201  
FXUS02 KWBC 050614  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
113 AM EST WED DEC 05 2018  
 
VALID 12Z SAT DEC 08 2018 - 12Z WED DEC 12 2018  
 
...SIGNIFICANT STORM SYSTEM TO BRING HEAVY RAIN AND POTENTIAL  
WINTER WEATHER TO AREAS FROM THE SOUTHERN PLAINS TO THE EASTERN  
U.S...  
 
   
..OVERVIEW
 
 
SPLIT FLOW ACROSS THE CONUS AT THE START OF THE MEDIUM RANGE WILL  
SUPPORT A SIGNIFICANT LOW PRESSURE SYSTEM MOVING ALONG THE GULF  
COAST STATES. THE SYSTEM WILL LIKELY INTERACT WITH A NORTHERN  
STREAM SHORTWAVE LATE SUN INTO MON AS IT CROSSES THE COASTAL  
SOUTHEAST AND MOVES INTO THE ATLANTIC OCEAN, BEFORE FOLLOWING A  
GENERAL NORTHEASTWARD MOTION INTO EARLY NEXT WEEK OFF THE EASTERN  
SEABOARD. FARTHER WEST THE UPPER RIDGE WILL BEGIN TO MOVE EASTWARD  
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.  
 
ENSEMBLE GUIDANCE SUGGESTS PERHAPS A SIGNIFICANT PATTERN CHANGE  
ACROSS THE NORTHERN HEMISPHERE BEGINNING EARLY NEXT WEEK AND  
CONTINUING BEYOND THE FORECAST PERIOD. STRONG POSITIVE HEIGHT  
ANOMALIES WHICH HAVE BEEN PERSISTENT ACROSS SIBERIA FOR A COUPLE  
WEEKS SHOULD BEGIN TO BREAK DOWN, WITH DOWNSTREAM FLOW ACROSS THE  
NORTH PACIFIC BECOMING MUCH MORE PROGRESSIVE THAN SEEN RECENTLY.  
DEVELOPING NEGATIVE HEIGHT ANOMALIES ACROSS ALASKA AND INTO  
WESTERN NORTH AMERICA AS A RESULT OF THIS PATTERN CHANGE WILL  
FAVOR UPPER-LEVEL RIDGING CENTERED IN THE VICINITY OF HUDSON BAY,  
WHICH WOULD SIGNIFICANTLY REDUCE THE FLOW OF POLAR/ARCTIC AIR  
SOUTHWARD INTO THE CONUS, WHILE ALSO RESULTING IN PERIOD OF MUCH  
WETTER CONDITIONS FOR THE PACIFIC NORTHWEST THAN SEEN RECENTLY.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS REMAIN REASONABLY WELL-CLUSTERED DURING THE FIRST COUPLE  
DAYS OF THE FORECAST PERIOD (SAT-SUN), AND A BLEND OF THE 12Z/18Z  
DETERMINISTIC RUNS SERVED AS A GOOD FORECAST STARTING POINT.  
CONFIDENCE WITH RESPECT TO THE SOUTHERN STREAM SYSTEM IS  
RELATIVELY GOOD THROUGH SUN MORNING AS THE SURFACE LOW MOVES  
ACROSS THE GULF COAST STATES AND MODEL/ENSEMBLE SOLUTIONS ARE  
RELATIVELY WELL-CLUSTERED. FORECAST CONFIDENCE REMAINS A BIT LOWER  
AFTER 12Z SUN WHEN TIMING DIFFERENCES BETWEEN THE  
NORTHERN/SOUTHERN STREAM AMPLIFY. THE EVENTUAL OUTCOME SEEMS TO  
HINGE LARGELY ON INTERACTION WITH A RELATIVELY SMALL-SCALE  
NORTHERN STREAM SHORTWAVE CURRENTLY OVER THE NORTH PACIFIC. THIS  
FEATURE WILL DE-AMPLIFY AS IT ROUNDS THE APEX OF THE WESTERN  
CANADIAN RIDGE BEFORE AMPLIFYING ONCE AGAIN AS IT ENTERS CYCLONIC  
FLOW ACROSS CENTRAL CANADA FRI-SAT. A TRAILING SHORTWAVE (OF  
MID-LATITUDE PACIFIC ORIGIN) WILL BE YET ANOTHER SOURCE OF  
MODEL/FORECAST ERROR AS IT CATCHES UP AND OVERTAKES THE INITIAL  
NORTHERN STREAM SHORTWAVE, WHICH MAY ACT TO SLOW THE TROUGH AND  
TUG THE SYSTEM CLOSER TO THE COAST AS IT ATTEMPTS TO EXIT TO THE  
EAST-NORTHEAST OR NORTHEAST. TREND HAS BEEN FOR A SLOWER EXODUS  
NEXT MON/TUE BUT THAT MAY NOT BE ENOUGH TO OVERCOME THE COLD/DRY  
AIR TO THE NORTH. WITH ALL THE GIVEN UNKNOWNS, SPECIFICS WILL HAVE  
TO WAIT ANOTHER COUPLE OF FORECAST CYCLES BUT THE BEST AGREEMENT  
ON AT LEAST ACCUMULATING SNOWFALL LIES OVER SW VIRGINIA AND NW  
NORTH CAROLINA.  
 
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. THESE  
DIFFERENCES AMPLIFY BY TUE-WED AS THE SYSTEM CROSSES THE ROCKIES  
AND MOVES INTO THE CENTRAL U.S., WITH CENTRAL PLAINS CYCLOGENESIS  
LOOKING PROBABLE EVEN IF THE SPECIFICS ARE UNCLEAR. WITH THESE  
FACTORS CONSIDERED, INCREASED WEIGHT WAS PLACED TOWARD ENSEMBLE  
MEANS (ECENS/GEFS) MON ONWARD.  
 
   
..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 ACROSS PORTIONS OF THE  
LOWER MISSISSIPPI VALLEY AND SOUTHEAST SAT-SUN. ON THE NORTHERN  
PERIPHERY OF THE PRECIPITATION SHIELD, SUFFICIENT COLD AIR WILL BE  
IN PLACE TO PRODUCE WINTRY PRECIPITATION, WITH SOME AREAS SEEING  
ACCUMULATIONS OF SNOW AND/OR SOME ICE. 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 AND POTENTIALLY SIGNIFICANT SNOW MAY ALSO BE  
POSSIBLE EAST OF THE APPALACHIANS FROM THE CAROLINAS NORTH INTO  
THE SOUTHERN MID-ATLANTIC STATES. SEE THE WPC DAY 4-7 WINTER  
WEATHER OUTLOOKS FOR FURTHER PROBABILISTIC 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 (NEGATIVE ANOMALIES OF 5 TO 15 DEG F  
FOR HIGH TEMPERATURES) WILL PREVAIL ACROSS MUCH OF THE CENTRAL AND  
EASTERN U.S. SAT-SUN, WITH A SLOW MODERATION OF TEMPERATURES  
POSSIBLE BY EARLY NEXT WEEK BEGINNING ACROSS THE NORTHERN HIGH  
PLAINS AND GRADUALLY SPREADING EASTWARD IN ASSOCIATION WITH THE  
UPPER-LEVEL RIDGE.  
 
RYAN/FRACASSO  
 
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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