924  
FXUS02 KWBC 070631  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
231 AM EDT SUN OCT 07 2018  
 
VALID 12Z WED OCT 10 2018 - 12Z SUN OCT 14 2018  
   
..CENTRAL U.S. HEAVY RAINFALL FINALLY MOVES EASTWARD
 
 
...POTENTIAL T.C. FOURTEEN LIKELY TO AFFECT THE GULF COAST AND  
PORTIONS OF THE EAST...  
...MOISTURE FROM SERGIO SHOULD REACH INTO ARIZONA/SOUTHERN  
ROCKIES...  
 
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
EXPECT AN OVERALL PATTERN TRANSITION THAT WILL PROMOTE A TREND  
FROM VERY ACTIVE WEATHER TOWARD DECREASING COVERAGE AND INTENSITY  
OF PRECIPITATION ACROSS THE LOWER 48 STATES.  
 
WHILE AN INITIALLY STRONG WESTERN ATLANTIC RIDGE ALOFT  
WEAKENS/RECEDES, A STRENGTHENING SYSTEM TRACKING FROM NEAR THE  
UPPER MS VALLEY INTO EASTERN CANADA WILL PUSH A COLD FRONT INTO  
AND ACROSS THE EAST DURING WED-FRI. THE FRONT WILL BE ACCOMPANIED  
BY AREAS OF MODERATE-HEAVY RAINFALL, AND IS LIKELY TO INTERACT  
EVENTUALLY WITH POTENTIAL TROPICAL CYCLONE FOURTEEN WHICH BASED ON  
THE 0300 UTC NHC ADVISORY SHOULD BE NEARING THE FLORIDA PANHANDLE  
BY EARLY WED. AT THE SAME TIME ENERGY WILL BE FEEDING INTO THE  
ESTABLISHED WESTERN U.S. UPPER TROUGH. AS THE UPSTREAM RIDGE  
ALOFT GRADUALLY BUILDS TOWARD THE WEST COAST/WESTERN CANADA, SOME  
OF THE WESTERN ENERGY SHOULD PROCEED EASTWARD WHILE THE REST  
LINGERS OVER/NEAR THE SOUTHWEST. IT IS POSSIBLE THAT SOME OF THIS  
LATTER ENERGY MAY INTERACT WITH THE UPPER REFLECTION OF SERGIO IS  
IT REACHES THE REGION IN A FEW DAYS. BY THE END OF THE PERIOD  
CONSENSUS EVOLUTION THAT SHOWS A WEST COAST/WESTERN CANADA MEAN  
RIDGE, A MODEST WEAKNESS NEAR THE SOUTHWEST, AND MEAN TROUGHING  
SETTLING INTO THE EAST-CENTRAL U.S. CORRESPONDS WELL TO  
TELECONNECTION-FAVORED FLOW RELATIVE TO NEGATIVE HEIGHT ANOMALIES  
THAT HAVE BEEN FORECAST JUST SOUTH OF THE ALEUTIANS/ALASKA  
PENINSULA LATE IN THE PERIOD.  
 
AS FOR FORECAST SPECIFICS, GUIDANCE HAS BEEN DISPLAYING SOME  
DAY-TO-DAY TIMING OSCILLATION FOR THE SYSTEM TRACKING FROM THE  
UPPER MIDWEST/GREAT LAKES INTO CANADA. LATEST ARRAY OF SOLUTIONS  
IS A LITTLE FASTER THAN YESTERDAY AFTER HAVING TRENDED SLOWER  
VERSUS THE PREVIOUS DAY. FARTHER SOUTHWARD MODEL/ENSEMBLE SPREAD  
IS STILL VERY BROAD FOR POTENTIAL TROPICAL CYCLONE FOURTEEN.  
RECENT CMC/GFS RUNS HAVE REPRESENTED THE FASTER SIDE OF THE SPREAD  
BUT THE 00Z GFS HAS ADJUSTED CLOSER TO THE OFFICIAL TRACK FROM THE  
0300 UTC ADVISORY. ON THE OTHER HAND ECMWF/ECMWF ENSEMBLE RUNS  
HAVE BEEN EXTREMELY SLOW AND POSSIBLY EXHIBITING BIASES THAT  
EXISTED FOR SOME PREVIOUS TROPICAL SYSTEMS DURING AT LEAST A  
PORTION OF THEIR EXISTENCE. BY LATE WEEK THE 12Z ECMWF/ECMWF MEAN  
POSITION MAY NOT BE TOO FAR OFF FROM A POSSIBLE FRONTAL WAVE  
THOUGH.  
 
BROADLY SPEAKING GUIDANCE SEEMS TO BE TRENDING TOWARD AN  
INTERMEDIATE SCENARIO OVER/NEAR THE SOUTHWEST. THE ECMWF/ECMWF  
MEAN HAVE TONED DOWN THEIR EXPECTED TROUGHING OVER THE PAST DAY  
WHILE THE GFS HAS MADE A NOTABLE ADJUSTMENT TOWARD HAVING MORE  
LINGERING ENERGY INSTEAD OF QUICKLY RAISING HEIGHTS ALOFT AS IN  
PREVIOUS RUNS. THERE IS STILL CONSIDERABLE ENSEMBLE SPREAD AND  
THE SCALE OF ENERGY IS SMALL ENOUGH TO SUGGEST PREDICTABILITY IN  
THE LOWER HALF OF THE SPECTRUM. AN ADDITIONAL UNKNOWN IS THE  
DEGREE TO WHICH THIS ENERGY MAY INTERACT WITH THE UPPER REFLECTION  
OF SERGIO AS IT ARRIVES LATE IN THE WEEK.  
 
