086  
FXUS02 KWBC 290653  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
152 AM EST FRI DEC 29 2017  
 
VALID 12Z MON JAN 01 2018 - 12Z FRI JAN 05 2018  
   
..OVERVIEW
 
 
A PERSISTENT AND AMPLIFIED WESTERN RIDGE-EASTERN TROUGH PATTERN  
WILL MAINTAIN VERY COLD TEMPS OVER MANY AREAS EAST OF THE ROCKIES  
AND AT TIMES DANGEROUSLY LOW WIND CHILL VALUES DURING PERIODS OF  
STRONGER WINDS, WHILE KEEPING A MAJORITY OF THE WEST NEAR TO ABOVE  
NORMAL FOR TEMPS. THE COMBINATION OF A LEADING SHORTWAVE AND  
STRONG AMPLIFICATION OF CANADIAN FLOW MAY SUPPORT A SIGNIFICANT  
WESTERN ATLANTIC/EAST COAST STORM SYSTEM AROUND MID-LATE WEEK, AND  
A COUPLE SYSTEMS WITHIN A MEAN TROUGH ALOFT OVER THE EASTERN  
PACIFIC COULD BRING SOME MOISTURE INTO THE WEST COAST STATES.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
EVOLUTION OF THE POTENTIAL MID-LATE WEEK SYSTEM NEAR THE EAST  
COAST WILL DEPEND ON SPECIFICS OF A LEADING SHORTWAVE POISED TO  
ENTER THE NORTHERN ROCKIES/HIGH PLAINS AT THE START OF THE PERIOD  
EARLY DAY 3 MON AND MUCH STRONGER DYNAMICS FORECAST TO AMPLIFY  
INTO THE UPPER MIDWEST/GREAT LAKES AND VICINITY AROUND DAY 5 WED.  
THE LEADING SHORTWAVE ORIGINATES FROM NORTHERN BC, JUST SOUTH OF  
AN UPPER HIGH. THIS TYPE OF SHORTWAVE TENDS TO BE DIFFICULT TO  
RESOLVE WELL VERY FAR IN ADVANCE SO IT IS NOT SURPRISING THAT  
GUIDANCE THROUGH THE 12Z/18Z CYCLES SHOWED MEANINGFUL DIFFERENCES  
IN SHAPE/PROGRESSION BY THE TIME IT REACHES THE SOUTHEASTERN U.S.  
BY MIDWEEK. FOR THE UPSTREAM FLOW AMPLIFYING INTO THE EAST,  
ENSEMBLE MEANS HAVE GENERALLY SUGGESTED A STRONGER/SHARPER TREND  
OVER THE PAST 1-2 DAYS LEADING TO A GRADUAL INCREASE IN THE NUMBER  
OF INDIVIDUAL ENSEMBLE MEMBERS THAT BRING WESTERN ATLANTIC LOW  
PRESSURE CLOSE TO THE EAST COAST, OR SLIGHTLY INLAND OVER NEW  
ENGLAND IN THE CASE OF A MINORITY OF 12Z MEMBERS. THE DIFFICULTY  
WITH THE OVERALL EVOLUTION LIES WITH THE INTERACTION AMONG THE TWO  
FEATURES ALOFT. THE CURRENT MAJORITY OF OPERATIONAL SOLUTIONS  
WOULD HAVE THE LEADING SHORTWAVE SUPPORT AN INITIAL WAVE WELL  
OFFSHORE BEFORE THE UPSTREAM HEIGHT FALLS ULTIMATELY DEVELOP/BRING  
LOW PRESSURE CLOSER TO THE COAST. THE PAST COUPLE ECMWF RUNS ARE  
SOMEWHAT IN THE MINORITY ON TWO COUNTS, AS THEY SHOW MORE PHASING  
ALOFT AMONG THE TWO FEATURES AND BECOME FAIRLY EXTREME WITH THE  
AMPLITUDE OF THE OVERALL EASTERN TROUGH ALOFT BY WED. IN SPITE OF  
THOSE ISSUES THE ENSEMBLE MEMBER TRENDS AT LEAST SUPPORT PARTIAL  
INCLUSION OF THE ECMWF SOLUTION IN THE FORECAST BLEND. ON THE  
OTHER HAND 18Z AND NEW 00Z GFS RUNS MAY TAKE LOW PRESSURE TOO FAR  
OFFSHORE. THE 12Z ECMWF MEAN WAS CLOSE TO ITS PREVIOUS RUN WITH A  
TRACK JUST EAST OF THE 40N 70W BENCHMARK WHILE THE GEFS MEANS  
THROUGH 18Z LOOKED TOO WEAK GIVEN OPERATIONAL MODEL SIGNALS. NOTE  
THAT THE 00Z GEFS MEAN HAS ADJUSTED TO A SOLUTION MUCH MORE  
SIMILAR TO THE 12Z ECMWF MEAN. GIVEN THE OPERATIONAL MODEL  
SIGNALS, WOULD EXPECT DEVELOPMENT TO BE SOMEWHAT STRONGER THAN  
SHOWN IN THE CURRENT ENSEMBLE MEANS. ULTIMATE PREFERENCE WOULD  
ESSENTIALLY BE A SOMEWHAT STRONGER VERSION OF THE MEANS GIVEN THE  
DETAIL DIFFERENCES IN OPERATIONAL RUNS.  
 
