769  
FXUS02 KWBC 280700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST WED NOV 28 2018  
 
VALID 12Z SAT DEC 1 2018 - 12Z WED DEC 5 2018  
 
GENERAL PATTERN OVERVIEW  
------------------------  
 
THE FORECAST PERIOD FROM THIS WEEKEND THROUGH WEDNESDAY WILL BE  
ACTIVE WITH TWO DOMINANT SYSTEMS OF INTEREST. ONE VIGOROUS  
SHORTWAVE OVER THE CENTRAL PLAINS ON SATURDAY WILL PROGRESS  
NORTHEASTWARD TOWARDS THE GREAT LAKES, ACCOMPANIED BY A STRONG  
SURFACE LOW PRESSURE SYSTEM. ANOTHER STORM SYSTEM WILL REACH THE  
PACIFIC NORTHWEST BY SATURDAY, AND ASSOCIATED UPPER TROUGHING  
SHOULD AMPLIFY OVER THE WEST DURING THE WEEKEND AS THIS FEATURE  
DROPS TOWARDS THE SOUTHEAST, FOLLOWED BY EJECTION OF LEADING  
ENERGY WHICH MAY GENERATE ANOTHER SURFACE LOW OVER THE CENTRAL  
U.S. BY NEXT TUESDAY. DURING THE FIRST HALF OF NEXT WEEK THE  
SPECIFICS OF THE EASTERN PACIFIC PATTERN BECOME SOMEWHAT UNCERTAIN  
WITH INDIVIDUAL MODELS/ENSEMBLES DIFFERING OVER HOW FLOW MAY  
SEPARATE WITHIN THE OVERALL TROUGH THAT WILL BE HEADING INTO THE  
UPPER RIDGE THAT APPROACHES THE WEST COAST.  
 
MODEL GUIDANCE EVALUATION AND PREFERENCES  
-----------------------------------------  
 
MODELS AND ENSEMBLE MEANS ARE IN RELATIVELY GOOD AGREEMENT ON  
SATURDAY ACROSS MOST OF THE CONTINENTAL U.S., INCLUDING THE  
REPRESENTATION OF THE TWO MAIN STORM SYSTEMS THAT WILL AFFECT THE  
U.S. DURING THIS FORECAST PERIOD. HOWEVER, MODEL SPREAD INCREASES  
SIGNIFICANTLY STARTING AS EARLY AS SUNDAY REGARDING THE EVOLUTION  
OF THE SECOND LOW PRESSURE SYSTEM OVER THE WESTERN U.S. WITH THE  
12Z ECMWF DEPICTING CONSIDERABLY MORE UPPER LEVEL RIDGING OVER THE  
SOUTHWESTERN U.S. COMPARED TO THE MODEL CONSENSUS, AND THIS  
BECOMES EVEN MORE APPARENT BY MONDAY MORNING. THIS HAS MAJOR  
IMPLICATIONS ON THE SURFACE CYCLOGENESIS AND EVENTUAL LOW TRACK  
ACROSS THE CENTRAL AND EASTERN U.S. FOR THE FIRST HALF OF NEXT  
WEEK. THE 12Z ECMWF IS INDICATING A BUILDING SURFACE HIGH OVER  
MUCH OF THE CENTRAL U.S. BY THE DAY 5 PERIOD ON MONDAY THAT IS  
WILDLY DIFFERENT FROM ITS PREVIOUS 00Z RUN, WHEREAS THE  
CMC/GFS/JMA/00Z TUESDAY ECMWF ARE SUPPORTING THE IDEA OF A LOW  
PRESSURE SYSTEM THAT WILL LIFT TOWARDS THE NORTHEAST. THERE HAS  
BEEN MORE RUN-TO-RUN CONSISTENCY WITH THE PAST FEW RUNS OF THE GFS  
WITH THIS FEATURE.  
 
