647  
FXUS02 KWBC 261551  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1150 AM EDT WED SEP 26 2018  
 
VALID 12Z SAT SEP 29 2018 - 12Z WED OCT 03 2018  
 
...POTENTIALLY HEAVY RAIN NEXT WEEK FOR THE SOUTHWEST DUE TO  
HURRICANE ROSA...  
 
...EXTENDED PERIOD OF BELOW AVERAGE TEMPERATURES EXPECTED FROM THE  
NORTHERN ROCKIES TO THE UPPER MIDWEST...  
 
   
..PATTERN OVERVIEW
 
 
HIGHLY AMPLIFIED FLOW PATTERN EXPECTED TO PERSIST FROM THE NORTH  
PACIFIC ACROSS NORTH AMERICA DURING THE MEDIUM RANGE, WITH A  
BLOCKING RIDGE CENTERED OVER ALASKA, MEAN TROUGHING OFF/ALONG THE  
U.S. WEST COAST AND JUST WEST OF HUDSON BAY, AND A PERSISTENT  
SUBTROPICAL RIDGE ANCHORED ACROSS THE SOUTHEASTERN STATES. THIS  
PATTERN WILL FAVOR SOUTHWARD TRANSPORT OF POLAR AIR INTO THE CONUS  
NORTHERN TIER, FROM THE NORTHERN ROCKIES EAST TO THE GREAT LAKES,  
WITH CONTINUED LATE SUMMER-LIKE CONDITIONS FROM THE GULF COAST TO  
THE SOUTHEAST, AND A PERSISTENT STRONG FRONTAL ZONE IN BETWEEN  
THESE AIR MASSES. THE EXTRATROPICAL TRANSITION OF TYPHOON TRAMI  
ACROSS EAST ASIA OVER THE WEEKEND SHOULD SUPPORT CONTINUED/FURTHER  
UPPER FLOW AMPLIFICATION DOWNSTREAM, SUGGESTING A CONTINUATION OF  
THE HIGHLY AMPLIFIED/BLOCKED FLOW REGIME ACROSS NORTH AMERICA  
THROUGH AND BEYOND THE MEDIUM RANGE PERIOD.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
CONSENSUS BLEND OF THE LATEST 00Z/06Z GUIDANCE OFFERED A  
REASONABLE STARTING POINT TO THE FORECAST FOR THE FIRST FEW DAYS  
(THIS WEEKEND). A NUMBER OF SHORTWAVES ARE EXPECTED TO TRAVERSE  
THE NORTHERN STREAM FLOW, AND MODEL SOLUTIONS SHOW REASONABLY GOOD  
CLUSTERING WITH RESPECT TO THESE DURING DAYS 3-5 (SAT-MON) WITH  
SOME EXPECTED TIMING DIFFERENCES. ADDITIONALLY, A CUTOFF UPPER LOW  
IS EXPECTED TO REACH THE WEST COAST SATURDAY EVENTUALLY OPENING  
INTO A WAVE AS IT INTERACTS WITH NORTHERN STREAM ENERGY ACROSS THE  
NORTHERN ROCKIES, AND QUICKLY EJECTING EASTWARD INTO THE ZONAL  
FLOW BETWEEN ANOMALY CENTERS EAST OF 100W.  
 
GREATER UNCERTAINTY IS INTRODUCED INTO THE FORECAST DURING DAYS  
5-7 SURROUNDING THE AMPLIFICATION OF ADDITIONAL UPPER-LEVEL ENERGY  
OFF THE WEST COAST, WITH POTENTIAL FORMATION OF ANOTHER CUTOFF LOW  
NEAR NORTHERN CALIFORNIA EARLY TUESDAY. THE EVOLUTION/DEPTH OF  
THIS TROUGHING WILL HELP DETERMINE THE FATE OF CURRENT HURRICANE  
ROSA IN THE EAST PACIFIC. THE NATIONAL HURRICANE CENTER SHOWS A  
TRACK WESTWARD OVER THE NEXT FEW DAYS BUT THEN TURNING NORTHWARD  
THIS WEEKEND AND EAST OF NORTH NEXT WEEK PER THE LATEST ADVISORY.  
ENSEMBLE CONSENSUS FOR TUES/WED SHOWS ROSA WEAKENING AS IT  
TRAVERSES COOLER WATER BUT LIFTING NORTHEASTWARD TOWARD BAJA  
CALIFORNIA. THE ECMWF AND ITS ENSEMBLES SHIFTED IN THEIR 00Z RUN  
TOWARD THE GFS/GEFS SOLUTION SHOWING THIS FORECAST RECURVATURE  
TOWARD NW MEXICO.  
 
   
..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 10-20 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 IS EXPECTED TO SPREAD INTO PORTIONS OF  
THE SOUTHWESTERN U.S. BY EARLY NEXT WEEK DEPENDING ON THE EVENTUAL  
TRACK OF ROSA. DESPITE A WEAKENING TROPICAL SYSTEM, THE POTENTIAL  
EXISTS FOR MODEST AND PERHAPS LOCALLY HEAVY RAINFALL IN  
SOUTHEASTERN CALIFORNIA INTO ARIZONA INTO THE FOUR CORNERS REGION.  
RAINFALL AMOUNTS WILL BE ADJUSTED OVER THE NEXT SEVERAL DAYS AS  
THE ATMOSPHERIC PLAYERS COME INTO BETTER FOCUS (I.E., INCOMING  
UPPER TROUGH AND RELATIVE PRECIPITABLE WATER/IVT VALUES), BUT THE  
WPC DETERMINISTIC QPF WILL REMAIN SOMEWHAT CONSERVATIVE FOR NOW  
GIVEN A FIVE OR SIX-DAY LEAD TIME.  
 
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  
 

 
 
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