072  
FXUS02 KWBC 310654  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
153 AM EST SUN DEC 31 2017  
 
VALID 12Z WED JAN 03 2018 - 12Z SUN JAN 07 2018  
   
..OVERVIEW
 
 
DOWNSTREAM FROM A WESTERN NORTH AMERICA MEAN RIDGE, STRONG EASTERN  
U.S. TROUGH AMPLIFICATION WILL SUPPORT RAPIDLY DEEPENING WESTERN  
ATLANTIC LOW PRESSURE WHICH MAY BECOME QUITE INTENSE BY THE TIME  
IT REACHES THE VICINITY OF THE CANADIAN MARITIMES BY AROUND EARLY  
FRI. FLOW AROUND THIS SYSTEM WILL REINFORCE THE MUCH BELOW NORMAL  
TEMPERATURES THAT HAVE BEEN PREVALENT OVER MOST AREAS EAST OF THE  
ROCKIES. OVER THE EASTERN PACIFIC/WESTERN U.S., A LEADING  
SHORTWAVE/EMBEDDED UPPER LOW OFF THE WEST COAST WILL STEADILY  
WEAKEN WED-THU AS IT HEADS INTO THE MEAN RIDGE. THEN UPSTREAM  
TROUGH ENERGY SHOULD APPROACH THE WEST COAST BY LATE IN THE WEEK  
WITH FAIRLY HIGH UNCERTAINTY OVER THE ULTIMATE SHAPE/PROGRESSION  
OF THE ENERGY FROM THE WEST COAST INTO THE CENTRAL U.S.. OVERALL  
THIS PROGRESSION OF FEATURES WILL BRING AN INCREASE OF MOISTURE  
INTO THE WEST AND EVENTUALLY A WARMING TREND OVER THE PLAINS.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
NOT SURPRISING GIVEN THE COMPONENTS INVOLVED, GUIDANCE STILL  
VARIES WITH RESPECT TO EVOLUTION OF RAPIDLY INTENSIFYING WESTERN  
ATLANTIC LOW PRESSURE MID-LATE WEEK. GFS/ECMWF/CMC RUNS THROUGH  
THE 12Z/18Z CYCLES HAVE BEEN SIGNALING THE POTENTIAL THAT THE  
SURFACE LOW COULD REACH A DEPTH IN THE 950'S/LOW 960'S MB BY THE  
TIME IT REACHES THE CANADIAN MARITIMES AROUND EARLY FRI. RECENT  
UKMET RUNS, WHICH HAVE TENDED TO BE ON THE WESTERN FRINGE OF THE  
GUIDANCE SPREAD, DEEPEN ONLY INTO THE 970'S MB. IN THE NEW 00Z  
CYCLE THUS FAR, THE GFS IS STILL EXTREMELY DEEP WHILE THE UKMET  
HAS ADJUSTED EASTWARD TO NEAR THE CONSENSUS TRACK. BOTH 00Z  
UKMET/CMC RUNS ONLY DEEPEN TO THE 970'S MB. THE DEEPEST SOLUTIONS  
SEEM TO HAVE A COMMON THEME OF INTENSIFYING THE LEADING SOUTHEAST  
U.S. SHORTWAVE AS IT CROSSES THE ATLANTIC, ON A SMALL ENOUGH SCALE  
TO TEMPER CONFIDENCE IN A DAYS 3-5 FORECAST. HOWEVER TRENDS OVER  
THE PAST DAY HAVE BEEN TOWARD A SHARPER, SLIGHTLY WESTWARD, AND  
MORE NEGATIVELY TILTED AXIS FOR FLOW DIGGING INTO THE EASTERN U.S.  
FROM CANADA-- WHICH DOES INCREASE THE POTENTIAL FOR A STRONG  
SYSTEM. AT THIS TIME PREFER A MODEL/ENSEMBLE MEAN COMPROMISE DUE  
TO LACK OF CONFIDENCE IN THE FINE DETAILS WHICH WILL STILL TAKE  
SOME TIME TO WORK OUT. AS FOR TRACK THERE HAS BEEN DECENT  
CONTINUITY IN CONSENSUS SHOWING INCIPIENT LOW PRESSURE OVER THE  
NORTHERN BAHAMAS EARLY WED WITH A MOST LIKELY PATH EAST OF THE  
40N/70W BENCHMARK AND INTO THE CANADIAN MARITIMES. SOME  
INDIVIDUAL ENSEMBLE MEMBERS AND THE 00Z CMC STILL SUGGEST A  
SURFACE CENTER (WHETHER CONSOLIDATED OR ONE OF TWO SEPARATE  
CENTERS) COULD GET CLOSE TO THE NEW ENGLAND COAST.  
 
