182  
FXUS02 KWBC 271543  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1143 AM EDT THU SEP 27 2018  
 
VALID 12Z SUN SEP 30 2018 - 12Z THU OCT 04 2018  
 
...LOCALLY HEAVY RAIN 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 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 PER THE LATEST NATIONAL HURRICANE CENTER TRACK.  
ADDITIONALLY, THE EXTRATROPICAL TRANSITION OF TYPHOON TRAMI ACROSS  
EASTERN ASIA OVER THE WEEKEND SHOULD SUPPORT CONTINUED/FURTHER  
UPPER FLOW AMPLIFICATION DOWNSTREAM OVER THE NORTH PACIFIC AND  
ALASKA, SUGGESTING A CONTINUATION OF THE HIGHLY AMPLIFIED/BLOCKED  
FLOW REGIME ACROSS NORTH AMERICA THROUGH AND BEYOND THE MEDIUM  
RANGE PERIOD.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
A DETERMINISTIC BLEND SUFFICED FOR THE FIRST FEW DAYS OF THE  
FORECAST (SUN-TUE) WITH THE MOST AGREEMENT EAST OF 105W. A NUMBER  
OF WEAK UPPER-LEVEL SHORTWAVES ARE EXPECTED TO TRAVERSE THE  
NORTHERN TIER DURING THE PERIOD, AND DETERMINISTIC SOLUTIONS  
SHOWED SOME TIMING/AMPLITUDE DIFFERENCES IN THEIR RECENT RUNS BUT  
A BLENDED SOLUTION WAS STILL PREFERRED WITH THESE FEATURES GIVEN  
THE SHIFTING OF THE GUIDANCE. ADDITIONALLY, A CLOSED 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 SHOULD QUICKLY EJECT EASTWARD INTO THE  
FASTER WSW FLOW EAST OF THE ROCKIES. AN ADDITIONAL CLOSED 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, BUT THE DETERMINISTIC MODELS HAVE BEEN QUICKER  
THAN THE ENSEMBLE MEANS SUGGEST, LIKELY RELATED TO STRONGER/SLOWER  
MEMBERS PULLING THE MEAN TROUGH AXIS BACK WESTWARD. A NEARLY EVEN  
ENSEMBLE/DETERMINISTIC BLEND HAS BEEN A DECENT POSITION OVER THE  
LAST FEW DAYS WITH A SLOW TREND EASTWARD IN THE ENSEMBLE MEANS AND  
A SLOW TREND WESTWARD IN THE DETERMINISTIC MODELS. AS THIS FEATURE  
DIGS SOUTHWARD OFF/ALONG THE CALIFORNIA COAST, IT SHOULD HELP  
STEER HURRICANE ROSA IN A GENERAL NORTHEASTWARD DIRECTION TOWARD  
THE BAJA CALIFORNIA PENINSULA THOUGH IN A WEAKENING STATE.  
 
DURING DAYS 6-7 (WED-THU), 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 OUT OF SW CANADA  
AND UPSTREAM GULF OF ALASKAN FLOW. WITH UNCERTAINTY IN SEVERAL  
PARTS OF THE FLOW, OPTED TO RELY MOSTLY ON THE ENSEMBLE MEANS FOR  
THE FORECAST WHICH WHICH HAVE BEEN MUCH MORE STABLE (BOTH THE GEFS  
AND ECENS), BUT HAVE DEFINITELY TRENDED TOWARD STRONGER RIDGING IN  
THE SOUTHEAST AND DEEPER TROUGHING IN THE WEST. THIS WILL HAVE AN  
AFFECT ON THE TRACK OF ROSA AND WHICH AREAS MAY SEE THE THREAT FOR  
HEAVY PRECIPITATION.  
 
   
..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 SLOWLY  
WEAKENS. 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 SOMEWHAT CONSERVATIVE  
FOR NOW GIVEN THAT THE EVENT IS STILL SEVERAL DAYS OUT.  
 
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