288  
FXUS02 KWBC 031559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST MON DEC 03 2018  
 
VALID 12Z THU DEC 06 2018 - 12Z MON DEC 10 2018  
 
...COLD PATTERN OVER THE CONTIGUOUS U.S. WITH A SIGNIFICANT STORM  
SYSTEM LIKELY TO AFFECT AT LEAST THE SOUTHERN HALF OF THE U.S.  
LATE WEEK INTO THE WEEKEND...  
 
   
..OVERVIEW
 
 
THE UPPER LEVEL FLOW ACROSS THE CONUS DURING THE MEDIUM RANGE  
PERIOD CONTINUES TO BE IN A TRANSITION ZONE FROM AN  
AMPLIFIED/BLOCKED FLOW ACROSS THE PACIFIC TO PROGRESSIVE/ZONAL  
FLOW IN THE ATLANTIC. A RIDGE BUILDING ACROSS THE WESTERN U.S.  
WILL ASSIST IN TRANSPORTING COLD/ARCTIC AIR IN CANADA TO THE  
CENTRAL AND EASTERN U.S. ESPECIALLY DURING THE SECOND HALF OF THE  
MEDIUM RANGE PERIOD. WITH THE MEAN TROUGH CENTERED OVER THE  
NORTHERN TIER STATES/GREAT LAKES REGION, A NUMBER OF SHORT WAVES  
WILL MOVE ACROSS THIS REGION. FARTHER SOUTH, AN ACTIVE SOUTHERN  
STREAM WILL CARRY PACIFIC ENERGY FROM NEAR CALIFORNIA EASTWARD  
ACROSS THE SOUTHERN STATES. AS THIS ENERGY REACHES THE SOUTHERN  
ROCKIES/PLAINS, A WAVE OF LOW PRESSURE IS EXPECTED TO DEVELOP  
ALONG THE POLAR FRONT NEAR THE WESTERN GULF COAST BY SATURDAY.  
MODELS AND ENSEMBLES CONTINUE TO SHOW VARYING DEGREES OF  
PHASING/INTERACTIVE WITH THIS SOUTHERN STREAM WAVE WITH THE  
NORTHERN STREAM ENERGY. HOWEVER, ALL MODELS AND ENSEMBLES INDICATE  
A SURFACE LOW DEVELOPING IN THE WESTERN GULF COAST REGION AND  
MOVING ALONG THE EAST COAST BY THE END OF THE WEEKEND. THIS WILL  
GIVE WAY TO AN ACTIVE, WET PERIOD FOR THE SOUTHERN PLAINS AND  
SOUTHEAST THROUGHOUT THE WEEKEND. BY THE END OF THE MEDIUM RANGE  
PERIOD (EARLY NEXT WEEK), GUIDANCE STILL SUGGESTING THE RIDGE  
ACROSS THE WESTERN U.S. WILL SHIFT EASTWARD, GIVING WAY TO HEIGHT  
FALLS ALONG THE WEST COAST. THIS SHOULD USHER IN STORMIER  
CONDITIONS.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS CONTINUE TO HANDLE THE NORTHERN AND SOUTHERN STREAM  
FEATURES WELL DURING DAYS 3 -5 (THU-SAT). THE MEAN TROUGH WILL  
CENTER OVER THE GREAT LAKES AND NORTHERN TIER STATES, WITH A  
TROUGH DIGGING ACROSS CALIFORNIA AND WILL MOVE INTO THE SOUTHWEST  
STATES. BY SATURDAY, THIS TROUGH WILL PUSH ACROSS THE  
SOUTH-CENTRAL U.S. AS AN UPPER RIDGE BUILDS ACROSS THE WESTERN  
U.S. A BLEND OF THE 00Z ECMWF/00Z GFS WAS USED DURING THIS TIME  
PERIOD. BY DAY 5 ONWARD, DIFFERENCES ESPECIALLY WITH THE DETAILING  
OF THE SHORTWAVE ENERGY DROPPING INTO THE NORTHERN U.S. BEGIN TO  
EMERGE AND THIS HAS AN IMPACT ON THE EVOLUTION OF THE DEVELOPING  
SURFACE LOW IN THE SOUTHERN STREAM. THE 00Z ECMWF AND THE 00Z/06Z  
GFS SHOW A SHORT WAVE SLIDING DOWN THE UPPER RIDGE OVER THE  
WESTERN U.S. HOWEVER, THE ECMWF IS MORE AGGRESSIVE WITH DIGGING  
THIS SHORT WAVE ACROSS THE MISSISSIPPI VALLEY. DOWNSTREAM, THE 06Z  