OVER THE EASTERN PACIFIC/WEST COAST, THERE IS INITIALLY MODERATE  
SCATTER OF SOLUTIONS FOR AN UPPER LOW WELL OFFSHORE CALIFORNIA.  
OVER TIME THERE IS REASONABLE AGREEMENT IN PRINCIPLE THAT THIS  
FEATURE SHOULD WEAKEN AS IT HEADS INTO THE WESTERN U.S. MEAN RIDGE  
BUT WITH SOME DIFFERENCES IN TIMING. BY DAY 5 WED THE ENSEMBLE  
MEANS TENDED TO BRING THE ENERGY IN A LITTLE FASTER THAN THE  
AVERAGE OF OPERATIONAL MODELS WITH INDIVIDUAL MEMBERS SHOWING  
CONSIDERABLE SPREAD. TYPICAL BIASES IN AN AMPLIFIED PATTERN WOULD  
TEND TO FAVOR LEANING SOMEWHAT TO THE SLOWER HALF OF THE SPECTRUM  
AND THE 00Z GFS/UKMET/CMC PROVIDE ADDED SUPPORT TO THAT IDEA.  
AMPLIFYING ENERGY IMMEDIATELY UPSTREAM WILL RELOAD THE EAST  
PACIFIC MEAN TROUGH ALBEIT WITH A FAIR AMOUNT OF UNCERTAINTY IN  
WHAT FORM AND LONGITUDE THAT WILL TAKE. LATEST SOLUTIONS RANGE  
FROM THE PAST COUPLE ECMWF RUNS THAT ARE WELL WEST OF THE  
GEFS/ECMWF MEANS, TO AN ELONGATED TROUGH OR EMBEDDED CLOSED LOW,  
OR IN WHAT APPEARS TO BE THE LEAST LIKELY SCENARIO A PROGRESSIVE  
POSITIVELY TILTED TROUGH IN THE 12Z CMC MEAN. THE 12Z GFS ALSO  
BECAME QUESTIONABLY PROGRESSIVE TOWARD THE END OF THE PERIOD. FOR  
THIS LATTER PART OF THE FORECAST THE GEFS/ECMWF MEANS APPEAR TO  
PROVIDE THE BEST INTERMEDIATE STARTING POINT.  
 
BASED ON THE ABOVE CONSIDERATIONS THE UPDATED FORECAST  
INCORPORATED OPERATIONAL GUIDANCE FROM THE 12Z-18Z GFS AND 12Z  
ECMWF/UKMET/CMC DURING THE FIRST HALF OF THE PERIOD AND THEN  
STEADILY INCREASED ENSEMBLE MEAN WEIGHT (MORE ECMWF MEAN THAN  
GEFS) SO THAT THE MEANS CONSTITUTED 70 PCT OF THE BLEND BY DAY 7  
FRI.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
LOCATIONS NEAR THE EAST COAST WILL NEED TO MONITOR EVOLUTION OF  
THE POTENTIAL MID-LATE WEEK STORM. CURRENT CONFIDENCE IN PRECISE  
IMPACTS FOR PRECIP IS IN THE LOWER HALF OF THE SPECTRUM WITH  
INDIVIDUAL SOLUTIONS RANGING FROM SIGNIFICANT SNOWFALL OVER THE  
MID ATLANTIC/NORTHEAST TO VERY LITTLE PRECIP. SOMEWHAT LESS  
SENSITIVE TO PRECISE STORM TRACK WILL BE A LIKELY PERIOD OF BREEZY  
TO WINDY CONDITIONS IN THE WAKE OF THE SYSTEM. MEANWHILE EXPECT A  
CONTINUATION OF LAKE EFFECT SNOW EPISODES. FLORIDA AND SOUTHERN  
TEXAS MAY SEE SOME RAINFALL WITH ONE OR MORE FRONTAL  
PASSAGES/EASTERLY LOW LEVEL FLOW. WITH UNCERTAINTY IN TIMING AND  
EXTENT, EXPECT MOISTURE TO MOVE INTO THE WEST COAST STATES WITH  
SNOW LEVELS LIKELY TO BE FAIRLY HIGH. FAVORED TERRAIN OVER  
NORTHERN/CENTRAL CALIFORNIA SHOULD SEE THE HIGHEST TOTALS FROM  
THIS RAIN/HIGH ELEVATION SNOW.  
 
THE OVERALL PATTERN WILL SUPPORT WELL BELOW NORMAL TEMPS OVER MOST  
LOCATIONS FROM THE EASTERN SLOPES OF THE ROCKIES TO EAST COAST,  
WITH SOME DAY TO DAY FLUCTUATIONS IN ANOMALIES WITH THE  
PROGRESSION OF INDIVIDUAL ARCTIC SURGES. DAY 3 MON WILL LIKELY  
SEE THE GREATEST COVERAGE OF READINGS 15-30F BELOW NORMAL ALONG  
WITH SOME MIN/COLD MAX DAILY RECORDS. FOR THE REST OF THE WEEK  
THERE WILL STILL BE CONSIDERABLE COVERAGE OF TEMPS 10-20F OR SO  
BELOW NORMAL, WITH LOWEST ANOMALIES TOWARD LATE WEEK TENDING TO BE  
OVER THE EAST WHILE THE CENTRAL U.S. MODERATES SOMEWHAT. THE  
COMBINATION OF COLD AIR AND BRISK WINDS WILL PRODUCE VERY LOW WIND  
CHILLS AT SOME LOCATIONS. OVER THE WEST, NORTHERN AREAS SHOULD BE  
WITHIN A FEW DEGS F OF NORMAL WHILE CENTRAL/SOUTHERN LOCATIONS  
WILL LIKELY BE 5-15F ABOVE NORMAL.  
 
RAUSCH  
 

 
 
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