TAKING THESE CONSIDERATIONS INTO ACCOUNT, THE 00Z TUESDAY RUN OF  
THE ECMWF WAS SUBSTITUTED FOR THE 12Z RUN FOR THE ENTIRE FORECAST  
PERIOD SINCE IT IS CLOSER TO BOTH WPC CONTINUITY AND THE EXISTING  
MODEL CONSENSUS. THE FIRST HALF OF THE FORECAST INCORPORATED  
MAINLY A BLEND OF THE PREVIOUS ECMWF, CMC, AND GFS, AND THE SECOND  
HALF LEANING MORE TOWARD THE GEFS MEAN ALONG WITH SOME OF THE 00Z  
EC MEAN AND 18Z GFS FOR PLACEMENT OF MAIN FEATURES. GIVEN THE  
UNCERTAINTY REGARDING THE SECOND LOW PRESSURE SYSTEM OVER THE  
SOUTH-CENTRAL U.S. EARLY NEXT WEEK, NOTEWORTHY REVISIONS TO THE  
FORECAST MAY BE REQUIRED IF MORE MODELS TREND IN THE DIRECTION OF  
THE 12Z TUESDAY ECMWF, AND CONFIDENCE IS QUITE LIMITED FOR THIS  
PORTION OF THE FORECAST.  
 
SENSIBLE WEATHER AND HAZARDS  
----------------------------  
 
AN ACTIVE WEATHER PATTERN IS EXPECTED ACROSS THE EASTERN HALF OF  
THE NATION THIS WEEKEND AS THE STRONG SURFACE LOW TRACKS  
NORTHEASTWARD TOWARDS THE GREAT LAKES AND THE NORTHEAST. NUMEROUS  
SHOWERS AND THUNDERSTORMS ARE FORECAST TO DEVELOP IN THE WARM  
SECTOR OF THIS LOW FROM THE CENTRAL GULF COAST TO THE OHIO VALLEY,  
WITH THE POTENTIAL FOR A FEW STRONG STORMS. HEAVY RAINFALL ACROSS  
PARTS OF THE DEEP SOUTH AND NORTHERN FLORIDA MAY LEAD TO SOME  
LOCALIZED FLOODING CONCERNS. FARTHER TO THE NORTH ACROSS THE  
NORTHERN PLAINS AND THE UPPER MIDWEST, A BAND OF MODERATE TO HEAVY  
SNOW IS EXPECTED TO DEVELOP WITHIN THE DEFORMATION ZONE ON THE  
NORTHWEST SIDE OF THE LOW, WITH SOME LOCATIONS POSSIBLY GETTING  
OVER SIX INCHES OF ACCUMULATION BEFORE THE STORM SYSTEM DEPARTS  
THE REGION. THERE MAY ALSO BE ADDITIONAL IMPACTS FROM THE NEXT  
SYSTEM TRACKING THROUGH THE SOUTH-CENTRAL U.S. WITH HEAVY RAIN AND  
STRONG STORMS NEAR THE GULF COAST AND INTO THE SOUTHEAST STATES,  
BUT GIVEN THE UNCERTAINTY IN THE EVOLUTION OF THAT SECOND SYSTEM,  
PREDICTABILITY REMAINS LIMITED REGARDING THE MAGNITUDE AND  
PLACEMENT OF THE HEAVIEST PRECIPITATION. A THIRD SYSTEM  
APPROACHING CALIFORNIA BY THE END OF THE FORECAST PERIOD NEXT  
WEDNESDAY SHOULD BRING ANOTHER ROUND OF RAIN AND MOUNTAIN SNOW  
FROM CENTRAL CALIFORNIA TO WESTERN WASHINGTON.  
 
IN TERMS OF TEMPERATURES, READINGS ARE EXPECTED TO BE ON THE ORDER  
OF 10 TO 20 DEGREES ABOVE NORMAL AHEAD OF THE WEEKEND STORM SYSTEM  
FROM THE GULF COAST TO THE GREAT LAKES ON SATURDAY, AND THEN 5 TO  
15 DEGREES ABOVE NORMAL FOR THE EAST COAST ON SUNDAY. MUCH COLDER  
WEATHER WILL SETTLE IN ACROSS THE WESTERN U.S. COURTESY OF THE  
BUILDING UPPER LEVEL TROUGH, WITH ANOMALIES ON THE ORDER OF 10 TO  
20 DEGREES BELOW NORMAL TO BEGIN THE WORK WEEK, LEADING TO SOME  
SUBZERO OVERNIGHT LOWS IN THE NORMALLY COLDER LOCATIONS. WITH THE  
EXCEPTION OF FLORIDA, MOST OF THE CONTINENTAL U.S. SHOULD HAVE  
BELOW AVERAGE TEMPERATURES BY THE MIDDLE OF NEXT WEEK.  
 
D. HAMRICK  
 
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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