456  
FXUS02 KWBC 140708  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
208 AM EST THU DEC 14 2017  
 
VALID 12Z SUN DEC 17 2017 - 12Z THU DEC 21 2017  
   
..OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODELS AND ENSEMBLES OFFER GREATER THAN NORMAL FORECAST SPREAD AND  
UNCERTAINTY FOR THE 3-7 DAY FORECAST PERIOD. THERE ARE TWO MAIN  
FORECAST CLUSTER SCENARIOS EVIDENT. THE MAIN FORECAST PROBLEM  
EMINATES FROM THE DIFFERENT HANDLING OF SHORT RANGE BASED TROUGH  
ENERIES DIGGING ALOFT OVER THE WRN US. RECENT GFS/CANADIAN RUNS  
AND TO A LESSER EXTENT GEFS/NAEFS MEANS DEVELOP A PHASED/AMPLIFIED  
SWRN US UPPER TROUGH THIS WEEKEND. THIS SRN STREAM SPLIT FEATURE  
AND ASSOCIATED SURFACE SYSTEM EJECTS ACROSS THE US SRN TIER  
MON-FRI TO FOCUS MODERATE TO HEAVY RAINS/THUNDERSTORMS OVER THE  
GULF COAST STATES/SERN US/SRN MID-ATLANTIC AS GULF MOISTURE IS  
ADVECTS INLAND. MEANWHILE IN THIS SCENARIO...SEPARATE NRN STREAM  
FLOW MID-UPPER LEVEL TROUGH ENERGIES AND ASSOCIATED AND PCPN  
SWATHS WITH MORE COLD AIR TO WORK WITH PROGRESS OVERTOP THEN TEND  
TO REMERGE TO SOME DEGREE WITH EJECTING SRN STREAM FLOW OVER THE  
E-CENTRAL STATES EARLY-MID NEXT WEEK BEFORE FULLY REDOMINATING  
LATER WEEK. IN STARK CONTRAST...RECENT ECMWF RUNS AND TO A LESSER  
EXTEND ECMWF ENSEMBLE MEANS SHOW MORE WEEKEND SPLIT FLOW OVER THE  
WEST AND SPLIT SOME TROUGHING BACK OVER THE ERN PACIFIC AND ALLOWS  
MORE PROGRESSIVE TROUGHING/PCPN SWATH SPLIT RAPIDLY EASTWARD  
CENTRAL AND ERN US MID-LATITUDES MON-TUE. IT DOES NOT SEEM  
REASONABLE AMID UNCERTAINTY TO FAVOR EITHER CAMP OF THOUGHT. A  
BLEND OF SOMEWHAT MORE COMPATABLE GEFS/NAEFS/ECMWF ENSEMBLES  
PROVIDES A COMPROMISE SOLUTION THAT SEEMS TO TAKE INTO ACCOUNT  
SOME OF EACH CAMP AND MAINTAINS MAX WPC FORECAST CONTINUITY. THE  
WPC PRODUCT SUITE WAS DERIVED FROM THAT BLEND. LATEST 00 UTC  
GFS/ECMWF GUIDANCE DOES SEEM TO BETTER SOLUTION CONVERGE/TIMING  
WED-THU SHOWING INCREASINGLY WET FLOW INTO/ACROSS THE NWRN  
STATES...INCLUDING SOME HEAVIER PAC NW RAINS...WITH ALSO BOLSTERED  
FORECAST CONFIDENCE IN A GENERAL SUBSEQUENT INLAND WINTERY THREAT  
TRACK ACROSS THE WRN US TO THE ROCKIES BY NEXT THU.  
 
SCHICHTEL  
 

 
 
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