091  
FXUS02 KWBC 190627  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
127 AM EST TUE DEC 19 2017  
 
VALID 12Z FRI DEC 22 2017 - 12Z TUE DEC 26 2017  
 
...ABOVE AVERAGE CONFIDENCE IN THE POTENTIAL FOR AN OUTBREAK OF  
ARCTIC AIR ACROSS THE CENTRAL U.S. BY THE WEEKEND...  
 
   
..OVERVIEW
 
 
CONFIDENCE REMAINS HIGH FOR A MAJOR PATTERN CHANGE ACROSS NORTH  
AMERICA OVER THE NEXT SEVERAL DAYS. A SPLIT FLOW PATTERN VERY  
EARLY IN THE PERIOD WILL QUICKLY UNDERGO A SIGNIFICANT  
AMPLIFICATION, WITH AN EASTERN PACIFIC RIDGE AXIS EXTENDING FROM  
JUST OFF THE U.S. WEST COAST NORTH TO THE ARCTIC, AND A DOWNSTREAM  
MEAN TROUGH FROM NEAR HUDSON BAY SOUTHWEST INTO THE CONUS. THIS  
SETUP WILL RESULT IN A FLOW OF AIR FROM THE HIGH LATITUDES OF THE  
ARCTIC SOUTHWARD THROUGH CANADA AND INTO THE  
PLAINS/MIDWEST/ROCKIES. TELECONNECTIONS ASSOCIATED WITH THE AREA  
OF MAXIMUM POSITIVE HEIGHT ANOMALIES ACROSS NORTHWEST CANADA (+2  
TO +3 STANDARD DEVIATIONS ON THE GEFS MEAN) SUPPORT THE IDEA SHOWN  
BY MOST GUIDANCE, WITH THE NORTHERN/CENTRAL PLAINS AND UPPER  
MIDWEST THE FAVORED AREAS FOR THE COLDEST TEMPERATURES BY THE  
WEEKEND. FARTHER EAST, THE SUBTROPICAL RIDGE IS FORECAST TO  
STRENGTHEN, CENTERED OVER THE BAHAMAS OR SOUTH FLORIDA (ALSO  
SUPPORTED BY TELECONNECTIONS TO THE NORTH PACIFIC RIDGE). THE  
CONTRAST BETWEEN THE FLOW OF ARCTIC AIR SOUTHWARD INTO THE CENTRAL  
U.S. AND WARM, MOIST SUBTROPICAL AIR TRANSPORTED NORTHWARD INTO  
THE SOUTHERN/SOUTHEASTERN U.S. WILL SUPPORT A PERSISTENT AND SHARP  
FRONTAL ZONE FROM THE GULF OF MEXICO NORTHEASTWARD INTO THE  
SOUTHEAST AND JUST OFF THE MID-ATLANTIC COASTLINE. SOME  
UNCERTAINTY REMAINS REGARDING EXACTLY WHERE THIS FRONTAL ZONE SETS  
UP (DISCUSSED MORE BELOW), WHICH WILL HAVE MAJOR IMPLICATIONS FOR  
THE WEATHER ACROSS THE EASTERN U.S. BY THE WEEKEND. EXPECT THIS  
FRONT TO SERVE AS A FOCUS FOR POTENTIAL ROUNDS OF PRECIPITATION,  
WITH SOME POTENTIAL FOR WINTER WEATHER ON THE NORTHERN FRINGES  
GIVEN THE SHARP THERMAL GRADIENT. ADDITIONALLY, MODELS SUGGEST THE  
POTENTIAL FOR A WAVE OF LOW PRESSURE TO DEVELOP ALONG THE FRONT BY  
SUN-MON ALONG THE SOUTHEAST/MID-ATLANTIC COAST.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
DURING DAYS 3-5, MODELS SHOW SOME TIMING DIFFERENCES WITH RESPECT  
TO THE SHORTWAVE AND ASSOCIATED SURFACE LOW MOVING FROM THE  
CENTRAL PLAINS TO THE GREAT LAKES AND INTO SOUTHEAST CANADA. IN  
GENERAL, A 12Z ECMWF/12Z GFS BLEND, WITH SMALL COMPONENTS OF THE  
RESPECTIVE ENSEMBLE MEANS INCLUDED, WAS THE PREFERRED APPROACH  
DURING THIS TIME PERIOD. MEANWHILE, CONFIDENCE IS HIGH THAT A  
LARGE POSITIVELY-TILTED TROUGH WILL BEGIN TO AMPLIFY ACROSS THE  
CENTRAL U.S. BY SAT AS A SHORTWAVE CROSSES THE ROCKIES AND  
AMPLIFIES. MEANWHILE, A DEEP UPPER LOW INITIALLY NEAR HUDSON BAY  
WILL DRIFT SOUTHWARD TOWARD THE UPPER MIDWEST.  
 