GUIDANCE SPREAD HAS NARROWED CONSIDERABLY FOR THE INITIAL EAST  
PACIFIC TROUGH/EMBEDDED UPPER LOW QUICKLY WEAKENING AS IT HEADS  
INTO THE WESTERN U.S. MEAN RIDGE. MULTI-DAY TRENDS HAVE BEEN  
TOWARD SLOWER PROGRESSION, WITH OPERATIONAL MODEL RUNS AND GEFS  
MEAN TENDING TO REFLECT THE SLOWER TIMING BETTER THAN THE ECMWF  
MEAN. ON THE OTHER HAND MODELS AND ENSEMBLES STILL VARY  
CONSIDERABLY WITH EXACTLY HOW UPSTREAM TROUGH ENERGY MAY SPLIT AS  
IT APPROACHES THE WEST COAST BY DAY 5 FRI, WITH FURTHER INCREASING  
DIVERGENCE OF SOLUTIONS AS ONE OR MORE SHORTWAVES PROGRESS INTO  
THE WESTERN HALF OF THE U.S. AND/OR SOUTHERN CANADA DURING THE  
WEEKEND. THE MOST NOTABLE TRENDS OVER THE PAST 24 HOURS HAVE BEEN  
TOWARD SLOWER PROGRESSION OF THE OVERALL TROUGH IN THE ECMWF/CMC  
MEANS AND FOR GFS/GEFS MEANS TO BRING SOMEWHAT MORE ENERGY  
INTO/THROUGH THE WEST. THIS IS BRINGING GUIDANCE CLOSER IN  
PRINCIPLE EVEN THOUGH SPECIFICS ARE STILL QUITE DIVERSE. AS  
GUIDANCE SPREAD INCREASES LATE IN THE PERIOD THE PREFERENCE GOES  
MORE TOWARD A GEFS/ECMWF MEAN COMPROMISE AS THE BEST PLACEHOLDER  
UNTIL THERE IS GREATER CLARITY. BY DAY 7 WOULD ULTIMATELY EXPECT  
A MORE DEFINED PATTERN OVER THE WEST THAN CURRENTLY INDICATED, AND  
SOME OPERATIONAL RUNS PLUS 12Z CMC MEAN SUGGEST THAT ENERGY ALOFT  
MAY SUPPORT A SOUTHERN CANADA-NORTHERN TIER U.S. LOW/FRONT WITH  
FASTER TIMING THAN THE GEFS/ECMWF MEANS.  
 
