196  
FXUS02 KWBC 160655  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
154 AM EST SAT DEC 16 2017  
 
VALID 12Z TUE DEC 19 2017 - 12Z SAT DEC 23 2017  
   
..OVERVIEW
 
 
MODELS/ENSEMBLES ARE IN GENERAL AGREEMENT THAT THE LARGE SCALE  
PATTERN ACROSS NORTH AMERICA WILL BECOME MORE AMPLIFIED OVER THE  
NEXT WEEK. STRONGLY NEGATIVE HEIGHT ANOMALIES JUST NORTH OF HAWAII  
WILL HAVE DOWNSTREAM EFFECTS OF ONCE AGAIN AMPLIFYING A RIDGE JUST  
OFF THE NORTH AMERICAN WEST COAST (NORTH TO THE ARCTIC), WITH  
TROUGHING PREVALENT ACROSS THE CENTRAL U.S. IN SOME WAYS THIS  
SHOWS SIMILARITIES TO THE AMPLIFIED PATTERN EXPERIENCED 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 SOUTHERN PLAINS INTO THE  
GREAT LAKES AS ARCTIC AIR IS ABLE TO PLUNGE SOUTH INTO THE NORTH  
CENTRAL CONUS WHILE COINCIDENTALLY, WARM AND MOIST AIR FROM THE  
GULF OF MEXICO SPREADS NORTH INTO THE LOWER MS VALLEY AND  
SOUTHEAST.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
A FAIRLY SIGNIFICANT DEGREE OF UNCERTAINTY EXISTS EVEN AT THE  
BEGINNING OF THE MEDIUM RANGE PERIOD ON DAY 3 (TUE) WITH RESPECT  
TO THE UPPER LOW ACROSS THE SOUTHWESTERN U.S. AND HOW QUICKLY IT  
MOVES EASTWARD. THE 12Z ECMWF ALONG WITH THE ECENS MEAN HAD BEEN  
SUGGESTING A SLOWER PROGRESSION, WHILE THE 18Z GFS WAS FASTER.  
GIVEN A NOTED TREND AMONG ENSEMBLE GUIDANCE, THE WPC FORECAST  
FAVORED THE SLOWER PROGRESSION SHOWN BY THE 12Z ECMWF/UKMET/ECENS.  
WITH THE 00Z RUN THE GFS HAS ALSO TRENDED SUBSTANTIALLY IN THIS  
DIRECTION AS WELL, ALBEIT KEEPING THE SHORTWAVE A BIT LESS  
AMPLIFIED ON DAY 4 (WED) ACROSS THE SOUTHERN PLAINS. NEXT  
AMPLIFYING TROUGH ENTERS THE NORTHWEST ON DAY 3, AND  
MODELS/ENSEMBLE SHOW GOOD CONSENSUS THAT THE WAVE WILL RAPIDLY  
AMPLIFY ACROSS THE GREAT BASIN BY DAY 5 (THU). THE GFS AND ECMWF  
BOTH SUGGEST THAT ANOTHER CUTOFF LOW WILL FORM ACROSS THE  
SOUTHWEST BY THU-SAT, WITH THE 12Z CMC THE ONLY DETERMINISTIC  
SOLUTION TO KEEP AN OPEN WAVE THAT MOVES MORE QUICKLY EAST.  
ENSEMBLE SUPPORT FAVORS SOMETHING MORE LIKE THE ECMWF/GFS HERE,  
BUT THERE IS ENOUGH SPREAD THAT ENSEMBLE MEANS DO NOT YET SHOW A  
CLOSED UPPER LOW. GIVEN THIS PATTERN, BY LATE NEXT WEEK EXPECT A  
RAPID SUCCESSION OF LOW AMPLITUDE SHORTWAVES TO EJECT FROM THE  
SOUTHWESTERN U.S. TROUGH/UPPER LOW INTO THE SOUTHERN PLAINS,  
CREATING THE POTENTIAL FOR MULTIPLE WAVES ALONG THE FRONTAL  
BOUNDARY ALONG THE GULF COAST. WHILE THE EXISTENCE OF THESE WAVES  
IS LIKELY GIVEN THE PATTERN, CONFIDENCE IN THE SPECIFICS OF SUCH  
FEATURES AND THE TIMING OF ANY FRONTAL WAVES AND ASSOCIATED PRECIP  
IS LOW, GIVEN THE LOW PREDICTABILITY OF SUCH SMALL SCALE FEATURES  
AT NEARLY A WEEK OUT.  
 
