184  
FXUS02 KWBC 301600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST SAT DEC 30 2017  
 
VALID 12Z TUE JAN 02 2018 - 12Z SAT JAN 06 2018  
 
...ARCTIC AIR SPREADING EAST OF THE ROCKIES WILL MAINTAIN WELL  
BELOW NORMAL TEMPERATURES...  
 
   
..PATTERN OVERVIEW
 
 
AN UPPER TROUGH ANCHORED OVER THE EASTERN HALF OF THE COUNTRY  
SHOULD MAINTAIN ITS PRESENCE INTO THE FIRST WEEK OF THE NEW YEAR.  
500-MB HEIGHT DEPARTURES AT TIMES WILL DIP BELOW 2 STANDARD  
DEVIATIONS BELOW AVERAGE FOR EARLY JANUARY. INITIAL SPLIT FLOW  
OVER THE CENTRAL/EASTERN U.S. SHOULD LEAD TO EVENTUAL PHASING OVER  
THE WESTERN ATLANTIC SPAWNING A POWERFUL LOW TAKING AIM AT NOVA  
SCOTIA AND NEW BRUNSWICK LATE THURSDAY. CONTINUED COLD ADVECTION  
IN THE WAKE WILL ASSURE A COLD PERIOD FOR THE NEXT WORK WEEK.  
MEANWHILE...A PAIR OF SYSTEMS ARE EXPECTED TO AFFECT THE WEST  
COAST WITH THE INITIAL ONE EXPECTED TO SHEAR WHILE PUSHING INTO  
NORTHERN CA ON WEDNESDAY NIGHT. IN ITS WAKE...ADDITIONAL LONGWAVE  
TROUGHING TAKES SHAPE WITHIN A POTENTIAL SPLIT FLOW REGIME ACROSS  
THE EASTERN PACIFIC. THIS NEGATIVE HEIGHT ANOMALY IS FORECAST TO  
REACH THE WESTERN U.S. BY DAY 6/JANUARY 5 AS THE ENTIRE  
SYNOPTIC-SCALE PATTERN SHIFTS EASTWARD. CONSEQUENTLY...A MEAN  
RIDGE WILL LIKELY BE POSITIONED OVER THE INTERMOUNTAIN  
WEST/ROCKIES BY DAY 7/JANUARY 6.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
COMPARED TO PREVIOUS DAYS...THE VOLATILITY OF THE FORECAST HAS  
DIMINISHED CONSIDERABLY AS THE GUIDANCE HAVE COME INTO MUCH BETTER  
AGREEMENT. REGARDING THE SPLIT FLOW ACROSS THE CENTRAL/EASTERN  
U.S. ON TUESDAY...THE 00Z UKMET CONTINUES TO BE A WESTWARD OUTLIER  
WITH THIS FEATURE AS WELL AS OTHER UPSTREAM SYNOPTIC SYSTEMS. THE  
SOUTHERN STREAM SHORTWAVE IS EXPECTED TO CROSS THE FL PANHANDLE ON  
WEDNESDAY MORNING WHILE THE NORTHERN BRANCH OF THE JET FAVORS A  
TROUGH PUSHING INTO THE GREAT LAKES. EVENTUAL PHASING OF THESE TWO  
STREAMS IS LIKELY OVER THE WESTERN ATLANTIC WITH THE EASTWARD  
TREND OF THIS SCENARIO HOLDING UP FROM YESTERDAY. RAPID  
CYCLOGENESIS SHOULD UNFOLD WEST OF BERMUDA LATE WEDNESDAY/EARLY  
THURSDAY LEADING TO A CYCLONE POSSIBLY NEARING 960-MB PRESSURES BY  
THURSDAY NIGHT IN THE VICINITY OF NOVA SCOTIA/NEW BRUNSWICK. THE  
06Z GFS DID TREND A BIT WEST OF THE 00Z GFS WHICH PLACES IT MUCH  
CLOSER TO THE 00Z ECMWF SOLUTION. MEANWHILE...THE 00Z CMC IS EAST  
OF THIS CONSENSUS BEFORE DEVELOPING A SECOND WAVE UNDER A  
STRENGTHENING SHORTWAVE. WHILE THERE IS STILL A LOT OF ENSEMBLE  
SPREAD AS SEEN IN THE SURFACE LOW PLOTS...MOST PASS EAST OF THE  
INFAMOUS 40N/70W BENCHMARK AND INTO NOVA SCOTIA. IN ITS WAKE...A  
REINFORCEMENT OF THE MEAN TROUGH WILL BE ESTABLISHED AS ADDITIONAL  
ARCTIC AIR DIGS TOWARD THE SOUTHERN U.S.  
 
A MEAN UPPER TROUGH WILL INITIALLY BE POSITIONED OVER THE WEST  
COAST WHILE A SLOW MOVING UPPER LOW APPROACHES UPSTREAM. AS  
MENTIONED EARLIER...THE 00Z UKMET IS A WESTERN OUTLIER HERE WITH  
NO DISCERNIBLE TRENDS AMONG THE GLOBAL GUIDANCE. RECENT MODELS  
SUPPORT A SHEARING SHORTWAVE REACHING NORTHERN CA BY LATE  
WEDNESDAY BEFORE FLATTENING OUT COMPLETELY UPON IMPACT WITH THE  
UPPER RIDGE. FARTHER UPSTREAM...MODEL SPREAD IS DECIDEDLY HIGHER  
AS THE GUIDANCE ATTEMPTS TO RESOLVE SPLIT FLOW EVOLVING OVER THE  
NORTHEASTERN/EASTERN PACIFIC. THERE HAS BEEN AN EASTWARD TREND  
SEEN IN THE OPERATIONAL ECMWF AS WELL AS MANY OF ITS CORRESPONDING  
ENSEMBLES. THIS PLACES ITS SOLUTIONS AHEAD OF THE 06Z/00Z GEFS AND  
00Z NAEFS MEANS. ULTIMATELY THIS WILL AFFECT HOW QUICKLY  
PRECIPITATION WILL MOVE BACK TOWARD THE WEST COAST AS WELL AS THE  
POSITION OF THE DOWNSTREAM RIDGE.  
 
