302  
FXUS02 KWBC 221600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 AM EDT SAT SEP 22 2018  
 
VALID 12Z TUE SEP 25 2018 - 12Z SAT SEP 29 2018  
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
LATEST GUIDANCE CONTINUES TO AGREE WELL WITH THE IDEA OF AN  
AMPLIFIED PATTERN CONSISTING OF A CANADA/CONTINENTAL U.S. MEAN  
TROUGH BETWEEN STRONG RIDGES OVER THE WESTERN ATLANTIC-GULF OF  
MEXICO AND NORTHEASTERN PACIFIC (EVENTUALLY EXTENDING NORTHWARD  
INTO ALASKA AND THE ARCTIC). THIS PATTERN WILL LIKELY SUPPORT A  
SERIES OF FRONTS THAT WILL DROP SOUTHWARD/SOUTHEASTWARD FROM  
CANADA AND PROMOTE MULTIPLE DAYS OF BELOW NORMAL TEMPERATURES FROM  
THE EASTERN SLOPES OF THE NORTHERN ROCKIES TO THE UPPER MIDWEST.  
 
DURING DAYS 3-5 TUE-THU A BLEND OF THE 00Z GFS/ECMWF/UKMET/CMC  
REPRESENTED CONSENSUS WELL FOR FEATURES OF INTEREST, RESOLVING  
DETAIL DIFFERENCES THAT WERE WITHIN TYPICAL ERROR FOR THE TIME  
FRAME INVOLVED. ONE FRONT EXTENDING FROM EASTERN CANADA LOW  
PRESSURE WILL PROGRESS FROM THE GREAT LAKES/CENTRAL  
PLAINS/SOUTH-CENTRAL WEST AS OF EARLY TUE TOWARD THE EAST COAST  
WHILE LOW PRESSURE TRACKING OUT OF WESTERN CANADA AND POSSIBLY  
REACHING NEAR THE NORTHERN MINNESOTA BORDER BY THU WILL ANCHOR THE  
NEXT FRONT WHICH SHOULD EXTEND INTO THE CENTRAL PLAINS/ROCKIES AND  
BACK INTO THE NORTHWEST AT THAT TIME. FARTHER EAST A WEAK WESTERN  
ATLANTIC FEATURE AT THE SURFACE AND ALOFT MAY TRACK CLOSE TO THE  
NORTH CAROLINA COAST BEFORE RECURVING TO THE NORTHEAST. GFS  
PREFERENCE LEANED TOWARD THE 00Z RUN VERSUS THE 06Z VERSION AS THE  
LATTER WAS DEEPER THAN MOST OTHER SOLUTIONS FOR THE SYSTEM NEAR  
THE CANADIAN BORDER AS OF THU AND ON THAT SAME DAY MORE AMPLIFIED  
THAN CONSENSUS FOR SHORTWAVE ENERGY CROSSING THE GULF OF ALASKA.  
 
DAYS 6-7 FRI-SAT TRANSITIONED TO A MODEL/MEAN BLEND INCORPORATING  
SOMEWHAT MORE 00Z ECMWF/ECMWF MEAN RELATIVE TO THE 00Z GFS/06Z  
GEFS MEAN, AS SOLUTIONS DIVERGED FOR SPECIFICS OF THE  
PACIFIC/ALASKA PATTERN WITH EVENTUAL EFFECTS ON FLOW ACROSS CANADA  
AND THE LOWER 48. IN PARTICULAR THE DIFFERENCES AFFECT HOW MUCH  
SHORTWAVE ELONGATION OCCURS OVER SOUTHWESTERN CANADA AND  
NORTHWESTERN U.S. OVER THE PAST DAY OPERATIONAL GFS/ECMWF RUNS  
HAVE ESSENTIALLY TRADED PLACES, THE GFS TRENDING MORE AMPLIFIED  
AND THE ECMWF TRENDING FLATTER. GEFS TRENDS REFLECT THE  
OPERATIONAL RUNS IN LESS DRAMATIC FASHION. HOWEVER IN CONTRAST TO  
THE OPERATIONAL ECMWF, ITS MEAN HAS BEEN SHOWING A GRADUAL TREND  
TOWARD SLIGHTLY LOWER HEIGHTS OVER THIS REGION OVER THE PAST  
COUPLE DAYS. TELECONNECTIONS RELATIVE TO STRONG POSITIVE HEIGHT  
ANOMALIES OVER THE GULF OF ALASKA IN THE D+5 THROUGH D+8 MULTI-DAY  
MEANS ALSO SHOW POTENTIAL FOR SOME MODEST ELONGATED UPPER  
TROUGHING OVER THE NORTHWEST. THE AFOREMENTIONED BLEND PROVIDED  
THE BEST ACCOUNT FOR EVIDENCE SUPPORTING A LITTLE SHARPENING OF  
SHORTWAVE ENERGY NEAR THE NORTHWEST BUT NOT TO THE EXTENT OF  
LATEST GFS SOLUTIONS. IT ALSO YIELDED AN INTERMEDIATE POSITION  
FOR THE UPPER LOW OFF THE CALIFORNIA COAST. FARTHER EAST EXPECT  
THE CENTRAL U.S. FRONT ON THU TO CONTINUE INTO THE WESTERN  
ATLANTIC/SOUTHEAST BY SAT WHILE THE NEXT FRONT MAY DROP INTO THE  
NORTHERN STATES. SOUTHWARD PROGRESSION OF THESE FRONTS WILL BE  
SENSITIVE TO DETAILS OF FLOW ALOFT SO THERE WILL LIKELY BE SOME  
RUN TO RUN CHANGES FOR THIS ASPECT OF THE FORECAST.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS
 