LATEST MODELS/ENSEMBLES ARE STARTING TO BECOME A LITTLE MORE  
SIMILAR FOR THE HANDLING OF NORTH PACIFIC ENERGY THAT SHOULD ENTER  
WESTERN NORTH AMERICA BY LATE WEEK AND THEN FEED INTO THE MEAN  
TROUGH LIKELY REACHING THE CENTRAL/EAST-CENTRAL STATES NEXT  
WEEKEND. THIS ENERGY WOULD SUPPORT A CLIPPER TYPE SYSTEM THAT  
TRACKS NEAR THE UPPER GREAT LAKES BY EARLY DAY 7 SUN.  
 
THE UPDATED FORECAST STARTED WITH AN OPERATIONAL MODEL BLEND ON  
DAY 3 WED FOLLOWED BY GRADUALLY INCREASING WEIGHT OF 18Z GEFS/12Z  
ECMWF MEANS SO THAT DAY 7 SUN HAD 70 PERCENT TOTAL MEANS WITH THE  
18Z GFS/12Z ECMWF COMPRISING THE REST OF THE BLEND. THE INITIAL  
BLEND WAS ADJUSTED TO ACCOUNT FOR TROPICAL SYSTEM FORECASTS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE VIGOROUS STORM SYSTEM TRACKING NORTHEASTWARD FROM THE UPPER MS  
VALLEY WILL ANCHOR A COLD FRONT THAT WILL BE ACCOMPANIED BY AREAS  
OF MODERATE-HEAVY RAINFALL AS IT CROSSES THE EASTERN U.S.  
MEANWHILE POTENTIAL TROPICAL CYCLONE FOURTEEN IS FORECAST TO BE  
NEAR THE FLORIDA PANHANDLE EARLY WED AND THEN CONTINUE  
NORTHEASTWARD AS THE FRONT APPROACHES. THIS SYSTEM WILL PROVIDE  
ADDITIONAL MOISTURE INPUT WITH ASSOCIATED RAINFALL MOST LIKELY TO  
OCCUR OVER PARTS OF THE SOUTHEAST/MID-ATLANTIC. HOWEVER NOTE THAT  
THERE IS STILL SIGNIFICANT UNCERTAINTY FOR EXACT TRACK/TIMING OF  
POTENTIAL T.C. FOURTEEN AND THUS FOR PRECISE LOCATION/INTENSITY OF  
RAINFALL. EXPECT DRIER CONDITIONS TO MOVE INTO THE EAST LATE WEEK  
INTO THE WEEKEND. IN THE COLD SECTOR, PARTS OF THE EXTREME  
NORTHERN PLAINS MAY SEE A PERIOD OF SNOW EARLY IN THE PERIOD.  
 
THE GREAT BASIN AND NORTHERN-CENTRAL ROCKIES SHOULD TREND DRIER  
AFTER A PERIOD OF SNOW AND LOWER ELEVATION RAIN THAT WILL CROSS  
THE REGION IN THE WED-THU TIME FRAME. FARTHER SOUTHWARD THERE ARE  
STILL SOME QUESTION MARKS FOR TIMING BUT A REASONABLE CONSENSUS  
EXISTS TOWARD SERGIO'S MOISTURE PRIMARILY AFFECTING THE SOUTHEAST  
HALF OF ARIZONA AND PARTS OF THE SOUTHERN ROCKIES. THE AXIS OF  
HEAVIEST RAINFALL WOULD BE SOMEWHAT SOUTHEAST OF LOCATIONS THAT  
RECEIVED THE MOST RAIN FROM ROSA. SOME OF THE MOISTURE FROM  
SERGIO SHOULD EXTEND INTO THE PLAINS BUT THUS FAR THERE HAS BEEN  
POOR GUIDANCE AGREEMENT/CONTINUITY FOR LOCATION AND INTENSITY OF  
RAINFALL.  
 
MUCH BELOW NORMAL TEMPERATURES COVERING AREAS FROM THE INTERIOR  
WEST INTO NORTHERN-CENTRAL PLAINS WED-THU SHOULD GRADUALLY  
MODERATE AND PUSH SLOWLY SOUTHEASTWARD WITH TIME. SOME MINUS  
20-30F ANOMALIES FOR HIGHS ARE POSSIBLE EARLY IN THE PERIOD OVER  
THE NORTHERN HALF OF THE HIGH PLAINS AND VICINITY. ALSO EARLY IN  
THE PERIOD THE EASTERN U.S. WILL BE EXTREMELY WARM WITH MIN  
TEMPERATURES PARTICULARLY ANOMALOUS (20-25F ABOVE NORMAL AT SOME  
LOCATIONS). NUMEROUS LOCATIONS MAY SEE DAILY RECORDS FOR WARM  
MINS. COLD FRONTAL PASSAGE WILL BRING EASTERN U.S. TEMPERATURES  
DOWN CLOSE TO OR EVEN A LITTLE BELOW NORMAL BY NEXT WEEKEND.  
EXPECT THE WEST COAST STATES TO SEE SLIGHTLY ABOVE NORMAL READINGS  
BY NEXT WEEKEND AS UPPER RIDGING APPROACHES.  
 
RAUSCH  
 
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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