109  
FXUS02 KWBC 260542  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
142 AM EDT WED SEP 26 2018  
 
VALID 12Z SAT SEP 29 2018 - 12Z WED OCT 03 2018  
 
...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 MORE 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
 
 
FORECAST CONFIDENCE IS RELATIVELY GOOD FOR ROUGHLY THE FIRST HALF  
OF THE PERIOD, AND A ECMWF/GFS/UKMET BLEND SERVED AS A GOOD  
CONSENSUS SOLUTION/FORECAST STARTING POINT. A NUMBER OF SHORTWAVES  
ARE EXPECTED TO TRAVERSE THE NORTHERN STREAM FLOW, AND MODEL  
SOLUTIONS SHOW REASONABLY GOOD CONSENSUS WITH RESPECT TO THESE  
DURING DAYS 3-5 (SAT-MON). ADDITIONALLY, A CUTOFF UPPER LOW IS  
EXPECTED TO REACH THE WEST COAST DURING THAT TIME FRAME,  
EVENTUALLY OPENING INTO A WAVE AS IT INTERACTS WITH NORTHERN  
STREAM ENERGY ACROSS THE NORTHERN ROCKIES, AND QUICKLY EJECTING  
EASTWARD. THIS IDEA IS ALSO GENERALLY WELL-REPRESENTED BY THE  
CONSENSUS SOLUTION DESCRIBED ABOVE.  
 
GREATER UNCERTAINTY IS INTRODUCED INTO THE FORECAST DURING DAYS  
6-7 SURROUNDING THE AMPLIFICATION OF ADDITIONAL UPPER-LEVEL ENERGY  
OFF THE WEST COAST, WITH POTENTIAL FORMATION OF ANOTHER CUTOFF  
LOW. THE ECMWF HAS COME AROUND TO A SLOWER/MORE CUTOFF SOLUTION  
(WITH ADDITIONAL SUPPORT FROM THE FV3) COMPARED TO PREVIOUS RUNS,  
WHILE THE GFS HAS TRENDED IN THE OPPOSITE DIRECTION. GIVEN THE  
SUPPORT FOR CONTINUED OR EVEN INCREASED HEMISPHERIC FLOW  
AMPLIFICATION, CONTINUE TO FAVOR A MORE AMPLIFIED TROUGH OR CUTOFF  
LOW ALONG/OFF THE WEST COAST EARLY-MID NEXT WEEK, WITH A SLOWER  
EASTWARD PROGRESSION. THE ECENS/GEFS MEANS SHOW THIS IDEA WELL,  
AND WEIGHTING OF THOSE SOLUTIONS WAS QUICKLY INCREASED DURING THE  
DAY 6-7 (TUE-WED) TIME FRAME. ONE IMPORTANT CONSIDERATION IS THAT  
THE EVOLUTION OF THIS FEATURE WILL LIKELY PLAY A SIGNIFICANT ROLE  
IN THE EVENTUAL TRACK OF TROPICAL STORM (FORECAST BECOME  
HURRICANE) ROSA. THE AMPLIFICATION OF AN UPPER TROUGH/LOW OFF THE  
CALIFORNIA COAST WOULD FAVOR A NORTHWARD MOVEMENT FOR ROSA BY THE  
WEEKEND OR EARLY NEXT WEEK, BUT THE MAJOR QUESTION IS TO WHAT  
DEGREE THIS OCCURS, AND ALSO TO WHAT DEGREE MOISTURE FROM THE  
SYSTEM IS ABLE TO MOVE INTO THE SOUTHWESTERN U.S., POTENTIALLY  
INCREASING PRECIPITATION FOR THAT REGION. CONFIDENCE REMAINS QUITE  
LOW IN THIS ASPECT OF THE FORECAST; PLEASE REFER TO PRODUCTS  
ISSUED BY THE NATIONAL HURRICANE CENTER FOR THE LATEST OFFICIAL  
INFORMATION ON ROSA.  
 
   
..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 MAY SPREAD INTO PORTIONS OF THE  
SOUTHWESTERN U.S. BY NEXT WEEK DEPENDING ON THE EVENTUAL TRACK OF  
ROSA. 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).  
 
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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