THE LATEST FORECAST CONSISTED OF PRIMARILY OPERATIONAL GUIDANCE  
EARLY (12Z-18Z GFS/12Z ECMWF AND LESS 12Z CMC) FOLLOWED BY  
INCREASING 18Z GEFS/12Z ECMWF MEAN INPUT SO THAT MODEL/MEAN  
WEIGHTING WAS ABOUT 50/50 DAY 5 FRI AND 70 PERCENT TOWARD THE  
MEANS BY DAY 7 SUN. THIS SOLUTION PROVIDED BETTER THAN AVERAGE  
CONTINUITY WITH MOST ASPECTS OF THE FORECAST.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE DEEPENING UPPER TROUGH OVER THE EAST WILL REINFORCE THE VERY  
COLD TEMPERATURES OVER THE EASTERN HALF OF THE COUNTRY. AREAS  
FROM THE GREAT LAKES/OH VALLEY INTO APPALACHIANS AND NORTHEAST  
WILL LIKELY SEE THE GREATEST ANOMALIES WITH ONE OR MORE DAYS OF  
TEMPS 20-30F BELOW NORMAL. PARTS OF NEW ENGLAND COULD SEE COLDER  
READINGS THAN THAT AROUND FRI-SAT AS THE CORE OF THE UPPER TROUGH  
PASSES OVER. FRI-SAT SHOULD BE THE MOST LIKELY DAYS FOR DAILY  
RECORD LOW/COLD HIGH READINGS. POTENTIALLY VERY INTENSE LOW  
PRESSURE TRACKING OFFSHORE THE EAST COAST AND REACHING THE  
CANADIAN MARITIMES BY EARLY FRI MAY BRING SNOW INTO THE NORTHEAST  
WITH PERHAPS A BRIEF PERIOD OF LIQUID PRECIP NEAR THE COAST  
DEPENDING ON LOW TRACK. IT WILL BE A VERY CLOSE CALL WITH RESPECT  
TO ANY PRECIP ALONG THE SOUTHEAST/MID ATLANTIC COAST. WHILE  
GUIDANCE HAS VARIED OVER THE DAYS FOR PRECIP IMPACTS, A PERIOD OF  
STRONG WINDS HAS BEEN MORE CONSISTENTLY FORECAST FOR THE  
NORTHEAST, GENERATING DANGEROUS WIND CHILLS AND BLOWING AROUND ANY  
SNOW THAT FALLS. FARTHER WEST, COLD CYCLONIC FLOW WILL MAINTAIN  
LAKE EFFECT SNOW THROUGH AT LEAST LATE WEEK THOUGH PERHAPS  
TAPERING OFF SOMEWHAT NEXT WEEKEND. FARTHER SOUTH, THE FLORIDA  
PENINSULA AND IN PARTICULAR THE EAST COAST OF THE STATE SHOULD SEE  
A PERIOD OF MIDWEEK RAIN AHEAD OF THE SHORTWAVE CROSSING THE  
SOUTHEAST.  
 
WITHIN THE AREA OF MOISTURE SPREADING INTO THE WEST, FAVORED  
TERRAIN FROM NORTHERN CALIFORNIA INTO THE PACIFIC NORTHWEST HAS  
THE BEST PROBABILITY OF SEEING THE HIGHEST PRECIP TOTALS DURING  
THE PERIOD. THE MAJORITY OF ACTIVITY WILL LIKELY BE ON THE LIGHT  
TO MODERATE SIDE BUT THERE IS STILL POTENTIAL FOR SOME LOCALLY  
ENHANCED FOCUS DEPENDING ON EVOLUTION OF ENERGY ALOFT. AREAS OF  
RAIN AND HIGH ELEVATION SNOW ARE POSSIBLE LATER IN THE PERIOD OVER  
THE INTERIOR WEST/ROCKIES, WITH FAIRLY HIGH UNCERTAINTY FOR  
SPECIFICS. PROGRESSION OF ANY WESTERN ENERGY AND LOW LEVEL RETURN  
FLOW AROUND EASTERN U.S. HIGH PRESSURE MAY BEGIN TO SPREAD  
MOISTURE INTO THE CENTRAL U.S. TOWARD THE END OF THE WEEKEND.  
CENTRAL/SOUTHERN PARTS OF THE WEST WILL SEE TEMPS 5-15F ABOVE  
NORMAL WITH SOME COOLING POSSIBLE LATE. PROGRESSION OF FLOW ALOFT  
WILL PROMOTE A GRADUAL WARMING TREND OVER THE PLAINS AND MS VALLEY  
WITH THESE REGIONS POSSIBLY SEEING NEAR TO ABOVE NORMAL TEMPS BY  
SAT OR SUN.  
 
RAUSCH  
 

 
 
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