GFS BEGINS TO CLOSE OFF AN UPPER LEVEL LOW ACROSS THE LOWER  
MISSISSIPPI VALLEY AND IS ALSO FASTER WITH THE PROGRESSION OF THE  
SURFACE LOW ON SUNDAY. BY MONDAY MORNING, THE ECMWF SHOWS A CLOSED  
UPPER LOW OVER THE MID-ATLANTIC WITH THE GFS SHOWING AN OPEN WAVE.  
THIS IS REFLECTED AT THE SURFACE--WITH THE ECMWF SHOWING THE  
SURFACE LOW CLOSER TO THE EAST COAST AND MUCH DEEPER. BECAUSE OF  
THESE DIFFERENCES, INCREASED THE USAGE OF THE ENSEMBLE MEANS  
(ECENS/GEFS). WITH THE POSITIONING OF THE SURFACE LOW BY MONDAY  
MORNING, COMPROMISED BETWEEN THE 00Z ECMWF AND 06Z GFS--WHICH PUTS  
IT JUST OFF THE NORTH CAROLINA COASTLINE. THE ENSEMBLE MEANS ALSO  
AGREE WITH SHOWING A DEEPENING TROUGH ALONG THE WEST COAST.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE SOUTHERN STREAM SYSTEM WILL INITIALLY BRING LOCALLY MODERATE  
TO LOCALLY HEAVY PRECIPITATION (RAIN AND HIGH ELEVATION SNOW) TO  
PARTS OF CENTRAL AND SOUTHERN CALIFORNIA INTO THE SOUTHWEST. AREAS  
NEAR COASTAL SOUTHERN CALIFORNIA AND FAVORED TERRAIN OVER ARIZONA  
SHOULD SEE THE HIGHEST TOTALS WITH THIS EVENT. FARTHER EAST,  
EXPECT AREAS OF HEAVY RAINFALL TO DEVELOP OVER THE SOUTHERN PLAINS  
THU NIGHT-FRI AND THEN CONTINUE EASTWARD INTO THE WEEKEND. COLD  
AIR IN PLACE ON THE NORTHERN PERIPHERY OF THE SYSTEM WILL LIKELY  
PRODUCE WINTRY PRECIPITATION IN THE NORTHERN PART OF THE  
PRECIPITATION SHIELD, STARTING OVER THE SOUTHERN PLAINS THU-FRI  
AND REACHING THE APPALACHIANS AND EAST COAST DURING THE WEEKEND.  
CURRENTLY THE BEST POTENTIAL FOR SIGNIFICANT SNOWFALL EXISTS FROM  
THE SOUTHERN/CENTRAL PLAINS INTO THE SOUTHERN/CENTRAL APPALACHIANS  
AND PARTS OF THE MID-ATLANTIC. NORTHERN STREAM SYSTEMS AFFECTING  
THE NORTHEAST QUADRANT OF THE NATION WILL BE RELATIVELY STARVED  
FOR MOISTURE BUT LAKE EFFECT ENHANCED SNOWFALL WILL BE POSSIBLE.  
SEE THE LATEST DAY 4-7 WINTER WEATHER OUTLOOKS FOR DETAILS ON  
AREAS EXPECTED TO SEE THE GREATEST WINTER WEATHER THREAT IN THE  
MEDIUM RANGE.  
 
BELOW AVERAGE TEMPERATURES WILL PREVAIL ACROSS MUCH OF THE CONUS  
DURING THE MEDIUM RANGE. THE MOST EXTREME ANOMALIES SHOULD BE OVER  
THE CENTRAL U.S. BEHIND THE FRONT THAT DROPS SOUTHWARD MID-LATE  
WEEK. SOME LOCATIONS MAY SEE HIGHS AT LEAST 15-20F BELOW AVERAGE.  
THE EASTERN U.S. WILL GENERALLY SEE HIGHS 5-15F BELOW NORMAL WITH  
SOME DAY-TO-DAY VARIABILITY. THE WEST WILL BEGIN THE PERIOD WED  
WITH HIGHS 5-10F BELOW AVERAGE, BUT TEMPERATURES WILL MODERATE  
THROUGH THE WEEKEND AS UPPER RIDGING SLOWLY SHIFTS EAST.  
TEMPERATURES MAY REACH SEVERAL DEGREES ABOVE AVERAGE ACROSS  
PORTIONS OF THE CENTRAL/NORTHERN ROCKIES AND THE NORTHERN HIGH  
PLAINS BY SUN-MON.  
 
REINHART/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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