UNCERTAINTY BEGINS TO INCREASE AFTER SUN WITH RESPECT TO HOW  
QUICKLY HEIGHTS WILL FALL ACROSS THE EASTERN U.S. THE ECMWF HAS  
BEEN CONSISTENT IN KEEPING A STRONGER SUBTROPICAL RIDGE IN PLACE,  
WHICH DELAYS THE ARRIVAL OF THE COLD FRONT INTO THE EASTERN U.S.,  
WHILE THE GFS AND CMC PUSH THE FRONT THROUGH MOST OF THE EASTERN  
U.S. BY SUN, BEFORE STALLING IT TEMPORARILY FROM THE NORTHERN GULF  
OF MEXICO TO THE CAROLINAS. THIS SCENARIO SEEMS TO BE FAVORED TO  
SOME DEGREE BY BOTH THE GEFS AND ECENS MEANS, BOTH OF WHICH  
SUGGEST THE LOWEST PRESSURES SUN-MON WILL BE LOCATED ALONG AN AREA  
GENERALLY ALONG THE EAST/SOUTHEAST U.S. COAST. THUS, WHILE THE  
ECMWF SCENARIO CANNOT BE RULED OUT, AM NOT FAVORING THE SLOWER  
FRONTAL PROGRESSION AT THIS TIME. TO FURTHER COMPLICATE MATTERS,  
THE GFS HAS SHOWN A LARGE DEGREE OF RUN-TO-RUN VARIABILITY, WITH  
THE 18Z RUN IN PARTICULAR BRINGING THE CORE OF THE ARCTIC AIR  
EASTWARD TOWARD THE CENTRAL APPALACHIANS BY TUE. THIS SCENARIO WAS  
ALSO NOT PREFERRED AT THIS TIME. TELECONNECTIONS WOULD SUPPORT  
SOME MODEST MAINTENANCE OF THE SUBTROPICAL RIDGE WITH THE CORE OF  
THE COLD AIR DIRECTED SOUTHWARD ACROSS THE CENTRAL U.S., AT LEAST  
INITIALLY. THUS, GIVEN THE UNCERTAINTY AMONG THE GUIDANCE, THE  
FORECAST FOR DAYS 6-7 WAS BASED LARGELY ON A BLEND OF THE 12Z  
ECENS/GEFS MEANS, WITH ONLY SMALL COMPONENTS OF THEIR RESPECTIVE  
DETERMINISTIC SOLUTIONS INCLUDED. WHILE THIS SCENARIO DOES BRING  
THE ARCTIC FRONT INTO THE EASTERN U.S. BY TUE, THE CORE OF THE  
COLD AIR WOULD REMAIN DIRECTED INTO THE CENTRAL U.S. GIVEN THE  
POSITION OF THE SURFACE HIGH ACROSS THE NORTHERN HIGH PLAINS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE SURFACE LOW TRACKING ACROSS THE GREAT LAKES FRI-SAT WILL BE  
ACCOMPANIED BY A BAND OF SNOW ON THE NORTH SIDE OF THE SYSTEM.  
SNOW WILL ALSO BE POSSIBLE OVER NEW ENGLAND FRI/SAT WHERE WARM  
ADVECTION WILL BE THE LIKELY MECHANISM. THE AREA EXTENDING FROM  
FROM LOWER MS/TN VALLEYS INTO THE SOUTHERN APPALACHIANS HAS THE  
HIGHEST PROBABILITY OF SEEING HEAVY RAINFALL DURING THE MEDIUM  
RANGE. MODELS SHOW A GROWING CONSENSUS FOR AN AREA OF VERY  
HEAVY/EXCESSIVE RAINFALL POTENTIAL FRI-SAT ACROSS THIS AREA IN  
ASSOCIATION WITH A SLOW-MOVING FRONT WHICH IS FORECAST TO  
EVENTUALLY STALL OVER THE SOUTHEAST BY SUN. ADDITIONAL OVERRUNNING  
PRECIPITATION WILL BE POSSIBLE NORTH OF THE STALLED FRONT ON  
SUNDAY, PERHAPS LOCATED A BIT TO THE SOUTHEAST OF THE FRI-SAT  
HEAVY RAINFALL AREA, ALTHOUGH CONFIDENCE IN THE EXACT FRONTAL  
PLACEMENT REMAINS A BIT BELOW AVERAGE FOR THE REASONS DESCRIBED  
ABOVE. IF A FRONTAL WAVE DEVELOPS ALONG THE SOUTHEAST/MID-ATLANTIC  
COASTLINE AS FORECAST BY SUN-MON, THIS WOULD SPREAD AN ORGANIZED  
ROUND OF PRECIP ACROSS AREAS FROM THE CAROLINAS NORTHWARD,  
ALTHOUGH AT THIS TIME THERMAL FIELDS ARE TOO UNCERTAIN TO  
SPECULATE MUCH ON RAIN/SNOW AREAS. NONETHELESS, SOME POTENTIAL FOR  
WINTER WEATHER WOULD EXIST, ESPECIALLY FOR THE APPALACHIANS AND  
INTERIOR MID-ATLANTIC/NORTHEAST IN THIS SCENARIO. IF THE CURRENT  
FORECAST WERE TO VERIFY, THIS WOULD LIKELY MEAN A COLD RAIN FOR  
THE MAJOR CITIES ALONG THE I-95 CORRIDOR GIVEN THAT THE ARCTIC  
FRONT WOULD STILL BE TO THE WEST OF THE REGION AND NO OBVIOUS  
SOURCE OF LOW-LEVEL COLD AIR IS EVIDENT. THIS FORECAST IS STILL  
SUBJECT TO SIGNIFICANT CHANGES, HOWEVER, GIVEN THE DESCRIBED  
UNCERTAINTY.  
 
