046  
FXUS02 KWBC 101601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST MON DEC 10 2018  
 
VALID 12Z THU DEC 13 2018 - 12Z MON DEC 17 2018  
   
..HEAVY PRECIPITATION THREAT FOR THE NORTHWEST
 
   
..HEAVY RAIN THREAT FOR THE LOWER MS VALLEY THROUGH THE EAST
 
 
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS AND ENSEMBLES THIS MORNING CONTINUE TO INDICATE THAT A LOW  
PRESSURE SYSTEM SHOULD BRING SIGNIFICANT PRECIPITATION ACROSS THE  
EASTERN HALF OF THE COUNTRY LATE THIS WEEK INTO THE WEEKEND.  
MEANWHILE, WEATHER SYSTEMS WILL MOVE RAPIDLY ACROSS THE PACIFIC  
NORTHWEST. CONSIDERABLE SPREAD IN THE GUIDANCE IS SHOWN REGARDING  
THE DEPTH AND INTENSITY OF THE LOW FORECAST TO MOVE ACROSS THE  
EASTERN U.S. THE ECMWF AND EC MEAN CONTINUE THE TENDENCY OF  
SLOWING DOWN THE LOW FOLLOWED BY CYCLOGENESIS JUST OFF THE EAST  
COAST. IN PARTICULAR, THE 00Z ECMWF SHOWED THE LOW CENTER  
POSITION LAGGING BEHIND THE 00Z EC MEAN POSITION BY A FEW HUNDRED  
MILES DURING THE WEEKEND! ON THE OTHER HAND, THE DISCREPANCY  
BETWEEN THE GFS AND GEFS LOW CENTER POSITIONS WERE MORE  
REASONABLE, PLACING THE LOW CENTER OVER THE MID-ATLANTIC ON  
SATURDAY, WHICH AGREE VERY WELL WITH THE 00Z EC MEAN POSITION AS  
WELL AS WPC CONTINUITY. THIS LENDS MORE CREDIBILITY TO THE  
GEFS/GFS SOLUTIONS REGARDING THIS LOW. BY DAY 7 MONDAY, THE ECMWF  
HAS BEEN INDICATING THE POSSIBILITY OF AN UPPER-LEVEL VORTICITY  
BREAKING OFF FROM A LONGWAVE TROUGH NEAR THE WEST COAST. THE GFS  
DID NOT INDICATE THAT BUT THE 06Z GFS BEGAN INDICATING THIS  
FEATURE. THEREFORE, THE WPC MEDIUM-RANGE MORNING GRID PACKAGE WAS  
DERIVED MAINLY FROM A BLEND OF 40% 00Z EC MEAN AND 60% 06Z GEFS,  
MIXING IN WITH VARIED PERCENTAGES OF THEIR DETERMINISTIC  
SOLUTIONS, WITH LITTLE OR NO 00Z ECMWF FOR DAYS 5 TO 7.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
PRECIPITATION/WIND WILL SUBSTANTIALLY INCREASE INTO THE NORTHWEST  
INTO CA FRI/SAT AND AGAIN BY MON/TUE AS HEIGHTS FALL WITH A  
SUCCESSION OF APPROACHING ENERGETIC PACIFIC SHORTWAVES/FRONTAL  
SYSTEMS. HIGHER PACIFIC NORTHWEST TERRAIN WILL OBSERVE THE  
HEAVIEST RAIN/MOUNTAIN SNOW (WITH VARYING SNOW LEVELS AS UPPER  
TEMPERATURES RISE AND FALL BETWEEN EACH SYSTEM), WITH ACTIVITY  
THEN WORKING TO THE NORTH-CENTRAL GREAT BASIN/ROCKIES.  
 
LEAD SHORTWAVE ENERGY DIGGING FROM THE ROCKIES TO THE SOUTHERN  
PLAINS THURSDAY WILL REINFORCE UPPER TROUGHING IN AN EMERGING  
SOUTHERN STREAM AND SPAWN SURFACE CYCLOGENESIS/FRONTOGENESIS.  
AMPLE GULF MOISTURE/WARM ADVECTION INCREASING AHEAD OF THE  
DEEPENING SYSTEM WILL HELP TO EXPAND THE PRECIPITATION SHIELD TO  
THE EAST AND ALSO WRAP TO THE NORTH OF THE SURFACE LOW.  
RELATIVELY SLOW MOVEMENT OF THE ORGANIZED SYSTEM AND FAVORABLE  
DYNAMICS SUPPORT A HEAVY RAIN THREAT FOR THE LOWER MS  
VALLEY/MID-SOUTH TO SOUTHEAST. HEAVY RAINFALL SPREADS TO THE EAST  
SIDE OF THE APPALACHIANS AND COASTAL PLAIN WITH ADDED INFUSION OF  
ATLANTIC MOISTURE. RUNOFF PROBLEMS WILL BE ENHANCED FOR AREAS  
WITH RECENT HEAVY SNOWFALL IN A LEAD-IN PATTERN WITH MODERATING  
TEMPERATURES. ICE IS POSSIBLE ACROSS THE CENTRAL APPALACHIANS  
THURSDAY NIGHT TO FRIDAY MORNING BEFORE THE DEEPENING LOW PULLS  
MILD AIR FROM THE SOUTH. SOME SNOW AND WINTER PRECIPITATION CAN  
BE EXPECTED TO WRAP BACK ACROSS THE MIDWEST/GREAT LAKES INTO  
INTERIOR NEW ENGLAND FROM THURSDAY AND INTO THE WEEKEND.  
 
KONG/SCHICHTEL  
 
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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