341  
FXUS02 KWBC 241601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST MON DEC 24 2018  
 
VALID 12Z THU DEC 27 2018 - 12Z MON DEC 31 2018  
 
...SIGNIFICANT WINTER STORM TO MOVE INTO THE UPPER MIDWEST  
THURSDAY AND GREAT LAKES ON FRIDAY...  
   
..OVERVIEW
 
 
A POWERFUL AND IMPACTFUL WINTER STORM WILL TRACK OUT OF THE  
CENTRAL PLAINS AFTER EARLY THURSDAY AND PROCEED THROUGH THE GREAT  
LAKES BY FRIDAY, BRINGING HEAVY WIND-DRIVEN SNOW OVER SOME AREAS  
IN THE COLD SECTOR AND LOCALLY HEAVY CONVECTIVE RAINS ACROSS THE  
SOUTH. THE TRAILING FRONT WILL MOVE OFF THE  
NORTHEAST/MID-ATLANTIC COAST SATURDAY BUT LIKELY LINGER OVER THE  
SOUTHEAST U.S. AND NORTHERN GULF OF MEXICO. ANOTHER SYSTEM  
DIGGING THROUGH THE WEST/SOUTHWEST IN THE WAKE OF THE MIDWEST  
STORM WILL SPREAD PRECIPITATION (MOSTLY SNOW) THROUGH THE GREAT  
BASIN/SOUTHWEST AND CENTRAL/SOUTHERN ROCKIES. LATE IN THE PERIOD  
GUIDANCE BEGINS TO DIFFER SIGNIFICANTLY REGARDING HOW UPSTREAM  
NORTHERN PACIFIC ENERGY MAY AMPLIFY INTO THE WEST.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
EARLY IN THE PERIOD THE PRIMARY FORECAST ISSUE IS WITH THE STORM  
TRACKING NORTHEASTWARD FROM THE CENTRAL PLAINS. THE OPERATIONAL  
GFS/GEFS RUNS ARE SOMEWHAT WEAKER WITH THE DYNAMICS ALOFT LEADING  
TO A SOMEWHAT FARTHER SOUTH SURFACE LOW TRACK AS OF EARLY DAY 3  
THU AND THEN STRAY FASTER THAN CONSENSUS BY EARLY DAY 4 FRI. THE  
00Z FV3 GFS IS MORE SIMILAR TO THE MAJORITY SOLUTION. A SMALL  
COMPONENT OF GFS INPUT WOULD NOT BE UNREASONABLE THOUGH, IN LIGHT  
OF EARLIER GUIDANCE THAT HAD SKEWED A LITTLE FARTHER SOUTH WITH  
THE SYSTEM EARLY THU AND SOME UNCERTAINTY OVER NORTHERN STREAM  
INFLUENCE BY FRI.  
 