 
THE COLD FRONT PROGRESSING FROM THE CENTRAL INTO EASTERN STATES  
TUE-WED WILL LIKELY BE ACCOMPANIED BY AREAS OF HEAVY RAINFALL  
GIVEN ACCESS TO MOIST LOW LEVEL FLOW FROM THE GULF. THE WEAK  
FEATURE NEAR THE CENTRAL EAST COAST AROUND MIDWEEK COULD  
ULTIMATELY PROVIDE A SOURCE OF ADDITIONAL MOISTURE FOR  
MID-ATLANTIC/NORTHEAST RAINFALL. THE MOST COMMON SIGNAL FOR  
HIGHEST RAINFALL TOTALS EXTENDS FROM THE MID-LOWER MISSISSIPPI  
VALLEY/TENNESSEE VALLEY INTO OHIO VALLEY WITH SOME LOCALIZED  
POTENTIAL ALSO OVER THE NORTHEAST. SOME CONVECTION MAY BE STRONG  
TO SEVERE. CONSULT LATEST SPC OUTLOOKS FOR DETAILS ON SEVERE  
THREATS. STALLING OF THE FRONT OVER THE SOUTH WILL KEEP A  
LINGERING POTENTIAL FOR RAIN OVER THAT REGION, WITH ARRIVAL OF THE  
NEXT FRONT (WHICH WILL BE DRIER THAN ITS PREDECESSOR UPSTREAM)  
ALSO PROMOTING AREAS OF ENHANCED RAINFALL. MEANWHILE LOW LEVEL  
UPSLOPE FLOW MAY GENERATE SOME PRECIPITATION OVER/NEAR THE  
NORTHERN ROCKIES, ESPECIALLY TOWARD THE END OF THE PERIOD.  
 
EXPECT COOL TEMPERATURES OVER THE NORTHERN PLAINS TO TREND LOWER  
OVER THE COURSE OF THE PERIOD, WITH HIGHS 5-15F BELOW NORMAL INTO  
MIDWEEK AND THEN 10-20F BELOW NORMAL LATER IN THE WEEK, POSSIBLY  
DROPPING TO 20-30F BELOW NORMAL BY SAT. ON THE OTHER HAND THE  
EAST AND WEST/SOUTHWEST WILL LIKELY SEE ABOVE NORMAL TEMPERATURES  
WITH SOME DAY-TO-DAY VARIABILITY. GREATEST ANOMALIES SHOULD BE IN  
THE PLUS 10-20F RANGE FOR MORNING LOWS FROM THE MIDWEST/GREAT  
LAKES INTO MID-ATLANTIC/NORTHEAST EARLY IN THE PERIOD BUT SOME  
LOCATIONS OVER THE SOUTHEAST THIRD OF THE U.S. MAY CONTINUE TO SEE  
SOME LOWS AT LEAST 10F ABOVE NORMAL THU-SAT. PLUS 5-10F ANOMALIES  
SHOULD BE MORE COMMON OVER THE WEST WITH ONLY LOCALIZED WARMER  
READINGS.  
 
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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