235  
FXUS02 KWBC 170639  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
139 AM EST SUN DEC 17 2017  
 
VALID 12Z WED DEC 20 2017 - 12Z SUN DEC 24 2017  
 
...MODELS SUGGESTING THE POTENTIAL FOR AN OUTBREAK OF ARCTIC AIR  
ACROSS THE NORTHERN PLAINS AND UPPER MIDWEST BY NEXT WEEKEND...  
 
   
..OVERVIEW
 
 
MODELS/ENSEMBLES CONTINUE TO SHOW A STRONG SIGNAL TOWARD  
AMPLIFICATION OF THE LARGE SCALE FLOW OVER NORTH AMERICA OVER THE  
NEXT WEEK. MEAN TROUGHING EXTENDING FROM THE BERING SEA TO HAWAI'I  
WILL HELP TO AMPLIFY A RIDGE JUST OFF THE WEST COAST, EXTENDING  
WELL NORTH TO THE ARCTIC. MEANWHILE, POSITIVELY TILTED MEAN  
TROUGHING WILL BECOME ESTABLISHED FROM NEAR HUDSON BAY THROUGH THE  
CENTRAL U.S. THIS PATTERN SHOWS SIMILARITIES TO PREVIOUS FLOW  
AMPLIFICATION ACROSS THE CONUS IN RECENT WEEKS, BUT A STARK  
DIFFERENCE THIS TIME SEEMS TO BE THE WESTERN ATLANTIC SUBTROPICAL  
RIDGE ATTEMPTING TO BUILD INTO THE SOUTHEASTERN STATES. THIS  
PATTERN WOULD FAVOR A PERSISTENT, SHARP, AND WAVY BAROCLINIC ZONE  
FROM THE SOUTH CENTRAL U.S. OR NORTHERN GULF OF MEXICO INTO THE  
EASTERN U.S. AS ARCTIC AIR IS ABLE TO PLUNGE SOUTH INTO THE NORTH  
CENTRAL CONUS AND WARM, MOIST AIR ON THE WESTERN SIDE OF THE  
SUBTROPICAL RIDGE ATTEMPTS TO SPREAD NORTH INTO THE SOUTHEASTERN  
STATES. SEVERAL MODEL SOLUTIONS SUGGEST THE POTENTIAL FOR AN  
OUTBREAK OF EXTREMELY COLD TEMPERATURES ACROSS THE NORTHERN PLAINS  
AND UPPER MIDWEST NEXT WEEKEND. THE LARGE SCALE FLOW PATTERN WOULD  
CERTAINTY SUPPORT SUCH AN EVENT, WITH 500 HPA FLOW ESSENTIALLY  
EXTENDING FROM THE NORTH POLE SOUTHWARD TOWARD THE NORTH CENTRAL  
CONUS. THE DEGREE TO WHICH THIS IS A POSSIBILITY WILL DEPEND ON  
ADDITIONAL DETAILS WHICH ARE DISCUSSED BELOW.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODELS HAVE COME INTO BETTER AGREEMENT SURROUNDING THE SYSTEM  
INITIALLY ACROSS THE LOWER MS VALLEY ON DAY 3 (WED). THE ECMWF HAD  
THE BEST HANDLE ON THIS FEATURE LAST NIGHT AND MOST GUIDANCE HAS  
TRENDED TO THAT SOLUTION. THUS, EXPECT SHORTWAVE INITIALLY OVER  
THE SOUTHERN PLAINS EARLY WED TO MOVE EAST OFF THE SOUTHEAST COAST  
BY THU. THE ECMWF REMAINS VERY WELL CENTERED WITHIN THE ENSEMBLE  
SPREAD IN TERMS OF THE SURFACE LOW TRACK, AND THE 12Z CMC WAS VERY  
CLOSE TO THE ECMWF. THE GFS CONTINUES TO DEPICT A SURFACE LOW ON  
THE SOUTHEAST EDGE OF THE CONSENSUS. ADDITIONAL UPPER TROUGH  
AMPLIFIES ACROSS THE WEST ON DAY 3. BY DAYS 4-5 (THU-FRI), SOME  
SPREAD REMAINS AS TO WHETHER UPPER-LEVEL ENERGY CUTS OFF ACROSS  
THE SOUTHWEST OR EVEN NORTHERN MEXICO, OR MOVES MORE QUICKLY EAST  
AS AN OPEN WAVE. WHILE DETERMINISTIC GUIDANCE YESTERDAY SEEMED TO  
FAVOR A CUTOFF SOLUTION, A SIGNIFICANT TREND WAS NOTED OVER THE  
PAST DAY TOWARD KEEPING A MORE OPEN WAVE. THIS TREND WAS THE  
STRONGEST IN THE ECMWF, WHICH IS NOW THE QUICKEST SOLUTION TO MOVE  
THE WAVE EAST INTO THE SOUTHERN PLAINS ON FRI. THE 00Z GFS HAS  
TRENDED TOWARD THE ECMWF WITH RESPECT TO THIS SYSTEM, NOW MOVING  
THE WAVE EAST A BIT FASTER THAN ITS 18Z RUN. GIVEN THE DESCRIBED  
CONSIDERATIONS, A HEAVILY ECMWF-BASED BLEND WAS FAVORED DURING  
DAYS 3-4, WITH SMALLER COMPONENTS OF THE CMC/GFS ALSO INCLUDED.  
 
