAST WAS LED BY THE 18Z/12Z  
GFS, 12Z UKMET, AND 18Z GEFS MEAN BUT AGAIN DID KEEP NON-ZERO  
CONTRIBUTIONS OF THE 12Z ECMWF/ECMWF ENSEMBLE MEAN IN THE MIX.  
GIVEN THIS ASSESSMENT, THE FORECAST CONFIDENCE IS BELOW AVERAGE  
FROM DAY 6/TUESDAY ONWARD.  
 
   
..SENSIBLE WEATHER AND HAZARDS
 
 
AS MENTIONED IN THE HEADLINE, A STRIKING SIGNAL IN THE GUIDANCE IS  
EVIDENT ACROSS THE SOUTHERN PLAINS EASTWARD INTO THE OZARKS REGION  
THIS WEEKEND. WHILE IT IS DIFFICULT TO TRUST SOME OF THE MORE  
ROBUST SOLUTIONS (6 TO 10 INCH 24-HOUR AMOUNTS), THE THREAT  
REMAINS THERE. THE MANUAL QPF DOWNPLAYS THIS SCENARIO BUT COULD  
SEE FUTURE FORECASTS RAISE NUMBERS IF THE GFS JOINS THE HEAVIER  
CAMP. GRADUALLY THIS RAINFALL THREAT WILL SHIFT NORTHEASTWARD,  
PARTICULARLY FOCUSING ALONG THE STALLED BACKDOOR FRONT ACROSS THE  
CAROLINAS AND MID-ATLANTIC REGION. MOVING INTO NEXT WEEK, WHILE  
CONSIDERABLE TIMING ISSUES HAVE BEEN WELL DOCUMENTED, SOME HEAVY  
PRECIPITATION THREAT WOULD BE POSSIBLE ALONG THE ADVANCING  
BAROCLINIC ZONE.  
 
THE BIG STORY WITH TEMPERATURES WILL BE THE CONSIDERABLE COOL DOWN  
ACROSS THE NORTH-CENTRAL U.S. EVENTUALLY INTO THE MIDDLE OF THE  
COUNTRY NEXT WEEK. BY DAY 6/7, SEPTEMBER 25/26, FORECAST ANOMALIES  
WILL BE AROUND 15 TO 20 DEGREES BELOW CLIMATOLOGY OVER THE  
NORTHERN ROCKIES INTO THE NORTHERN/CENTRAL PLAINS. IF THIS WERE TO  
VERIFY, HIGHS WOULD ONLY BE IN THE LOW TO MID 50S THESE DAYS  
ALLOWING FOR A TRUE FEEL OF AUTUMN.  
 
RUBIN-OSTER  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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