687  
FXUS02 KWBC 251543  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1142 AM EDT TUE SEP 25 2018  
 
VALID 12Z FRI SEP 28 2018 - 12Z TUE OCT 02 2018  
 
...CHILLY TEMPERATURES 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 WESTERN/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 CANADIAN 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. NEWLY  
FORMED TROPICAL STORM ROSA IN THE EAST PACIFIC IS FORECAST TO MOVE  
NORTHWARD OVER THE NEXT FIVE DAYS PER THE LATEST NHC TRACK WITH  
THE POTENTIAL TO BE DRAWN NORTHWARD/NORTHEASTWARD AHEAD OF THE  
WESTERN TROUGH. HOWEVER, THE ENSEMBLES ARE SPLIT BETWEEN LIFTING  
TOWARD BAJA AND THE GULF OF CALIFORNIA VS REMAINING JUST FAR  
ENOUGH SOUTH TO LINGER IN THE OPEN WATER. 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  
SOLUTION WAS 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  
(SUNDAY), 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 (BETWEEN  
125-130W PER THE GFS RUNS), OR OPEN INTO A BROADER WAVE AND MOVE  
INLAND MORE QUICKLY (PER THE 00Z ECMWF). THOUGH THE AMPLIFIED FLOW  
ALREADY IN PLACE COULD FAVOR THE SLOWER AND MORE CLOSED-OFF  
GFS-LIKE SOLUTIONS, THE MAJORITY OF THE ENSEMBLES WERE FARTHER  
WEST THAN THE GFS DETERMINISTIC RUNS WITH AT LEAST THE SOUTHERN  
PORTION OF THE TROUGH WHICH WOULD THEN ALLOW THE NORTHERN PORTION  
TO SLIDE EASTWARD QUICKLY (SOMEWHAT LIKE THE ECMWF). GIVEN THE  
LACK OF CONSENSUS, OPTED TO TREND TOWARD AN ENSEMBLE MEAN BLEND BY  
NEXT MONDAY AND ESPECIALLY TUESDAY. THIS WOULD KEEP ROSA AND THE  
BULK OF ITS MOISTURE OUT OF THE SOUTHWEST, BUT THE POTENTIAL  
CERTAINLY EXISTS FOR GREATER IMPACTS. PLEASE CONSULT THE NATIONAL  
HURRICANE CENTER FOR THE LATEST INFORMATION.  
 
   
..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 (40S AND 50S). 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/UPPER MISSISSIPPI VALLEY TO THE GREAT LAKES.  
SUFFICIENTLY 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 SLOWLY WEAKENING FRONTAL BOUNDARY FAILS TO  
CLEAR THE REGION. MOISTURE MAY SPREAD INTO PORTIONS OF THE  
SOUTHWESTERN U.S. BY NEXT WEEK BUT WILL BE CONSERVATIVE FOR NOW  
WITH POPS JUST ABOVE CLIMO AND GENERALLY LIGHT QPF. HOWEVER, HEAVY  
RAIN IS POSSIBLE IF THE SYSTEM TAKES A MORE NORTHERLY TRACK AHEAD  
OF THE TROUGH TO THE NORTH BUT CONFIDENCE IS LOW AT THIS TIME AS  
TO WHERE THAT MAY OCCUR (IF AT ALL).  
 
FRACASSO/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  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page