731  
FXUS02 KWBC 131906  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
205 PM EST WED DEC 13 2023  
 
VALID 12Z SAT DEC 16 2023 - 12Z WED DEC 20 2023  
 
...A WET PATTERN IS LIKELY OVER THE SOUTH/SOUTHEAST AND  
POTENTIALLY UP THE EAST COAST THIS WEEKEND INTO NEXT WEEK...  
 
   
..OVERVIEW
 
 
A SOUTHERN STREAM UPPER LOW TRAVERSING THE SOUTH-CENTRAL TO  
SOUTHEASTERN U.S. INTERACTING WITH NORTHERN STREAM ENERGY THIS  
WEEKEND INTO EARLY NEXT WEEK WILL PROVIDE SUPPORT FOR A SURFACE  
LOW TO STRENGTHEN AND SPREAD WIDESPREAD HEAVY RAINFALL ACROSS  
FLORIDA ON SATURDAY. HEAVY RAIN MAY SPREAD UP THE EAST COAST INTO  
EARLY NEXT WEEK, BUT ITS PLACEMENT REMAINS QUITE UNCERTAIN GIVEN  
MODEL DIFFERENCES IN THE UPPER LEVELS AND THE SURFACE LOW TRACK.  
BEHIND THESE FEATURES, DRY UPPER RIDGING WILL BUILD INTO THE  
INTERIOR WEST AHEAD OF A MEAN EASTERN PACIFIC TROUGH AXIS THAT  
SHOULD PUSH PERIODIC PRECIPITATION INTO THE WEST COAST STATES.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THERE IS FAIRLY GOOD AGREEMENT ON THE TWO STREAMS OF ENERGY ACROSS  
THE MIDWEST AND THE SOUTH, THE INTERACTION OF WHICH SHOULD INDUCE  
CYCLOGENESIS ACROSS THE SOUTHEAST GULF OF MEXICO ON SATURDAY WHICH  
TRACKS ACROSS FLORIDA INTO SUNDAY. THERE REMAINS HOWEVER A LOT OF  
UNCERTAINTY IN THE DETAILS OF THIS INTERACTION, AND ALSO WITH  
ADDITIONAL WEAKER ENERGY IN THE GULF. THE MODELS STRUGGLE RUN TO  
RUN AND MODEL TO MODEL ON THE INTERACTION BETWEEN THESE THREE  
FEATURES WHICH HAVE HUGE IMPLICATIONS FOR EXACT SURFACE LOW  
TRACK/PLACEMENT AND HEAVY RAINFALL POTENTIAL ACROSS THE SOUTHEAST  
AND INTO THE MID-ATLANTIC AND THE NORTHEAST AS WELL. BASED ON THE  
12Z GUIDANCE FOR TODAY THOUGH, THERE DOES SEEM TO BE SOME TREND  
TOWARDS A SURFACE LOW TRACK CLOSER TO THE COAST (OR POSSIBLY EVEN  
INLAND) ACROSS THE SOUTHEAST AND MID-ATLANTIC. BY MONDAY-TUESDAY,  
ADDITIONAL ENERGY DROPPING IN FROM THE MIDWEST LOOKS TO REINFORCE  
SOME KIND OF TROUGHING STILL OFF THE SOUTHEAST COAST, WITH SOME  
SUPPORT FOR SURFACE LOW PRESSURE TO LINGER OFF THE COAST INTO NEXT  
WEEK. THE 06Z GFS FAVORED THIS SOLUTION MOST, BUT ITS NEW 12Z RUN  
TODAY BACKED OFF ON THIS IDEA. THE CMC AND ECMWF SHOW THE LOW  
RACING NORTHWARD UP THE EAST COAST. THE VARIOUS ENSEMBLE GUIDANCE  
OFFERS A LOT OF SPREAD ALL THE WAY UP THE EAST COAST, BUT BOTH THE  
EC AND GEFS MEANS DO SUPPORT SOME SORT OF WEAK AREA OF LOW  
PRESSURE LINGERING OFF THE COAST. THE WPC FORECAST FOR TODAY  
TRENDED MOSTLY TOWARDS THE ENSEMBLE MEANS THE SECOND HALF OF THE  
PERIOD, WITH SOME BLENDING OF THE PREVIOUS SHIFT CONTINUITY TOO,  
BUT THIS IS A VERY LOW CONFIDENCE FORECAST AND HIGHLY DEPENDING ON  
EXACT STREAM/ENERGY INTERACTIONS WHICH COULD TAKE UNTIL THE SHORT  
RANGE PERIOD TO FULLY RESOLVE.  
 