FROM DAY 5 ONWARD, GENERAL CONSENSUS AMONG THE GUIDANCE IS FOR A  
BROAD AND DEEP TROUGH TO DIG ACROSS THE NORTH CENTRAL U.S.,  
DOWNSTREAM OF THE VERY STRONG RIDGE OFF THE WEST COAST. A  
SHORTWAVE ENTERING THE NORTHERN ROCKIES FRI NIGHT SHOULD BE THE  
BEGINNING OF THIS TROUGH AMPLIFICATION, WITH ADDITIONAL SHORTWAVE  
ENERGY REACHING THE NORTHERN PLAINS ON SUN FURTHER DEEPENING THE  
TROUGH. THE POTENTIAL EXISTS WITH SUCH AN AMPLIFIED PATTERN FOR  
THE TRANSPORT OF AIR FROM THE ARCTIC HIGH LATITUDES SOUTHWARD INTO  
THE NORTH CENTRAL U.S. THIS DEGREE TO WHICH THIS CAN OCCUR WILL  
LARGELY DEPEND ON THE EVOLUTION OF AN UPPER LOW INITIALLY NORTH OF  
HUDSON BAY ON DAY 3, WHICH THEN DRIFTS SOUTHWARD BY THE WEEKEND -  
ALTHOUGH MODELS DIFFER ON THE EXTENT. THE 12Z ECMWF AND 00Z GFS  
ARE MORE AGGRESSIVE WITH MOVING THE UPPER LOW SOUTHWARD, WHICH  
WOULD BRING THE POTENTIAL FOR AN EXTREMELY COLD AIR MASS INTO THE  
NORTHERN PLAINS AND UPPER MIDWEST BY NEXT WEEKEND. ENSEMBLES HAVE  
SHOWN A MODEST SHIFT IN SUPPORT TOWARD THIS IDEA AS WELL, WITH  
ECENS/GEFS MEAN 500 HPA HEIGHTS ACROSS THE UPPER MIDWEST  
DECREASING FOR SEVERAL CONSECUTIVE RUNS. GIVEN THE GRADUALLY  
INCREASING UNCERTAINTY, THE ECENS/GEFS MEANS WERE GIVEN INCREASING  
WEIGHT THROUGH TIME IN THE FORECAST AFTER DAY 5.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE NORTHERN/CENTRAL ROCKIES WILL BE UNSETTLED EARLY IN THE PERIOD  
AS A STRONG LOW PRESSURE SYSTEM CROSSES THE REGION ON WED. FARTHER  
EAST, HEAVY RAIN ONGOING AT THE START OF DAY 3 ALONG A  
STATIONARY/WARM FRONT ACROSS THE LOWER MS VALLEY/SOUTHEAST IS  
EXPECTED TO SPREAD EAST IN ASSOCIATION WITH AN AREA OF LOW  
PRESSURE, MOVING OFF THE SOUTHEAST U.S. COAST BY THU. ANOTHER  
EPISODE OF SHOWERS AND THUNDERSTORMS APPEARS POSSIBLE FRI-SAT AS  
ANOTHER COLD FRONT CROSSES THE MS/OH VALLEYS AND THE SOUTHEAST.  
THE FRONT MAY STALL IN THE GULF OF MEXICO WITH OVERRUNNING  
MOISTURE MAINTAINING PERIODS OF PRECIPITATION OVER THE SOUTHERN  
STATES INTO NEXT WEEKEND. A COUPLE ROUNDS OF SNOW ARE POSSIBLE  
ACROSS THE NORTHERN PLAINS/UPPER MIDWEST THU-SAT, THE FIRST WITH A  
SURFACE LOW MOVING FROM THE CENTRAL PLAINS ON THU TO THE GREAT  
LAKES ON FRI WITH POTENTIAL FOR IT TO BE ACCOMPANIED BY FAIRLY  
BRISK WINDS DEPENDING ON SYSTEM STRENGTH. A CLIPPER SYSTEM QUICKLY  
CROSSING THE NORTHERN PLAINS/UPPER MIDWEST LATE FRI INTO SAT COULD  
BRING ANOTHER ROUND OF SNOW ALONG WITH A MUCH COLDER AIR MASS.  
 
TEMPERATURES WILL INITIALLY BE ABOVE AVERAGE ACROSS MUCH OF THE  
CENTRAL/EASTERN U.S. WED-THU BEFORE AN AREA OF POLAR HIGH PRESSURE  
BEGINS TO NOSE SOUTHWARD EAST OF THE ROCKIES THU INTO FRI,  
BRINGING BELOW AVERAGE TEMPERATURES TO THE NORTH CENTRAL U.S. AN  
ARCTIC FRONT IS EXPECTED TO USHER EVEN COLDER AIR INTO THE  
NORTHERN PLAINS AND UPPER MIDWEST BY NEXT WEEKEND, WITH  
TEMPERATURES (CONSERVATIVELY) 15 DEGREES TO PERHAPS EVEN EXCEEDING  
20 DEGREES BELOW AVERAGE FOR SOME AREAS. THE SUBTROPICAL RIDGE  
WILL KEEP ABOVE AVERAGE TEMPS IN PLACE ACROSS THE SOUTHEAST AND  
MUCH OF THE EASTERN SEABOARD INTO NEXT WEEKEND.  
 
RYAN  
 

 
 
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