282  
FXUS02 KWBC 250532  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
131 AM EDT TUE SEP 25 2018  
 
VALID 12Z FRI SEP 28 2018 - 12Z TUE OCT 02 2018  
 
...EARLY-SEASON COLD AIR EXPECTED FROM THE NORTHERN ROCKIES TO THE  
UPPER MIDWEST LATER THIS WEEK...  
 
   
..PATTERN OVERVIEW
 
 
EXPECT AN AMPLIFIED UPPER-LEVEL FLOW PATTERN TO PERSIST ACROSS  
NORTH AMERICA DURING THE MEDIUM RANGE PERIOD. AN  
ANOMALOUS/BLOCKING UPPER RIDGE WILL SHIFT NORTHWARD FROM THE NORTH  
PACIFIC ACROSS ALASKA LATE THIS WEEK INTO THE WEEKEND, WITH  
500-HPA HEIGHTS REACHING +4 STANDARD DEVIATIONS ACROSS INTERIOR  
ALASKA BY THE WEEKEND. THE RIDGE ALONG WITH DOWNSTREAM PERSISTENT  
TROUGHING JUST WEST OF HUDSON BAY WILL PROMOTE SOUTHWARD TRANSPORT  
OF POLAR AIR THROUGH CENTRAL CANADA AND INTO THE NORTH CENTRAL  
U.S. MEANWHILE, UPPER RIDGING INITIALLY CENTERED OFF THE SOUTHEAST  
WILL EXPAND WESTWARD ACROSS THE SOUTHERN U.S. AND GULF OF MEXICO.  
THE COMBINATION OF THESE FEATURES WILL FAVOR THE SETUP OF A  
PERSISTENT AND RELATIVELY STRONG BAROCLINIC ZONE ACROSS THE  
CENTRAL PLAINS BY LATE THIS WEEK. FARTHER WEST OVER THE PACIFIC,  
UPPER-LEVEL ENERGY WILL BEGIN TO UNDERCUT THE STRENGTHENING ALASKA  
RIDGE, WITH A NUMBER OF MODEL SOLUTIONS SUGGESTING THE POTENTIAL  
FOR RATHER STRONG HEIGHT FALLS OFF/ALONG THE PACIFIC NORTHWEST  
COAST BY EARLY NEXT WEEK. SPECIFICS OF THIS UNDERCUTTING FLOW AND  
WESTERN CANADA FLOW DOWNSTREAM FROM THE RIDGE AMPLIFYING INTO  
ALASKA WILL PLAY A ROLE IN DETERMINING THE TIMING OF AN UPPER LOW  
THAT SHOULD BE APPROACHING THE WEST COAST AS OF EARLY SAT. ONE  
ADDITIONAL FEATURE OF NOTE DURING THE MEDIUM RANGE WILL BE THE  
EXTRATROPICAL TRANSITION OF SUPER TYPHOON TRAMI ACROSS EASTERN  
ASIA, WHICH SHOULD PROMOTE AMPLIFICATION OF (OR AT LEAST  
MAINTENANCE OF ALREADY AMPLIFIED) FLOW DOWNSTREAM ACROSS THE NORTH  
PACIFIC AND NORTH AMERICA LATE IN THE MEDIUM RANGE AND BEYOND.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
DETERMINISTIC MODEL SOLUTIONS SHOWED RELATIVELY GOOD AGREEMENT FOR  
DAYS 3-5 (FRI-SUN), AND A ECMWF/GFS/CMC BLEND WAS USED AS A  
STARTING POINT FOR THE WPC FORECAST FOR THAT TIME PERIOD. MODEL  
SOLUTIONS SHOWED SOME DIFFERENCES WITH RESPECT TO FRONTAL POSITION  
BY SAT-SUN AS A COLD FRONT SWEEPS ACROSS THE EASTERN U.S. AND  
BECOMES STATIONARY ACROSS THE CENTRAL PLAINS, AND A CONSENSUS  
SOLUTIONS IS REPRESENTED WELL BY THE DESCRIBED BLEND. SOLUTIONS  
HAVE ALSO CONVERGED REASONABLY WELL WITH RESPECT TO THE UPPER LOW  
APPROACHING THE WEST COAST IN THIS TIME FRAME. AFTER DAY 5,  
FORECAST CONFIDENCE BEGINS TO DECREASE RATHER QUICKLY,  
