231  
FXUS02 KWBC 051549  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1048 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 WINTRY WEATHER  
TO AREAS OF THE SOUTHEAST AND 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 TO START THE WEEKEND. THE SYSTEM WILL LIKELY INTERACT  
WITH AND/OR BE INFLUENCED BY A NORTHERN STREAM SHORTWAVE LATE SUN  
INTO MON AS IT CROSSES THE COASTAL SOUTHEAST AND MOVES INTO THE  
ATLANTIC OCEAN. HOW DEEP IT GETS AND HOW CLOSE TO THE COAST IT  
WILL TRACK REMAIN TO BE IRONED OUT, BUT IT SHOULD GENERALLY EXIT  
NORTHEASTWARD 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 00Z/06Z  
DETERMINISTIC RUNS SERVED AS A GOOD FORECAST STARTING POINT.  
CONFIDENCE WITH RESPECT TO THE SOUTHERN STREAM SYSTEM WAS  
RELATIVELY GOOD THROUGH SUN MORNING AS THE SURFACE LOW MOVES  
ACROSS THE GULF COAST STATES AND MODEL/ENSEMBLE SOLUTIONS WERE  
RELATIVELY WELL-CLUSTERED. FORECAST CONFIDENCE REMAINS A BIT LOWER  
AFTER 12Z SUN WHEN TIMING DIFFERENCES (AND TRACK) BETWEEN THE  
NORTHERN/SOUTHERN STREAM AMPLIFY. THE EVENTUAL OUTCOME SEEMS TO  
HINGE LARGELY ON INTERACTION WITH A SMALL-SCALE BUT INCREASINGLY  
IMPORTANT NORTHERN STREAM SHORTWAVE CURRENTLY OVER THE NORTHEAST  
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. THE DEEPENING  
SHORTWAVE OR EVENTUAL UPPER LOW WILL LIKELY ACT TO SLOW THE  
SOUTHERN STREAM TROUGH TO THE EAST 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 PER THE  
ENSEMBLE TRENDS WHICH WERE QUITE NOTABLE. THE 00Z ECMWF/CANADIAN  
AND FV3-GFS WERE CLOSE TO THE 00Z ECMWF ENSEMBLE MEAN WHICH WAS  
THE GUIDE FOR THE SYSTEM MON-TUE AS IT SLOWLY TRACKS INTO THE  
ATLANTIC. THE HIGHEST IMPACT AREAS REMAIN OVER SOUTHWESTERN  
VIRGINIA INTO WESTERN/NORTHWESTERN NORTH CAROLINA (SNOWFALL) WITH  
A TIGHT GRADIENT TO THE NORTH UP TOWARD THE MASON-DIXON LINE.  
SMALL CHANGES IN THE TRACK WILL HAVE LARGE IMPACTS ON THE  
"FRINGES" OF THE PRECIPITATION SHIELD (ESPECIALLY TO THE NORTH  
WHERE THE PTYPE WILL LIKELY BE SNOW AT LEAST AWAY FROM THE COAST).  
 
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. DISCOUNTED THE  
VERY SLOW 00Z ECMWF OFFSHORE BAJA AS THE ENSEMBLES (ECENS/GEFS)  
WERE ALIGNED QUITE WELL ALONG WITH THE 00Z FV3-GFS.  
 
   
..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 EASTWARD INTO THE 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.  
 
FRACASSO/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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