FARTHER NORTH, A PIECE OF ENERGY IS FORECAST TO SPLIT OFF FROM THE  
AMPLIFYING WESTERN TROUGH AND GET CARRIED EAST IN THE NORTHERN  
STREAM, CROSSING THE MIDWEST ON DAY 6 (FRI). MODELS SUGGEST THIS  
FEATURE WILL RESULT IN SURFACE FRONTAL WAVE DEVELOPMENT ACROSS THE  
CENTRAL PLAINS ON DAY 5, WITH THE SURFACE LOW QUICKLY TRACKING  
NORTHEASTWARD ACROSS THE GREAT LAKES ON DAY 6 AND INTO  
SOUTHEASTERN CANADA ON DAY 7 (SAT). AT THE SAME TIME, GUIDANCE  
SHOWS DECENT AGREEMENT THAT A RAPID SUCCESSION OF SHORTWAVE ENERGY  
ORIGINATING IN THE ARCTIC WILL DIVE SOUTHWARD ALONG THE EASTERN  
PERIPHERY OF THE RIDGE AND GRADUALLY AMPLIFY A BROAD TROUGH AXIS  
ACROSS THE UPPER MIDWEST BY NEXT WEEKEND.  
 
THE WPC FORECAST WAS HEAVILY BASED ON THE 12Z ECMWF/UKMET DURING  
DAYS 3-4, WITH A GRADUAL TREND TOWARD HEAVIER 12Z ECENS/18Z GEFS  
MEAN WEIGHTING FROM DAY 5 ONWARD.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE PACIFIC NORTHWEST AND NORTHERN/CENTRAL ROCKIES WILL BE  
UNSETTLED EARLY IN THE PERIOD AS A STRONG LOW PRESSURE SYSTEM  
MOVES INLAND TUE-WED. HEAVY RAINS ALONG THE COAST AND MOUNTAIN  
SNOWS WILL BE POSSIBLE. FARTHER EAST, THE POTENTIAL APPEARS TO BE  
GROWING FOR HEAVY RAINFALL FROM THE SOUTHERN PLAINS TO THE  
SOUTHEAST/SOUTHERN APPALACHIANS TUE-WED IN ASSOCIATION WITH A  
FRONTAL WAVE. A SECOND ROUND OF SHOWERS AND THUNDERSTORMS APPEARS  
IN THE CARDS FOR FRI-SAT AS ANOTHER COLD FRONT MOVES THROUGH THE  
SAME AREAS AND THEN POTENTIALLY STALLS IN THE GULF OF MEXICO WITH  
MOISTURE OVERRUNNING THE FRONT INTO THE SOUTHERN STATES. A COUPLE  
ROUNDS OF SNOW ARE POSSIBLE ACROSS THE NORTHERN PLAINS/UPPER  
MIDWEST THU-SAT, THE FIRST IN ASSOCIATION WITH A SURFACE LOW  
MOVING FROM THE CENTRAL PLAINS ON THU TO THE GREAT LAKES ON FRI. 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. TUE-THU BEFORE AN AREA OF ARCTIC HIGH  
PRESSURE BEGINS TO NOSE SOUTHWARD EAST OF THE ROCKIES THU INTO  
FRI, BRINGING BELOW AVERAGE TEMPERATURES TO THE NORTH CENTRAL U.S.  
THE AFOREMENTIONED SUBTROPICAL RIDGE WILL KEEP ABOVE AVERAGE TEMPS  
IN PLACE ACROSS THE SOUTHEAST INTO NEXT WEEKEND.  
 
RYAN  
 

 
 
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