GIVEN THE REMARKABLY GOOD AGREEMENT SEEN THROUGH DAY  
5/THURSDAY...WAS ABLE TO AFFORD HEAVY USE OF THE OPERATIONAL  
MODELS...IN PARTICULAR THE 06Z GFS/00Z ECMWF. BEYOND THIS  
PERIOD...ENSEMBLE MEANS STARTED TO WEIGH INTO THE BLEND MORE  
HEAVILY GIVEN THE GROWING SYNOPTIC-SCALE BLEND. UTILIZED A  
COMBINATION OF THE 06Z GEFS/00Z NAEFS MEANS WITH THE 00Z ECMWF  
ENSEMBLE MEAN WITH 30 PERCENT OF THE BLEND CONSISTING OF THE 06Z  
GFS/00Z ECMWF OUT TO DAY 7/JANUARY 6.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
MULTIPLE ARCTIC SURGES WILL CARRY TEMPERATURES ON THE ORDER TO 20  
TO 30 DEGREES BELOW CLIMATOLOGY TO MANY LOCATIONS EAST OF THE  
ROCKIES. THROUGHOUT THE PERIOD...THE COLDEST AIR WILL SETTLE FROM  
NORTHERN PLAINS EASTWARD ACROSS THE UPPER MIDWEST/MIDDLE MS VALLEY  
INTO THE GREAT LAKES AND NEW ENGLAND. HIGH TEMPERATURES MAY  
STRUGGLE TO GO ABOVE 0 DEGREES OVER AREAS OF MN AND THE INTERIOR  
NORTHEAST WHILE OVERNIGHT LOWS PLUNGE INTO THE NEGATIVE TEENS AND  
PERHAPS A -20S IN SOME LOCATIONS. OF COURSE AREAS ALONG THE GULF  
COAST WILL NOT BE SPARED EITHER AS LOW TEMPERATURES SURGE BELOW  
FREEZING AS FAR SOUTH AS SOUTH TX AND ALONG THE ENTIRE GULF COAST  
INTO CENTRAL FL. ON TUESDAY...MANY LOCATIONS WITHIN SOME OF THESE  
MENTIONED AREAS MAY SEE DAILY TEMPERATURE RECORDS BROKEN FOR BOTH  
OVERNIGHT LOWS AND CHILLY AFTERNOON HIGHS. IN ADDITION TO THE  
ARCTIC AIR SPREADING THROUGH...GUSTY WINDS AT TIMES WILL LOWER  
WIND CHILLS TOWARD SOME VERY DANGEROUS LEVELS. WHILE FRIGID  
CONDITIONS PERSIST EAST OF THE ROCKIES...READINGS AROUND 10 TO 15  
DEGREES ABOVE AVERAGE WILL BE COMMONPLACE OVER MANY AREAS OF THE  
WESTERN U.S...PARTICULARLY ACROSS THE CENTRAL GREAT BASIN INTO THE  
DESERT SOUTHWEST.  
 
AS CYCLONIC CONTINUES OVER THE GREAT LAKES...LAKE EFFECT SNOWS  
WILL LIKELY CONTINUE ALTHOUGH VERY LOW PRECIPITABLE WATER VALUES  
COULD BE A LIMITING FACTOR AT TIMES. HOWEVER...700-MB TEMPERATURES  
DROPPING DOWN INTO THE -20C TO -30C RANGE WILL SIGNIFICANTLY  
STEEPEN LOW-LEVEL LAPSE RATES LEADING TO VERY STRONG UPDRAFTS  
AIDING IN HEAVY SNOW PRODUCTION. FARTHER SOUTH AND EAST...MODERATE  
RAINFALL WILL BE POSSIBLE ALONG THE ADVANCING FRONT ACROSS FL  
WHILE WAVE DEVELOPMENT OVER THE WESTERN ATLANTIC SHOULD AFFORD AN  
INCREASE IN WINTRY PRECIPITATION CHANCES ACROSS AREAS OF NEW  
ENGLAND. ANY WESTWARD TREND IN THIS CYCLONE TRACK WOULD SPREAD  
SNOW FURTHER INTO THE NORTHEASTERN U.S. ACROSS THE WEST COAST...A  
RETURN TO A WET PATTERN IS ANTICIPATED GIVEN THE SERIES OF SYSTEMS  
APPROACHING FROM THE WEST. SNOW WILL BE LIKELY ACROSS THE HIGHER  
ELEVATIONS WHILE SOME WELL NEEDED RAIN SHOULD PUSH INTO  
SOUTHERN/CENTRAL CA WHICH HAS NOT SEEN MUCH PRECIPITATION DURING  
THEIR RAINY SEASON.  
 
RUBIN-OSTER  
 

 
 
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