013  
FXUS02 KWBC 041557  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1056 AM EST TUE DEC 04 2018  
 
VALID 12Z FRI DEC 07 2018 - 12Z TUE DEC 11 2018  
 
...SIGNIFICANT STORM SYSTEM TO BRING LOCALLY HEAVY RAIN AND  
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 NEAR THE WESTERN GULF COAST BY EARLY SATURDAY WITH A LARGE  
AREA OF POTENTIALLY MODEST AND LOCALLY HEAVY RAIN. THIS WILL THEN  
MOVE EASTWARD ACROSS THE GULF COAST STATES INTO EARLY SUNDAY AND  
SLOWLY DEEPEN. 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 JUST OFF THE EAST COAST ON MONDAY.  
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 AND USHERING IN A  
PERIOD OF STORMY CONDITIONS FOR THAT REGION.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS ARE REASONABLY WELL-CLUSTERED DURING THE FIRST COUPLE DAYS  
OF THE MEDIUM RANGE (FRI-SAT), AND A BLEND OF THE 00Z/06Z  
DETERMINISTIC RUNS 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 IN THE DETAILS AFTER  
12Z SUNDAY 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. STRONG SURFACE HIGH PRESSURE (1036-1040MB)  
ACROSS THE MIDWEST WILL LIKELY OUTRUN THE SURFACE LOW TO THE SOUTH  
WHICH WILL PLAY A ROLE IN HOW FAR NORTH THE PRECIPITATION SHIELD  
WILL GET (WORKING AGAINST THE TYPICALLY SEEN NW DRIFT IN  
DPROG/DT). 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 MODEST 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. WITH  
THESE FACTORS CONSIDERED, INCREASED WEIGHT WAS PLACED TOWARD  
ENSEMBLE MEANS (MOSTLY ECENS WITH SOME GEFS) AFTER SUN AS THE  
00Z/06Z GFS/GEFS LAGGED BEHIND THE 00Z ECMWF/CANADIAN/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 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 A COUPLE INCHES OF SNOW  
AND/OR SOME 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 AND  
POTENTIALLY SIGNIFICANT SNOW 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 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 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 INTO THE HIGH PLAINS WILL SEE GRADUALLY WARMING  
TEMPERATURES THROUGH THE WEEKEND AS UPPER RIDGING SLOWLY SHIFTS  
EAST.  
 
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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