644  
FXUS02 KWBC 270601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 AM EDT THU SEP 27 2018  
 
VALID 12Z SUN SEP 30 2018 - 12Z THU OCT 04 2018  
 
...HEAVY RAIN POSSIBLE NEXT WEEK ACROSS THE SOUTHWEST ASSOCIATED  
WITH 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. TROUGHING NEAR THE WEST COAST WILL LIKELY ACT TO  
STEER HURRICANE ROSA NORTHEASTWARD TOWARD THE BAJA CALIFORNIA  
PENINSULA EARLY NEXT WEEK. ADDITIONALLY, 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
 
 
A BLEND OF THE ECMWF/GFS/CMC SERVED AS A REASONABLE STARTING POINT  
FOR THE FORECAST DURING DAYS 3-5 (SUN-TUE). A NUMBER OF WEAK  
UPPER-LEVEL SHORTWAVES ARE EXPECTED TO TRAVERSE THE NORTHERN TIER  
DURING THE PERIOD, AND DETERMINISTIC SOLUTIONS SHOWED REASONABLY  
GOOD CONSENSUS SURROUNDING THESE FEATURES. ADDITIONALLY, A CUTOFF  
UPPER LOW IS EXPECTED TO REACH THE WEST COAST SAT NIGHT,  
EVENTUALLY OPENING INTO A WAVE AS IT INTERACTS WITH NORTHERN  
STREAM ENERGY ACROSS THE NORTHERN ROCKIES, AND QUICKLY EJECTING  
EASTWARD. AN ADDITIONAL CUTOFF UPPER LOW IS FORECAST BY ALL  
GUIDANCE TO APPROACH THE WEST COAST NEXT MON-TUE. MODEL CONSENSUS  
WITH RESPECT TO THIS FEATURE IS ALSO REASONABLY GOOD, AT LEAST TO  
DAY 5. AS THIS FEATURE DIGS SOUTHWARD OFF/ALONG THE CALIFORNIA  
COAST, THIS SHOULD STEER HURRICANE ROSA IN A GENERAL NORTHEASTWARD  
DIRECTION TOWARD THE BAJA CALIFORNIA PENINSULA, PER THE LATEST  
FORECAST FROM THE NATIONAL HURRICANE CENTER.  
 
DURING DAYS 6-7, SPREAD BEGINS TO INCREASE WITH RESPECT TO THE  
DEGREE OF AMPLIFICATION THAT OCCURS WITH THE WESTERN UPPER  
LOW/TROUGH - DIFFERENCES THAT SEEM TO LARGELY COME DOWN TO THE  
DEGREE OF INTERACTION WITH NORTHERN STREAM ENERGY. THE ECMWF DIGS  
ADDITIONAL NORTHERN STREAM ENERGY SOUTH INTO THE WESTERN TROUGH,  
RESULTING IN FURTHER AMPLIFICATION AND A MUCH SLOWER PROGRESSION  
RELATIVE TO THE GFS, WHICH KEEPS THE NORTHERN STREAM SEPARATE AND  
MOVES THE TROUGH FROM CALIFORNIA INTO THE BASIN MORE QUICKLY. THE  
DEGREE TO WHICH THIS OCCURS WILL PLAY A SIGNIFICANT ROLE IN THE  
EVENTUAL TRACK OF ROSA AND IN WHICH AREAS MAY SEE THE THREAT FOR  
HEAVY PRECIPITATION. WHETHER THIS INTERACTION OF STREAMS OCCURS  
HAS EFFECTS THAT PROPAGATE DOWNSTREAM TO THE CENTRAL/EASTERN U.S.  
WITH A MORE AMPLIFIED/SLOWER WESTERN TROUGH RESULTING IN HIGHER  
HEIGHTS/STRONGER RIDGING FOR THOSE AREAS. ENSEMBLE MEANS SEEM TO  
FAVOR SOMETHING MORE ALONG THE LINES OF THE SLOWER/MORE AMPLIFIED  
SOLUTION, AND THIS IDEA IS ALSO SUPPORTED BY THE GENERAL  
AMPLIFICATION OF UPPER-LEVEL FLOW ACROSS MUCH OF THE NORTHERN  
HEMISPHERE. THUS, TRENDED AWAY FROM THE GFS AND TOWARD HEAVIER  
ECENS/GEFS MEAN WEIGHTING DURING DAYS 6-7.  
 
   
..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 10-25 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  
PRODUCE POTENTIALLY HEAVY RAINFALL FOR PORTIONS OF THE  
MIDWEST/GREAT LAKES LATE THIS WEEKEND INTO EARLY NEXT WEEK.  
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 AREAS OF HEAVY RAINFALL IN SOUTHEASTERN CALIFORNIA INTO  
ARIZONA AND 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) AND FORECAST CONFIDENCE INCREASES,  
BUT THE WPC DETERMINISTIC QPF WILL REMAIN AT LEAST SOMEWHAT  
CONSERVATIVE FOR NOW GIVEN THAT THE EVENT IS STILL SEVERAL DAYS  
OUT.  
 
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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