161  
FXUS02 KWBC 291607  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1106 AM EST FRI DEC 29 2017  
 
VALID 12Z MON JAN 01 2018 - 12Z FRI JAN 05 2018  
 
...EXTREMELY COLD TEMPERATURES CONTINUE TO SPREAD FROM THE ROCKIES  
EASTWARD...  
 
   
..PATTERN OVERVIEW
 
 
THE START OF THE NEW YEAR WILL CONTINUE TO FAVOR BELOW AVERAGE  
HEIGHTS ACROSS THE EASTERN HALF OF THE COUNTRY AS SEVERAL CLIPPER  
SYSTEMS BRING REINFORCING SHOTS OF ARCTIC AIR TO THE REGION. WHILE  
PRIMARILY NORTHERN STREAM DOMINANT...A SHORTWAVE ALONG THE  
CENTRAL/EASTERN GULF COAST ON WEDNESDAY MORNING MAY EVENTUALLY  
LEAD TO A PHASED SOLUTION SOMEWHERE ACROSS THE WESTERN ATLANTIC.  
THIS WILL AID IN THE DEVELOPMENT OF A SIGNIFICANT AREA OF LOW  
PRESSURE WHICH COULD AFFECT SOME SECTION OF EASTERN NORTH AMERICA.  
ACROSS THE WEST COAST...INITIAL RIDGING SHOULD BE IN PLACE WHILE A  
SPLIT FLOW REGIME DOMINATES THE OFFSHORE WATERS OF THE EASTERN  
PACIFIC. THERE IS INCREASING CONFIDENCE A SHORTWAVE WILL APPROACH  
THE CA COAST ON WEDNESDAY BEFORE SHEARING ON ITS PUSH INLAND.  
MID-LEVEL HEIGHTS ARE FORECAST TO REBOUND OVER THE WEST COAST  
WHILE A RE-ESTABLISHMENT OF THE LONGWAVE TROUGH TAKES PLACE OVER  
THE EASTERN PACIFIC.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
MODELS REMAIN IN GOOD AGREEMENT WITH A BROAD LONGWAVE TROUGH  
EXTENDING OVER THE GREAT LAKES/NORTHEASTERN U.S. ON NEW YEARS DAY.  
AN ARCTIC HIGH WILL SINK SOUTHWARD INTO THE NORTHERN/CENTRAL  
PLAINS WITH EXPECTED SEA-LEVEL PRESSURES IN THE 1050-1054 MB  
RANGE. THIS COULD APPROACH JANUARY RECORDS BASED ON THE IN-HOUSE  
CLIMATOLOGY. THIS DOME OF HIGH PRESSURE IS FORECAST TO CONTINUE  
ITS SURGE SOUTH AND EAST SPREADING ANOTHER BLAST OF ARCTIC AIR TO  
THE COUNTRY. WHILE THE UPPER PATTERN SHOULD BE HEAVILY DOMINATED  
BY THE NORTHERN STREAM...MODELS CONTINUE TO ADVERTISE A SHORTWAVE  
EJECTING OUT OF THE NORTHERN ROCKIES ON 01/1200Z TO BE A PLAYER BY  
MID-WEEK ACROSS THE SOUTHEASTERN U.S. RECENT MODEL RUNS SHOW  
FURTHER SEPARATION FROM THE NORTHERN STREAM AND A RESULTANT  
QUICKER EVOLUTION BASED ON MULTI-DAY ENSEMBLE SPAGHETTI PLOTS. THE  
EVENTUAL PHASING SCENARIO WOULD TAKE PLACE A BIT FURTHER NORTH AND  
EAST WHICH HAS CHANGED THE COASTAL LOW TRACK FROM THE PREVIOUS DAY  
OF MODEL RUNS. OPERATIONAL MODEL COMPARISONS CONTINUE TO SUGGEST  
THE 00Z UKMET IS A SLOW OUTLIER WHILE THE 00Z ECMWF AS WELL AS  
PRECEDING RUNS HAVE BEEN MORE AMPLIFIED THAN MOST OF THE ENSEMBLE  
MEMBERS. THE CORRESPONDING SURFACE ENSEMBLE LOW PLOTS CONTINUE TO  
INDICATE QUITE A BIT OF SCATTER BETWEEN AND WITHIN THE INDIVIDUAL  
GLOBAL MODELS. THERE ARE STILL PLENTY OF DEEP SURFACE SOLUTIONS  
WITH IMPACTS ANYWHERE FROM THE MID-ATLANTIC UP TO NEWFOUNDLAND  
DURING THE DAY 5-7...JANUARY 3-5 TIMEFRAME. IN THE WAKE OF THIS  
SYSTEM...PERSISTENT NORTHWESTERLY FLOW ALOFT WILL REMAIN A FIXTURE  
IN THE FORECAST WHICH CONTINUES TO FAVOR EXTREMELY COLD DAILY  
TEMPERATURES.  
 