TEMPERATURES ACROSS THE CENTRAL U.S. WILL BE ON THE DECLINE  
THROUGHOUT THE MEDIUM RANGE AS A COUPLE ARCTIC FRONTAL BOUNDARIES  
IN QUICK SUCCESSION SPREAD INCREASINGLY COLD AIR SOUTHWARD. THE  
COLDEST TEMP ANOMALIES WILL INITIALLY BE JUST EAST OF THE ROCKIES  
SAT-SUN, WITH HIGH TEMPS 15 TO 30 DEG F BELOW AVERAGE. BY MON-TUE,  
THE COLDEST ANOMALIES, ALSO 15 TO 30 DEG F BELOW AVERAGE, WILL  
SHIFT TO THE NORTHERN PLAINS AND UPPER MIDWEST, WHERE HIGH  
TEMPERATURES FOR SOME AREAS WILL REMAIN BELOW ZERO, AND LOWS MAY  
REACH -20 DEG F. MEANWHILE, ABOVE AVERAGE TEMPS WILL PERSIST  
ACROSS THE EASTERN U.S. UNTIL THE FIRST COLD FRONT IS ABLE TO  
PASS. HIGH TEMPS FRI-SAT ARE FORECAST TO 10-20 DEG F ABOVE AVERAGE  
FOR THE SOUTHEAST, OH VALLEY, AND MID-ATLANTIC.  
 
RYAN  
 

 
 
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