995  
FXUS02 KWBC 040657  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
157 AM EST TUE DEC 04 2018  
 
VALID 12Z FRI DEC 07 2018 - 12Z TUE DEC 11 2018  
 
...SIGNIFICANT STORM SYSTEM TO BRING POTENTIAL WINTER WEATHER TO  
AREAS FROM THE SOUTHERN PLAINS TO THE EASTERN U.S. LATE THIS  
WEEK...  
 
   
..OVERVIEW
 
 
UPPER-LEVEL FLOW ACROSS THE CONUS DURING THE MEDIUM RANGE  
CONTINUES TO LIE IN A TRANSITION ZONE FROM A HIGHLY  
AMPLIFIED/BLOCKED FLOW REGIME ACROSS THE PACIFIC TO MORE  
PROGRESSIVE FLOW IN THE ATLANTIC. UPPER RIDGING INITIALLY ACROSS  
WESTERN CANADA WITH DOWNSTREAM CYCLONIC NORTHWESTERLY FLOW FROM  
CENTRAL CANADA INTO THE EASTERN U.S. WILL ENSURE AN AMPLE SUPPLY  
OF COLD AIR. A NUMBER OF SHORTWAVES WILL TRAVERSE THIS  
NORTHWESTERLY FLOW, WHILE FARTHER SOUTH AN ACTIVE SOUTHERN STREAM  
TRANSPORTS PACIFIC ENERGY FROM BAJA CALIFORNIA EASTWARD ACROSS THE  
U.S. SOUTHERN STATES. IN RESPONSE TO THIS SOUTHERN STREAM ENERGY,  
A WAVE OF LOW PRESSURE IS EXPECTED TO DEVELOP ALONG THE POLAR  
FRONT ALONG THE WESTERN GULF COAST BY EARLY SAT, THEN MOVE EAST  
ACROSS THE GULF COAST STATES INTO EARLY SUN. GUIDANCE CONTINUES TO  
SHOW VARYING DEGREES OF PHASING/INTERACTION BETWEEN THIS SOUTHERN  
STREAM WAVE AND NORTHERN STREAM ENERGY, WHICH AFFECTS THE EVENTUAL  
TRACK OF THIS LOW PRESSURE SYSTEM AS IT MOVES OFF THE EAST COAST  
LIKELY ON SUN AND EITHER TAKES A MORE SUPPRESSED TRACK OUT TO SEA  
OR TURNS AND MOVES NORTHEASTWARD CLOSER TO THE COAST. FARTHER  
WEST, GUIDANCE CONTINUES TO SUGGEST THAT THE UPPER RIDGE WILL  
BEGIN TO MOVE EAST TOWARD THE ROCKIES/HIGH PLAINS BY EARLY NEXT  
WEEK, ALLOWING RELATIVELY STRONG HEIGHT FALLS TO REACH THE WEST  
COAST, USHERING IN A PERIOD OF STORMY CONDITIONS FOR THAT REGION.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS ARE REASONABLY WELL CLUSTERED DURING THE FIRST HALF OF THE  
MEDIUM RANGE, AND A BLEND OF THE ECMWF/GFS/FV3 SERVED AS A GOOD  
FORECAST STARTING POINT. CONFIDENCE WITH RESPECT TO THE SOUTHERN  
STREAM SYSTEM IS RELATIVELY GOOD THROUGH SAT NIGHT AS THE SURFACE  
LOW MOVES ACROSS THE GULF COAST STATES AND MODEL/ENSEMBLE  
SOLUTIONS ARE RELATIVELY WELL-CLUSTERED. FORECAST CONFIDENCE  
DECLINES AFTER 12Z SUN AS TO WHETHER THE LOW PRESSURE SYSTEM TAKES  
A MORE EASTERLY TREK OUT TO SEA OR NORTHEASTERLY CLOSE TO THE  
COAST, OR PERHAPS EVEN SLOWS/STALLS FOR A PERIOD OF TIME. THE  
EVENTUAL OUTCOME SEEMS TO HINGE LARGELY ON INTERACTION WITH A  
RELATIVELY SMALL-SCALE NORTHERN STREAM SHORTWAVE CURRENTLY WELL  
OUT OVER THE DATA SPARSE NORTH PACIFIC, EVIDENT ON WATER VAPOR  
SATELLITE IMAGERY NEAR 31N/175W. THIS FEATURE WILL DEAMPLIFY AS IT  
ROUNDS THE APEX OF THE WESTERN CANADA RIDGE BEFORE AMPLIFYING ONCE  
AGAIN AS IT ENTERS CYCLONIC FLOW ACROSS CENTRAL CANADA FRI-SAT.  
SOME CONSENSUS HAS EMERGED AS TO THE EVENTUAL TRACK OF THE  