RESOLVING THE PATTERN ACROSS THE EASTERN PACIFIC CONTINUES TO BE  
AN ISSUE AS IT HAS BEEN IN RECENT DAYS. A SOLID MODEL CONSENSUS IS  
NOTED DURING THE FIRST COUPLE OF DAYS WITH THE MEAN RIDGE ALONG  
THE WEST COAST. WITHIN THE SPLIT FLOW REGIME OFFSHORE...A SOUTHERN  
STREAM SHORTWAVE IS EXPECTED TO APPROACH THE CA COAST BY WEDNESDAY  
SOME TIME. COMPARING THE PAST FOUR RUNS OF THE GFS/ECMWF...THE GFS  
HAS SHOWN MUCH STRONGER RUN-TO-RUN CONTINUITY WHILE THE ECMWF HAS  
BEEN BOUNCING AROUND A BIT MORE. WHILE THE OPERATIONAL MODELS HAVE  
GENERALLY BEEN SLOWER...THERE ARE STILL MANY ENSEMBLE  
SOLUTIONS...PARTICULARLY OF THE ECMWF VARIETY WHICH SUGGEST A JUMP  
EASTWARD MAY BE POSSIBLE IN FUTURE MODEL RUNS. EVENTUALLY THE  
SHORTWAVE WILL SHEAR BY EARLY THURSDAY ACROSS THE CENTRAL GREAT  
BASIN WHILE RENEWED TROUGHING TAKES SHAPE OVER THE EASTERN  
PACIFIC. THE 558-MB ENSEMBLE SPAGHETTI PLOT SHOWS QUITE A BIT OF  
SPREAD OFFSHORE WITH THE ENSEMBLE MEANS DIFFERING IN  
TIMING/PLACEMENT. THE 00Z ECMWF ENSEMBLE MEAN IS THE QUICKEST  
RELATIVE TO THE 00Z GEFS/NAEFS MEANS WHICH ARE MORE IN PHASE WITH  
ONE ANOTHER. DETAILS WILL LIKELY CHANGE IN THE DAYS AHEAD GIVEN  
HOW MUCH NOISE THERE IS AMONG THE ENSEMBLES.  
 
