470  
FXUS02 KWBC 060658  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
157 AM EST THU DEC 06 2018  
 
VALID 12Z SUN DEC 09 2018 - 12Z THU DEC 13 2018  
 
...SIGNIFICANT WINTER WEATHER EVENT POSSIBLE 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 POTENTIAL 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 AN 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.  
 
STRONG HEIGHT FALLS ARE EXPECTED TO REACH THE WEST COAST ON MON,  
AS THE FIRST IN A RAPID SERIES OF SYSTEMS TO 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 SEEM TO 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). THE  
CRITICAL NORTHERN STREAM WAVE REMAINED OVER THE NORTHEAST PACIFIC  
OCEAN AS OF THE 00Z MODEL INITIALIZATION, AND SHOULD MOVE INTO  
BRITISH COLUMBIA LATER TODAY, HOPEFULLY CONTRIBUTING TO A  
REDUCTION IN SPREAD IN THE GUIDANCE. A BLEND OF DETERMINISTIC  
GUIDANCE (ECMWF/GFS/FV3/UKMET) WAS USED AS A STARTING POINT FOR  
THE FORECAST ON DAYS 3-4 (SUN-MON), EXCLUDING THE 12Z CMC WHICH  
WAS OVERLY AMPLIFIED WITH THE NORTHERN STREAM ENERGY, RESULTING IN  
A MUCH FARTHER NORTH TRACK OF THE SURFACE LOW ALONG THE SOUTHEAST  
U.S. COAST. THE RESULTING FORECAST SHOWS A SURFACE LOW TRACK CLOSE  
TO/ALONG THE GULF STREAM, A TRACK WHICH IS FAIRLY CLIMATOLOGICAL.  
 
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).  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE LOW PRESSURE SYSTEM ACROSS THE SOUTHEAST SUN-MON IS EXPECTED  
TO PRODUCE A POTENTIALLY 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 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.  
 
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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