125  
FXUS02 KWBC 240516  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
116 AM EDT MON SEP 24 2018  
 
VALID 12Z THU SEP 27 2018 - 12Z MON OCT 01 2018  
 
...EARLY-SEASON COLD AIR EXPECTED TO GRADUALLY SPREAD INTO AREAS  
FROM THE NORTHERN ROCKIES TO THE UPPER MIDWEST LATER THIS WEEK...  
 
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
AMPLIFIED UPPER-LEVEL FLOW PATTERN EXPECTED TO PERSIST ACROSS  
NORTH AMERICA DURING THE MEDIUM RANGE. AN ANOMALOUS/BLOCKING UPPER  
RIDGE IS EXPECTED TO SHIFT NORTHWARD FROM THE NORTH PACIFIC INTO  
ALASKA OVER THE NEXT WEEK, WITH 500-HPA HEIGHTS REACHING +3.5  
STANDARD DEVIATIONS ACROSS INTERIOR ALASKA BY LATE THIS WEEK. THE  
RIDGE ALONG WITH DOWNSTREAM PERSISTENT TROUGHING JUST WEST OF  
HUDSON BAY WILL PROMOTE SOUTHWARD TRANSPORT OF POLAR AIR THROUGH  
CENTRAL CANADA AND INTO THE NORTH CENTRAL U.S. MEANWHILE, UPPER  
RIDGING IS EXPECTED TO HOLD STRONG OR EVEN INTENSITY FROM THE  
SOUTHERN PLAINS TO THE SOUTHEAST. THE COMBINATION OF THESE  
FEATURES WILL FAVOR THE SETUP OF A PERSISTENT AND RELATIVELY  
STRONG BAROCLINIC ZONE ACROSS THE CENTRAL PLAINS BY LATER THIS  
WEEK. FARTHER WEST, 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.  
 
MODELS INITIALLY SHOWED GOOD CONSENSUS WITH RESPECT TO SHORTWAVE  
ENERGY AND A SURFACE LOW PRESSURE SYSTEM CROSSING THE MIDWEST ON  
DAY 3 (THU), WITH RELATIVELY SMALL TIMING/AMPLITUDE DIFFERENCES  
FOR AN ADDITIONAL SHORTWAVE AMPLIFYING AS IT DIVES SOUTH TOWARD  
THE PACIFIC NORTHWEST DAYS 4-5 (FRI-SAT). ADDITIONALLY, THE ECMWF  
HAS BACKED OFF A BIT ON ITS FAST SOLUTION WITH CUTOFF ENERGY  
REACHING THE NORTHWEST AND BECOMING ABSORBED INTO THE WESTERLIES,  
AND MOST DETERMINISTIC SOLUTIONS ARE NOW IN THE SAME VICINITY WITH  
THIS FEATURE THROUGH DAY 5. THUS, A MULTI-MODEL DETERMINISTIC  
BLEND (INCLUDING THE ECMWF/GFS/CMC) WAS USED A STARTING POINT FOR  
THE WPC FORECAST DAYS 3-5. BY DAYS 6-7 (SUN-MON), CONSENSUS HAS  
IMPROVED THAT A RELATIVELY DEEP UPPER LOW WILL UNDERCUT THE ALASKA  
RIDGE AND APPROACH THE PACIFIC NORTHWEST. WHILE MODELS AGREE ON  
THE EXISTENCE AND GENERAL TIMING OF THIS FEATURE, THERE HAS BEEN  
SIGNIFICANT RUN-TO-RUN VARIABILITY WITH RESPECT TO THE EXACT  
CHARACTER OF THIS FEATURE AND ANY POTENTIAL INTERACTIONS WITH  
NORTHERN STREAM ENERGY (WHICH COULD PLAY A ROLE IN THE AMPLITUDE  
OF UPPER FLOW/POTENTIAL HEIGHT FALLS DOWNSTREAM ACROSS THE NORTH  
CENTRAL/NORTHEASTERN U.S. IN GENERAL, THE DOWNSTREAM RESULT  
APPEARS TO BE RELATIVELY FLAT UPPER FLOW FARTHER EAST ACROSS THE  
CENTRAL/EASTERN STATES, AND AN INTENSIFYING LOW-LEVEL BAROCLINIC  
ZONE ACROSS THE CENTRAL U.S. BETWEEN A POLAR AIR MASS ACROSS THE  
NORTH CENTRAL STATES AND SUBTROPICAL AIR FARTHER SOUTH. GIVEN  
INCREASED UNCERTAINTY AND RUN-TO-RUN VARIABILITY BY DAYS 6-7,  
WEIGHTING OF ENSEMBLE MEANS (ECENS/NAEFS) WAS BOOSTED TO COMPRISE  
A MAJORITY OF THE FORECAST BLEND.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
LOCALLY HEAVY CONVECTION WILL BE POSSIBLE FROM THE GULF COAST TO  
THE SOUTHERN MID-ATLANTIC THU-FRI, FOCUSED ALONG A COUPLE COLD  
FRONTS. THE SECOND OF THESE FRONTS MAY ALSO PRODUCE AREAS OF  
SHOWERS FROM THE NORTHERN ROCKIES TO THE UPPER MIDWEST. BY THE  
WEEKEND, AS THE AFOREMENTIONED FRONTAL ZONE INTENSIFIES ACROSS THE  
CENTRAL U.S., AND ADDITIONAL UPPER SHORTWAVE ENERGY APPROACHES,  
EXPECT THE POTENTIAL FOR A ROUND OF MORE WIDESPREAD PRECIPITATION  
TO DEVELOP FROM THE NORTHERN ROCKIES INTO THE NORTHERN/CENTRAL  
PLAINS AND PORTIONS OF THE MIDWEST. SUFFICIENT COLD AIR MAY BE IN  
PLACE IN THE POLAR AIR MASS NORTH OF THE FRONTAL BOUNDARY TO  
SUPPORT AREAS OF SNOW MIXING INTO THE RAIN AT TIMES ACROSS THE  
NORTHERN ROCKIES AND PERHAPS EVEN EASTWARD INTO THE NORTHERN HIGH  
PLAINS. ADDITIONALLY, THE SYSTEM APPROACHING THE PACIFIC NORTHWEST  
BY THE WEEKEND MAY BEGIN TO SPREAD PRECIPITATION ACROSS AREAS FROM  
NORTHERN CALIFORNIA NORTHWARD.  
 
COOL TEMPERATURES WILL INCREASINGLY BECOME A STORY THROUGH THE  
WEEK AS A COUPLE COLD FRONTS SPREAD SUCCESSIVELY COOLER AIR INTO  
AREAS FROM THE NORTHERN ROCKIES TO THE UPPER MIDWEST. HIGH  
TEMPERATURES THU-FRI ARE EXPECTED TO BE 5 TO 15 DEG F BELOW  
AVERAGE ACROSS THESE AREAS, WITH COLDER TEMPERATURES, 15 TO 25 DEG  
BELOW AVERAGE, ARRIVING BY THE WEEKEND. HIGH TEMPERATURES ARE  
EXPECTED TO REMAIN IN THE 40S BY NEXT WEEKEND ACROSS A WIDE AREA  
OF THE NORTHERN PLAINS AND UPPER MIDWEST, WITH LOW DROPPING BELOW  
FREEZING FOR MANY OF THE SAME AREAS. MEANWHILE, MUCH OF THE  
SOUTHEAST AND SOUTHEAST WILL REMAIN RELATIVELY WARM, WITH HIGHS 5  
TO 10 DEG ABOVE AVERAGE.  
 
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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