884  
FXUS02 KWBC 020649  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
148 AM EST SUN DEC 02 2018  
 
VALID 12Z WED DEC 05 2018 - 12Z SUN DEC 09 2018  
 
...COLD PATTERN WITH INCREASING WINTER WEATHER POTENTIAL FOR  
PORTIONS OF THE CENTRAL/EASTERN U.S. LATE THIS WEEK...  
 
   
..OVERVIEW
 
 
THE FLOW ACROSS THE CONUS DURING THE MEDIUM RANGE WILL LIE IN A  
TRANSITION ZONE FROM HIGHLY AMPLIFIED/BLOCKED FLOW EXTENDING FROM  
EAST ASIA ACROSS THE NORTH PACIFIC TO SOMEWHAT MORE PROGRESSIVE  
FLOW ACROSS THE ATLANTIC OCEAN. ANOMALOUS RIDGING ACROSS SIBERIA  
WILL DISPLACE COLD POLAR/ARCTIC AIR TOWARD CANADA, AND PERSISTENT  
RIDGING FROM WESTERN CANADA NORTH TO ALASKA WILL FAVOR TRANSPORT  
OF COLD AIR SOUTHWARD FROM CANADA INTO THE CENTRAL/EASTERN U.S. A  
NUMBER OF SHORTWAVES WILL TRAVERSE AN ACTIVE NORTHERN STREAM FROM  
THE MIDWEST TO THE NORTHEAST DURING THE PERIOD, WITH A STRONG COLD  
FRONT ACCOMPANYING THE MOST SIGNIFICANT OF THESE WAVES ACROSS THE  
PLAINS, OHIO VALLEY, AND NORTHEAST WED-FRI. FARTHER SOUTH, AN  
ACTIVE SOUTHERN STREAM WILL SPREAD PACIFIC ENERGY FROM CALIFORNIA  
EAST ACROSS THE SOUTHERN STATES. A WAVE OF LOW PRESSURE IS  
EXPECTED TO DEVELOP ALONG THE POLAR FRONT NEAR THE WESTERN GULF  
COAST BY SAT MORNING IN RESPONSE AS THE SOUTHERN STREAM WAVE  
APPROACHES. MODELS CONTINUE TO SUGGEST THE POSSIBILITY FOR PHASING  
INTERACTION THIS SOUTHERN STREAM WAVE WITH ADDITIONAL NORTHERN  
STREAM ENERGY BY SAT NIGHT/SUN, WITH POTENTIAL DEVELOPMENT OF A  
COASTAL LOW NEAR THE SOUTHEAST U.S. COAST.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS HANDLED BOTH NORTHERN AND SOUTHERN STREAM FEATURES  
SUFFICIENTLY DURING DAYS 3-4 (WED-THU) TO JUSTIFY USE OF A  
MULTI-MODEL DETERMINISTIC BLEND (ECMWF/GFS/FV3). STARTING ON DAY 5  
(FRI), FORECAST CONFIDENCE BEGINS A GRADUAL DOWNWARD SLIDE,  
ESPECIALLY SURROUNDING A COUPLE KEY AREAS. WHILE MOST SOLUTIONS  
AGREE THAT THE SOUTHERN STREAM WAVE SHOULD GRADUALLY DEAMPLIFY AS  
IT MOVES EAST, DIFFERENCES EMERGE AS TO THE TIMING OF THE  
DEAMPLIFICATION AS WELL AS THE TIMING OF THE EASTWARD PROGRESSION.  
UNCERTAINTY IS EVEN A BIT HIGHER IN THE NORTHERN STREAM BY LATE  
WEEK, AS MODELS HAVE SHOWN A FAIRLY LARGE DEGREE OF VARIABILITY  
AND RUN-TO-RUN INCONSISTENCY WITH A NUMBER OF SMALLER SCALE  
IMPULSES TRAVERSING THE MIDWEST/GREAT LAKES. BOTH OF THESE FACTORS  
PLAY A SUBSTANTIAL ROLE IN ANY POTENTIAL PHASING OF THE TWO  
STREAMS, WITH COMPOUNDING UNCERTAINTY SURROUNDING ANY SUCH  