FOR THE MOST PART...FAVORED AN EQUAL BLEND OF THE 06Z/00Z GFS AND  
00Z GEFS MEAN WITH THE 00Z ECMWF/ECMWF ENSEMBLE MEAN. BEYOND DAY  
5/JANUARY 3...STARTED TO LEAN MORE HEAVILY ON THE ENSEMBLE  
SOLUTIONS WITH SLIGHTLY MORE WEIGHTING TOWARD THE 00Z GEFS MEAN  
GIVEN THE DESIRE TO BE SLOWER IN THE EASTERN PACIFIC. DESPITE  
HEAVY USE OF ENSEMBLES...THESE MEANS WERE PLENTY DEEP ENOUGH TO  
SUPPORT A ROBUST AREA OF LOW PRESSURE OVER THE WESTERN  
ATLANTIC/EASTERN CANADA ON JANUARY 4/5.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE MAJOR WEATHER STORY WILL REMAIN THE EXTREME COLD INHABITING  
AREAS FROM THE ROCKIES EASTWARD. THE MORNINGS OF JANUARY 1/2 WILL  
FEATURE TEMPERATURE ANOMALIES IN THE 30 TO 35 DEGREE BELOW AVERAGE  
RANGE ACROSS THE NORTHERN ROCKIES EASTWARD INTO THE NORTHERN  
PLAINS/MIDDLE MS VALLEY. THIS EQUATES TO LOWS IN THE -20S WITH  
EVEN SOME POCKETS OF -30S ACROSS EASTERN MT. WHILE THE AIR MASS  
WILL MODERATE ON ITS MOVEMENT TOWARD THE SOUTH AND EAST...LOWS IN  
THE 20S ARE EXPECTED DOWN INTO SOUTH TX EASTWARD ALONG THE ENTIRE  
GULF COAST. DESPITE AMPLE SUNSHINE EXPECTED DURING THE  
DAYTIME...LITTLE RELIEF FROM THE COLD IS EXPECTED GIVEN THE  
PERSISTENT COLD ADVECTION AND LOW SOLAR ANGLE FOR EARLY JANUARY. A  
NUMBER OF DAILY TEMPERATURE RECORDS WILL LIKELY BE BROKEN FROM THE  
GREAT PLAINS EASTWARD...PRIMARILY FOR THE COLD MAXIMA EXPECTED.  
THIS IS PARTICULARLY THE CASE ON NEW YEARS DAY WITH SINGLE DIGITS  
AND TEENS EXPECTED ALONG THE I-95 CORRIDOR FROM NEW ENGLAND DOWN  
TO THE MID-ATLANTIC WHILE 20S ARE COMMONPLACE ALONG THE RED RIVER  
AND INTO THE LOWER MS VALLEY. OUT WEST...A PERSISTENT RIDGE WITH  
OCCASIONAL PACIFIC INFLUENCES SHOULD KEEP TEMPERATURES ON THE MILD  
SIDE WITH READINGS 10 TO 15 DEGREES ABOVE CLIMATOLOGY OVER THE  
CENTRAL GREAT BASIN AND DESERT SOUTHWEST.  
 
CYCLONIC FLOW ACROSS THE GREAT LAKES SHOULD CONTINUE TO KEEP LAKE  
EFFECT SNOW BANDS IN PLAY. SOME OF THESE ARCTIC SURGES WILL BRING  
700-MB TEMPERATURES DOWN INTO THE -25C TO -30C RANGE WHICH WILL  
SIGNIFICANTLY STEEPEN LAPSE RATES OVER ANY ICE-FREE LAKE SURFACES.  
FARTHER SOUTH...MODERATE TO LOCALLY HEAVY RAINFALL SHOULD BE  
POSSIBLE OVER THE STATE OF FL GIVEN A SLOW-MOVING FRONTAL ZONE  
COUPLED WITH ABOVE AVERAGE PRECIPITABLE WATER VALUES IN PLACE.  
DEPENDING ON THE EVENTUAL SURFACE LOW TRACK ACROSS THE WESTERN  
ATLANTIC...SOME WINTRY PRECIPITATION THREAT WILL BE POSSIBLE  
SOMEWHERE OVER THE NORTHEASTERN U.S. BY MID/LATE NEXT WEEK. ACROSS  
THE WEST...WIDESPREAD PRECIPITATION IS EXPECTED GIVEN THE  
SHORTWAVE APPROACHING THE COAST. THE BEST ACTION SHOULD BE ACROSS  
NORTHERN CA AND INTO THE NORTHERN/CENTRAL SIERRA WITH SNOW LIKELY  
OVER THE HIGHER ELEVATIONS. ADDITIONAL PRECIPITATION MAY BE  
LURKING OFFSHORE TOWARD THE END OF THE FORECAST AS THE NEXT TROUGH  
TAKES SHAPE UPSTREAM.  
 
RUBIN-OSTER  
 

 
 
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