OUT WEST, THERE IS A LOT OF UNCERTAINTY WITH TROUGHING OFF THE  
WEST COAST, BUT THE 12Z GUIDANCE FOR TODAY DOES SHOW BETTER  
SUPPORT FOR A CUT OFF UPPER LOW DEEPENING AS IT MOVES TOWARDS THE  
CALIFORNIA COAST MID-NEXT WEEK. LOTS OF QUESTIONS STILL ON  
PLACEMENT OF THIS FEATURE AND EFFECTS ON HEAVY RAINFALL POTENTIAL  
ACROSS PARTS OF CALIFORNIA AS WELL.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
AMPLE MOISTURE WITH PRECIPITABLE WATER VALUES LIKELY WELL OVER THE  
90TH PERCENTILE WILL BE IN PLACE ACROSS FLORIDA AS THE SOUTHERN  
STREAM UPPER LOW ENHANCES LIFT AND DEVELOPS A SURFACE LOW. THESE  
FACTORS WILL LEAD TO WIDESPREAD HEAVY RAINFALL OF 2-4 INCHES WITH  
LOCALLY HIGHER AMOUNTS LIKELY. A SLIGHT RISK CONTINUES ON DAY  
4/SATURDAY IN THE EXCESSIVE RAINFALL OUTLOOK, AS RAINFALL OF THIS  
MAGNITUDE WITH SOME HIGH RAIN RATES COULD CAUSE INSTANCES OF FLASH  
FLOODING. HEAVY RAIN AND FLOODING MAY BE A CONCERN FOR THE LOWER  
MISSISSIPPI VALLEY AS WELL ON SATURDAY IN THE VICINITY OF ANOTHER  
FRONTAL SYSTEM. A MARGINAL RISK CONTINUES TO COVER THIS POTENTIAL  
AS RECENT MODELS HAVE TRENDED UP WITH RAINFALL ONSHORE, THOUGH  
THERE IS STILL A CHANCE THAT MOST COULD FALL OFFSHORE IN THE  
CENTRAL GULF. BY SUNDAY, INCREASING MODEL SPREAD MAKES RAINFALL  
TOTALS AND PLACEMENT VERY UNCERTAIN. THE WPC PRECIPITATION  
FORECAST IS REFLECTIVE OF THE MODEL PREFERENCE FOR A LOW MOVING  
NORTHEASTWARD INTO THE FAR WESTERN ATLANTIC, FOCUSING RAINFALL  
FROM GEORGIA INTO THE CAROLINAS AND THE MID-ATLANTIC BUT THE  
LATEST 12Z GUIDANCE SHOWS BETTER SUPPORT FOR HEAVIER RAINFALL EVEN  
FARTHER INLAND. FOR THE DAY 5 ERO, THERE WAS ENOUGH AGREEMENT (AND  
SUPPORT FROM WFOS ILM, MHX, AND AKQ) FOR A SLIGHT RISK ACROSS FAR  
EASTERN NORTH CAROLINA TO SOUTHEAST VIRGINIA. A MARGINAL RISK  
SURROUNDS THIS EXTENDING FROM NORTHEAST FLORIDA, ACROSS MUCH OF  
THE SOUTHEAST, AND INTO THE EASTERN MID-ATLANTIC REGION WHERE THE  
INLAND EXTENT FOR HEAVY RAIN IMPACTS ARE MUCH MORE UNCERTAIN.  
HEAVY RAIN COULD ALSO SPREAD INTO THE THE NORTHEAST ON MONDAY  
DEPENDING ON THE SURFACE LOW TRACK, WITH MODELS AS OF NOW SHOWING  
SOME AGREEMENT ON THIS POTENTIAL ACROSS EASTERN NEW ENGLAND.  
 
PRECIPITATION CHANCES WILL BE INCREASING OVER THE WEEKEND FOR  
CALIFORNIA AS SHORTWAVE ENERGY SUPPORTING A POSSIBLE ATMOSPHERIC  
RIVER APPROACHES. RAIN AMOUNTS AND RATES ON SUNDAY COULD BE  
SUFFICIENT TO CAUSE SOME FLOODING CONCERNS, ESPECIALLY GIVEN SOME  
INSTABILITY COULD REACH COASTAL AREAS, SO A MARGINAL RISK IS IN  
PLACE IN THE ERO. PRECIPITATION LOOKS TO PERSIST ACROSS CALIFORNIA  
AND REACH THE PACIFIC NORTHWEST INTO MONDAY-TUESDAY, INCLUDING  
SOME HIGHER ELEVATION SNOW, THOUGH CONFIDENCE IN THE DETAILS  
REMAINS SOMEWHAT LOW. THE INTERIOR WEST TO THE PLAINS WILL BE THE  
DRY SPOT OVER THE LOWER 48 THROUGH THE FIRST HALF OF NEXT WEEK.  
 
THE NORTHERN TWO-THIRDS OF THE LOWER 48 SHOULD SEE NEAR TO ABOVE  
NORMAL TEMPERATURES THIS WEEKEND INTO EARLY NEXT WEEK, WITH  
TEMPERATURES 10-20F ABOVE AVERAGE PERIODICALLY AFFECTING THE  
NORTHERN/CENTRAL HIGH PLAINS INTO THE UPPER MIDWEST. THE WEST CAN  
ALSO EXPECT GENERALLY ABOVE NORMAL TEMPERATURES UNDERNEATH UPPER  
RIDGING. COOLER THAN NORMAL TEMPERATURES WILL BE LIMITED TO THE  
SOUTHERN PLAINS (IN TERMS OF HIGHS) EASTWARD INTO THE SOUTHEAST  
THIS WEEKEND, MODERATING CLOSER TO NORMAL BY EARLY NEXT WEEK,  
THOUGH FLORIDA COULD STILL SEE BELOW NORMAL HIGHS BEHIND MULTIPLE  
FRONTAL PASSAGES.  
 
SANTORELLI/TATE  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION FORECAST, EXCESSIVE RAINFALL OUTLOOK,  
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY  
MESSAGES ARE 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/#PAGE=ERO  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW  
 

 
 

 
 
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