BY MID-LATE PERIOD GUIDANCE BECOMES INCREASINGLY DIVERGENT, WITH  
RESPECT TO PACIFIC TROUGH ENERGY THAT REACHES 150-160W LONGITUDE  
BY FRI AND LIKELY SEPARATES TO SOME DEGREE BEFORE HEADING TOWARD  
NORTH AMERICA--AS WELL AS INFLUENCE OF THIS EVOLUTION ON THE  
LEADING FOUR CORNERS/SOUTHWEST/NORTHWESTERN MEXICO UPPER TROUGH.  
THE 00Z GFS AND MANY OF ITS GEFS MEMBERS OFFER ONE EXTREME WITH A  
MORE PHASED PACIFIC TROUGH AND MUCH MORE PERSISTENT WESTERN  
CANADA/NORTHWESTERN U.S. RIDGE, IN CONTRAST TO THE ECMWF/CMC AND  
THEIR ENSEMBLES WHICH ARE QUITE AGGRESSIVE WITH WESTERN U.S.  
TROUGH AMPLIFICATION BY DAYS 6-7 SUN-MON. THE COMBINATION OF  
EC/CMC GUIDANCE TENDING TO PROVIDE A BETTER LEAD ON THE EASTERN  
PACIFIC/WESTERN U.S. PATTERN RECENTLY ALONG WITH THE 06Z GFS TREND  
IN THEIR DIRECTION WOULD SUGGEST FAVORING THE ECMWF/CMC AND THEIR  
ENSEMBLES. HOWEVER TELECONNECTIONS RELATIVE TO D+8 NEGATIVE  
HEIGHT ANOMALIES NEAR 180 LONGITUDE AND POSITIVE ANOMALIES NEAR  
THE BRITISH ISLES PRESENT THE RISK THAT THE EASTERN PACIFIC/NORTH  
AMERICA PATTERN COULD END UP BEING SHIFTED SOMEWHAT EASTWARD FROM  
WHAT THE MAJORITY CLUSTER IS SHOWING. THUS WOULD ULTIMATELY FAVOR  
TRENDING THE FORECAST IN THE ECMWF/CMC DIRECTION BUT WITH SOME 06Z  
GFS/00Z NAEFS INPUT, BOTH OF WHICH PROVIDE SOMEWHAT MORE  
COMPATIBILITY FOR BLENDING THAN EITHER 00Z OR 06Z GEFS MEAN RUNS.  
NEEDLESS TO SAY CONFIDENCE IS VERY LOW FOR SPECIFICS OF  
SOUTHWESTERN U.S. ENERGY THAT MAY EJECT LATE IN THE PERIOD, WITH  
ECMWF/CMC RUNS DIFFERING GREATLY AMONG EACH OTHER AND RUN TO RUN  
IN SPITE OF BEING MORE SIMILAR/CONSISTENT WITH UPSTREAM EVOLUTION.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
PRECIPITATION IN ASSOCIATION WITH THE MAJOR PLAINS/MIDWEST SYSTEM  
WILL PUSH EASTWARD THURSDAY WITH SNOW (HEAVY AT TIMES WITH  
INCREASING WIND AND POSSIBLE BLIZZARD CONDITIONS) MAKING TRAVEL  
DIFFICULT IF NOT IMPOSSIBLE. SEVERAL INCHES OF SNOW ARE LIKELY  
OVER A LARGE AREA WITH OVER A FOOT QUITE POSSIBLE FOR SOME AREAS,  
ESPECIALLY THROUGH PARTS OF THE NORTH-CENTRAL PLAINS, MINNESOTA,  
AND NORTHERN WISCONSIN/WESTERN U.P. OF MICHIGAN AS THE SURFACE LOW  
TRACKS TOWARD LAKE MICHIGAN. PLEASE CONSULT THE LATEST WPC WINTER  
WEATHER OUTLOOK PROBABILITIES FOR A DAILY GRAPHICAL DEPICTION OF  
THE BEST CHANCE OF SNOW. IN THE WARM SECTOR, RAINFALL COULD BECOME  
LOCALLY HEAVY IN THE SOUTH AS THE SYSTEM INFUSES GULF MOISTURE  
NORTHWARD UP AND OVER THE WARM FRONT. SEVERAL INCHES OF RAIN ARE  
POSSIBLE IN SOME AREAS WITH A MORE WIDESPREAD 1-2+" LIKELY. THE  
MAIN LOW SHOULD EXIT THROUGH SOUTHERN ONTARIO AND NORTHERN NEW  
ENGLAND WHERE SOME ONSET FROZEN PRECIPITATION (PERHAPS APPRECIABLE  
FREEZING RAIN AS WARM AIR ADVANCES OVER IN SITU LOWER-LEVEL COLD  
AIR) IS LIKELY. NEXT SYSTEM REACHING THE WEST ON THE HEELS OF THE  
LEAD STORM WILL BRING MORE DYNAMICALLY DRIVEN SNOW AND SOME LOWER  
ELEVATION RAIN FROM THE NORTHWEST AND GREAT BASIN INTO THE FOUR  
CORNERS REGION. ANOTHER COLD FRONT WILL ENTER THE PACIFIC  
NORTHWEST WITH LIMITED MOISTURE EAST OF THE CASCADES. STATIONARY  
FRONT LINGERING OVER THE SOUTHEAST WILL LIKELY PROVIDE A FOCUS FOR  
FURTHER RAINFALL OVER THE SOUTHEASTERN QUADRANT OF THE LOWER 48  
AND POSSIBLY WINTRY WEATHER ON THE EXTREME NORTHERN FRINGE OF THE  
MOISTURE SHIELD. WAVE DETAILS ALONG THIS FRONT BECOME INCREASINGLY  
UNCERTAIN TOWARD LATE WEEKEND/EARLY NEXT WEEK.  
 
TEMPERATURES WILL BE COOLER THAN AVERAGE IN THE WEST AND PLAINS  
BEHIND THE DEPARTING PLAINS/MIDWEST STORM, WITH COLDEST ANOMALIES  
(10-20F BELOW AVERAGE) LIKELY FOCUSED OVER THE HIGH PLAINS AND  
VICINITY). ON THE OTHER HAND THE STORM WILL PULL MUCH ABOVE  
AVERAGE WARMTH (PLUS 10-30F ANOMALIES) INTO AREAS FROM THE  
MISSISSIPPI VALLEY EASTWARD UNTIL THE COLD FRONT MOVES THROUGH.  
WITH REINFORCING TROUGHING INTO THE WEST LATER IN THE WEEK,  
TEMPERATURES WILL STAY BELOW AVERAGE FROM THE GREAT BASIN/ROCKIES  
INTO THE PLAINS, ESPECIALLY OVER NEWLY SNOW-COVERED AREAS. COOLER  
AIR WILL FILTER INTO THE EAST NEXT WEEKEND AS THE FRONT SINKS INTO  
THE GULF AND NORTHERN FLORIDA. CENTRAL/SOUTHERN FLORIDA WILL STAY  
UNDER THE INFLUENCE OF SUBTROPICAL RIDGING AND ABOVE AVERAGE  
TEMPERATURES.  
 
RAUSCH/FRACASSO  
 
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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