SOUTHEAST U.S. LOW PRESSURE SYSTEM AFTER SUN, WITH SOME DEGREE OF  
ENSEMBLE CLUSTERING AROUND A LOW PERHAPS 50-75 MILES EAST OF THE  
OUTER BANKS BY MON MORNING. THIS SHOULD BE TAKEN SOMEWHAT WITH A  
GRAIN OF SALT, HOWEVER, GIVEN THE CURRENT LOCATION OF THE CRITICAL  
FEATURE AND THE COMPLEX DEGREE OF INTERACTION THAT WILL OCCUR WITH  
THE SOUTHERN STREAM SYSTEM. CONFIDENCE IS A BIT BETTER ALONG THE  
WEST COAST BY EARLY NEXT WEEK WITH BROAD CONSENSUS THAT STRONG  
HEIGHT FALLS SHOULD ARRIVE BY SUN-MON. WHILE TIMING IS SIMILAR  
ACROSS MOST GUIDANCE, SOLUTIONS CONTINUE TO DIFFER AS TO HOW  
AMPLIFIED THE TROUGH WILL BE. WITH THESE FACTORS CONSIDERED,  
INCREASED WEIGHT WAS PLACED TOWARD ENSEMBLE MEANS (ECENS/GEFS)  
AFTER SUN, WITH ALMOST EXCLUSIVE USE OF ENSEMBLES BY DAY 7 (TUE).  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE SYSTEM CROSSING THE SOUTHERN U.S. IS EXPECTED TO PRODUCE  
WIDESPREAD PRECIPITATION NORTH OF A SURFACE WARM FRONT AS ABUNDANT  
GULF OF MEXICO MOISTURE STREAMS NORTHWARD. MODELS CONTINUE TO SHOW  
A SIGNAL FOR POTENTIALLY HEAVY RAINFALL FROM PORTIONS OF THE  
SOUTHERN PLAINS TO THE LOWER MISSISSIPPI VALLEY ON FRI INTO EARLY  
SAT. ON THE NORTHERN PERIPHERY OF THE PRECIPITATION SHIELD,  
SUFFICIENT COLD AIR SHOULD BE IN PLACE TO PRODUCE WINTRY  
PRECIPITATION, WITH SOME AREAS SEEING POTENTIAL ACCUMULATIONS OF  
SNOW AND/OR ICE. AS THE SYSTEM SHIFTS EASTWARD SAT INTO SUN,  
PRECIPITATION WILL ALSO FOCUS ACROSS AREAS OF THE SOUTHEAST,  
TENNESSEE VALLEY, AND PERHAPS PORTIONS OF THE SOUTHERN  
MID-ATLANTIC REGION. MODELS HAVE SHOWN A PERSISTENT SIGNAL FOR A  
POTENTIALLY SIGNIFICANT WINTER WEATHER EVENT FOR PORTIONS OF THE  
SOUTHERN APPALACHIANS LATE SAT INTO SUN. DEPENDING ON THE EVENTUAL  
TRACK AND INTENSITY OF THE LOW PRESSURE SYSTEM, ACCUMULATING SNOWS  
MAY ALSO BE POSSIBLE EAST OF THE APPALACHIANS FROM THE CAROLINAS  
NORTH INTO THE SOUTHERN MID-ATLANTIC STATES, ALTHOUGH CONFIDENCE  
IN THIS ASPECT OF THE FORECAST REMAINS RELATIVELY LOW. SEE DAY 4-7  
WINTER WEATHER OUTLOOKS FOR FURTHER DETAILS ON THE WINTER WEATHER  
THREAT ASSOCIATED WITH THIS SYSTEM. ALONG THE WEST COAST, EXPECT  
AN INCREASE IN PRECIPITATION FROM SUN ONWARD AS HEIGHTS BEGIN TO  
FALL AND A COLD FRONT APPROACHES. CURRENT INDICATIONS ARE THAT  
AREAS FROM NORTHERN CALIFORNIA NORTH INTO COASTAL OREGON AND  
WASHINGTON WILL SEE THE POTENTIAL FOR THE HEAVIEST PRECIPITATION  
AMOUNTS (RAIN AND MOUNTAIN SNOW) THROUGH EARLY NEXT WEEK.  
 
BELOW AVERAGE TEMPERATURES WILL PREVAIL ACROSS MUCH OF THE CENTRAL  
AND EASTERN U.S. DURING THE MEDIUM RANGE, WITH HIGH TEMPERATURES  
RANGING FROM 5 TO 15 DEG F BELOW AVERAGE FOR MANY AREAS. THE  
ROCKIES WILL SEE GRADUALLY WARMING TEMPERATURES 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 FROM SUN  
ONWARD.  
 
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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