417  
FXUS02 KWBC 061544  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1044 AM EST THU DEC 06 2018  
 
VALID 12Z SUN DEC 09 2018 - 12Z THU DEC 13 2018  
 
...SIGNIFICANT WINTER WEATHER EVENT LIKELY FOR PORTIONS OF THE  
SOUTHERN APPALACHIANS AND FOOTHILLS THIS WEEKEND...  
 
   
..OVERVIEW
 
 
INTERACTION BETWEEN NORTHERN AND SOUTHERN STREAM SHORTWAVES  
ADVERTISED FOR DAYS SHOULD BE ONGOING ON DAY 3 (SUN) AS SURFACE  
LOW PRESSURE CROSSES THE FLORIDA PANHANDLE AND MOVES TO OFF THE  
OUTER BANKS BY MON MORNING, PRODUCING A WIDESPREAD PRECIPITATION  
(RAIN AND SNOW) EVENT ACROSS PORTIONS OF THE SOUTHEAST AND  
SOUTHERN MID-ATLANTIC STATES. A SECOND WAVE OF LOW PRESSURE (IN  
RESPONSE TO THE AMPLIFYING NORTHERN STREAM WAVE ACROSS THE  
SOUTHEAST) MAY LINGER OFF THE EASTERN SEABOARD THROUGH MON BEFORE  
EJECTING QUICKLY OUT TO SEA ON TUE. IN TERMS OF THE BIG PICTURE,  
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 A PERIOD OF MUCH  
WETTER CONDITIONS FOR THE PACIFIC NORTHWEST THAN SEEN RECENTLY.  
 
STRONG HEIGHT FALLS ARE EXPECTED TO REACH THE WEST COAST ON MON,  
AS THE FIRST IN A RAPID SERIES OF SYSTEMS WILL AFFECT THE PACIFIC  
NORTHWEST IN PARTICULAR. THE TENDENCY MAY CONTINUE FOR ENERGY TO  
SPLIT INTO NORTHERN AND SOUTHERN STREAM COMPONENTS, WITH THE  
NORTHERN STREAM WAVES GENERALLY MORE PROGRESSIVE RELATIVE TO THE  
SOUTHERN STREAM. HEIGHT FALLS SHOULD REACH THE CENTRAL U.S. TUE  
NIGHT INTO WED, REINFORCED BY THE RAPID SUCCESSION OF NORTHERN  
STREAM ENERGY. A RESULTANT LOW PRESSURE SYSTEM WILL LIKELY DEVELOP  
ACROSS THE PLAINS BY THE MIDDLE OF NEXT WEEK.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS CONTINUE TO SHOW VARIABILITY WITH RESPECT TO BOTH TIMING  
AND AMPLITUDE OF NORTHERN AND SOUTHERN STREAM WAVES ACROSS THE  
CENTRAL/EASTERN U.S. SUN-MON, BUT THE DEGREE OF VOLATILITY IN  
SOLUTIONS SEEMS TO BE SLOWLY DECREASING. MODELS SHOW IMPROVED  
AGREEMENT ON THE INITIAL SURFACE LOW CROSSING FLORIDA AND MOVING  
OFF THE OUTER BANKS SUN-MON, WITH A SECOND SURFACE LOW DEVELOPING  
OFF THE CAROLINAS LATE MON IN RESPONSE TO THE NORTHERN STREAM WAVE  
(WHICH NEVER FULLY PHASES WITH THE LEADING WAVE). A BLEND OF  
DETERMINISTIC GUIDANCE (00Z ECMWF/GFS/FV3/UKMET) WAS USED AS A  
STARTING POINT FOR THE FORECAST ON DAYS 3-4 (SUN-MON). THE  
RESULTING FORECAST SHOWS A SURFACE LOW TRACK CLOSE TO/ALONG THE  
GULF STREAM OFF NC/SC AND THEN EAST-NORTHEASTWARD, SUPPRESSED BY  
SURFACED HIGH PRESSURE AND A SOUTHEASTERN CANADIAN TROUGH.  
 
SPREAD BEGINS TO INCREASE ACROSS THE WESTERN U.S. BY DAY 5 (TUE)  
AS THE INCOMING TROUGH SEPARATES INTO THE TWO STREAMS, WITH MODELS  
SHOWING THE MOST SPREAD IN THE PROGRESSIVE NORTHERN STREAM.  
DIFFERENCES ONLY GROW THROUGH TIME IN THE NORTHERN STREAM, WITH  
EVEN RUN-TO-RUN INCONSISTENCIES IN THE ECMWF AND THE GFS BY THE  
MIDDLE OF NEXT WEEK. AS A RESULT, WEIGHTING OF ECENS AND GEFS  
ENSEMBLE MEANS WAS BOOSTED STARTING ON DAY 5 WITH CONTINUED  
INCREASES THROUGH DAY 7 (THU) AS ADDITIONAL NORTHERN STREAM ENERGY  
WILL UNDOUBTEDLY CHANGE THE SHAPE OF THE UPPER TROUGH AND LOCATION  
OF SURFACE FRONTS.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE LOW PRESSURE SYSTEM ACROSS THE SOUTHEAST SUN-MON IS EXPECTED  
TO PRODUCE A SIGNIFICANT WINTER WEATHER EVENT (BEGINNING LATE IN  
THE SHORT RANGE FORECAST PERIOD) FOR PORTIONS OF THE SOUTHERN  
APPALACHIANS AND POTENTIALLY THE FOOTHILLS, WITH MULTI-INCH  
SNOWFALL ACCUMULATIONS POSSIBLE. SEE THE WPC DAY 1-3 PWPF GRAPHICS  
AND THEN THE 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 RAPID SUCCESSION OF  
SHORTWAVES/FRONTAL SYSTEMS APPROACH. CURRENT INDICATIONS ARE THAT  
THE HIGHER TERRAIN OF WESTERN OREGON AND WASHINGTON WILL SEE THE  
POTENTIAL FOR THE HEAVIEST PRECIPITATION AMOUNTS (RAIN AND  
MOUNTAIN SNOW) THROUGH THE MIDDLE OF 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. SUN-MON, WITH A SLOW MODERATION OF TEMPERATURES  
POSSIBLE THROUGH EARLY NEXT WEEK BEGINNING ACROSS THE NORTHERN  
HIGH PLAINS AND GRADUALLY SPREADING EASTWARD AS THE AFOREMENTIONED  
UPPER-LEVEL PATTERN CHANGE BEGINS TO RESTRICT THE FLOW OF POLAR  
AIR SOUTHWARD INTO THE CONUS. BY NEXT WED/THU, TEMPERATURES ARE  
FORECAST TO BE 5-10 DEG F ABOVE AVERAGE FROM EASTERN MONTANA  
THROUGH THE DAKOTAS INTO MINNESOTA.  
 
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  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page