PARTICULARLY ACROSS THE WESTERN U.S. AS ADDITIONAL UPPER-LEVEL  
ENERGY UNDERCUTS THE ALASKA RIDGE AND APPROACHES THE WEST COAST BY  
SUN-MON, MODEL SOLUTIONS BEGIN TO DIVERGE SIGNIFICANTLY WITH  
RESPECT TO THE DEGREE OF AMPLIFICATION THAT OCCURS, AND WHETHER A  
CUTOFF UPPER LOW WILL LINGER/DEEPEN JUST OFFSHORE (PER THE GFS),  
OR OPEN INTO A BROADER WAVE AND MOVE INLAND MORE QUICKLY (PER THE  
ECMWF). GIVEN THE AMPLIFIED FLOW ALREADY IN PLACE, AND THE PATTERN  
FAVORING CONTINUED AMPLIFICATION, BELIEVE THAT THE ECMWF SOLUTION  
BY EARLY NEXT WEEK IS TOO QUICK TO MOVE STRONG HEIGHT FALLS INTO  
THE INTERIOR WESTERN U.S. WHETHER A SOLUTION AS AMPLIFIED/CUTOFF  
AS THE GFS VERIFIES REMAINS TO BE SEEN, BUT NONETHELESS, TRENDED  
HEAVILY TOWARD ENSEMBLE MEANS BY SUN-MON AND AWAY FROM OPERATIONAL  
SOLUTIONS (ESPECIALLY THE ECMWF). AMPLIFICATION TO THE DEGREE  
SHOWN BY THE GFS WOULD LIKELY STEER A POTENTIAL TROPICAL SYSTEM  
NORTHWARD TOWARD CENTRAL OR NORTHERN PORTIONS OF THE BAJA  
PENINSULA (WHILE A SOLUTION MORE LIKE THE ECMWF WOULD KEEP THE  
SYSTEM ON A WESTWARD TRACK WELL OUT OVER THE OPEN WATER OF THE  
PACIFIC).  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
WELL-BELOW AVERAGE TEMPERATURES WILL PREVAIL ACROSS AREAS FROM THE  
NORTHERN ROCKIES TO THE UPPER MIDWEST THROUGH MUCH OF THE MEDIUM  
RANGE AS COLD POLAR AIR IS TRANSPORTED SOUTHWARD INTO THE NORTH  
CENTRAL U.S. HIGH TEMPERATURES AS COLD AS 20-25 DEG F BELOW  
AVERAGE WILL BE POSSIBLE. A COUPLE WAVES OF LOW PRESSURE ALONG THE  
STRONG CENTRAL U.S. FRONTAL BOUNDARY WILL RESULT IN POTENTIALLY  
WIDESPREAD PRECIPITATION FOR PORTIONS OF THE CENTRAL/NORTHERN  
PLAINS AND MIDWEST. THE MOST COHERENT SIGNAL FOR POTENTIALLY HEAVY  
RAINFALL IN THE GUIDANCE IS EARLY NEXT WEEK FROM THE CENTRAL  
PLAINS TO THE GREAT LAKES. SUFFICIENT COLD AIR IN PLACE NORTH OF  
THE FRONTAL BOUNDARY MAY SUPPORT PRECIPITATION FALLING AS SNOW FOR  
SOME AREAS FROM THE NORTHERN ROCKIES TO THE NORTHERN HIGH PLAINS.  
EXPECT SHOWERS AND THUNDERSTORMS WITH LOCALLY HEAVY RAINFALL  
ACROSS THE GULF COAST AND SOUTHEAST THROUGH MUCH OF THE PERIOD AS  
A MOIST AIR MASS REMAINS IN PLACE AND A FRONTAL BOUNDARY FAILS TO  
CLEAR THE REGION. DEPENDING ON THE EVENTUAL TRACK OF A POTENTIAL  
EAST PACIFIC TROPICAL SYSTEM, MOISTURE MAY SPREAD INTO PORTIONS OF  
THE SOUTHWESTERN U.S. BY EARLY NEXT WEEK. HEAVY RAIN IS POSSIBLE  
IF THE SYSTEM TAKES A MORE NORTHERLY TRACK, ALTHOUGH CONFIDENCE IN  
THE SPECIFICS IS LOW AT THIS TIME.  
 
RYAN  
 
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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