INTERACTION. AS A TESTAMENT TO THE LARGE ARRAY OF POSSIBLE  
SOLUTIONS, EVEN THE USUALLY UNDER-DISPERSED GEFS BECOMES QUITE  
DISPERSED BY NEXT WEEKEND, WITH SIMILAR BEHAVIOR SHOWN BY THE  
ECENS AND CMCE ENSEMBLES. AS A SURFACE LOW DEVELOPS ALONG THE  
WESTERN GULF COAST FRI NIGHT, THE PRECISE EVOLUTION/TRACK OF THIS  
FEATURE WILL HINGE ENTIRELY ON THE EVENTUAL EVOLUTION OF THESE  
UPPER-LEVEL FEATURES. POSSIBILITIES RANGE FROM A CONSOLIDATED LOW  
PRESSURE AREA THAT MOVES NORTH TOWARD THE OHIO VALLEY TO A MORE  
EASTERLY MOVING LOW WITH ENERGY TRANSFERRING TO A NEW COASTAL  
SYSTEM ONCE PHASING OCCURS. GIVEN THESE CONSIDERATIONS, A  
TRANSITION TO MAJORITY WEIGHTING OF ENSEMBLE MEANS (ECENS/GEFS)  
WAS SHOWN FROM DAY 5 ONWARD, WITH THE SOLUTION BY DAY 7 (SUN)  
COMPRISED ALMOST ENTIRELY OF ENSEMBLE MEANS.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
SYSTEMS TRAVERSING THE NORTHERN STREAM ACROSS THE MIDWEST/GREAT  
LAKES WILL BE RELATIVELY STARVED FOR MOISTURE, BUT LAKE EFFECT  
ENHANCED SNOWFALL WILL BE POSSIBLE. HEAVY PRECIPITATION (RAIN AND  
MOUNTAIN SNOW) WILL BE POSSIBLE FOR PORTIONS OF CALIFORNIA AND THE  
SOUTHWEST WED-THU AS THE SOUTHERN STREAM SYSTEM MOVES ONSHORE. AS  
THIS SYSTEM REACHES THE CENTRAL U.S. ON FRI IT WILL BEGIN TO  
ADVECT DEEPER GULF OF MEXICO MOISTURE NORTHWARD, WHICH WILL BE  
ENHANCED AHEAD OF A DEVELOPING SURFACE LOW IN THE NORTHWESTERN  
GULF. THE RESULT IS EXPECTED TO BE A SHIELD OF WIDESPREAD AND  
POTENTIALLY HEAVY PRECIPITATION FRI-SAT FROM PORTIONS OF THE  
SOUTHERN PLAINS TO THE TENNESSEE VALLEY. WINTRY PRECIPITATION WILL  
BE POSSIBLE ON THE NORTHERN PERIPHERY OF THIS SYSTEM AS A COLD  
FRONT AND STRONG POLAR SURFACE HIGH PROVIDE AN ABUNDANT SOURCE OF  
COLD AIR. PRECIPITATION SHOULD SPREAD INTO THE  
APPALACHIANS/MID-ATLANTIC REGIONS BY SAT/SAT NIGHT AS WE SEE A NEW  
SURFACE LOW POTENTIALLY DEVELOPING ALONG THE SOUTHEAST COAST. 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.  
 
TEMPERATURES WILL BE BELOW NORMAL ACROSS MUCH OF THE CONUS THROUGH  
THE MEDIUM RANGE. THE CENTRAL U.S. WILL SEE THE LARGEST  
TEMPERATURE ANOMALIES THROUGH MUCH OF THE PERIOD, WHERE HIGH TEMPS  
ARE EXPECTED TO RANGE FROM 10 TO 20 DEG F BELOW AVERAGE.  
SURROUNDING AREAS FROM THE ROCKIES TO THE EASTERN SEABOARD WILL  
NOT ESCAPE THE COLD CONDITIONS, WITH HIGHS ACROSS A LARGE AREA  
RANGING FROM 5 TO 15 DEG BELOW 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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