471  
FXUS02 KWBC 010651  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
150 AM EST MON JAN 01 2018  
 
VALID 12Z THU JAN 04 2018 - 12Z MON JAN 08 2018  
   
..OVERVIEW
 
 
A DEEP/AMPLIFIED UPPER TROUGH OVER THE EAST WILL PROMOTE VERY COLD  
TEMPERATURES FROM LATE WEEK INTO THE WEEKEND. A RAPIDLY DEEPENING  
WESTERN ATLANTIC SYSTEM NEAR THE START OF THE PERIOD SHOULD BRING  
MEANINGFUL SNOW AT LEAST TO NEW ENGLAND AND LIKELY GENERATE A  
PERIOD OF STRONG WINDS OVER THE NORTHEAST. EXPECT EVENTUAL  
PROGRESSION OF EASTERN PACIFIC TROUGH ENERGY TO BRING SOME  
MOISTURE INTO THE WEST DURING THE PERIOD AND RESULT IN A  
FLATTENING OF MEAN FLOW ACROSS THE LOWER 48 BY SUN-MON, LEADING TO  
A PRONOUNCED WARMING TREND OVER THE CENTRAL U.S. AND THEN FARTHER  
EASTWARD. THERE IS STILL CONSIDERABLE UNCERTAINTY OVER THE  
SPECIFICS OF THIS PACIFIC ENERGY THOUGH.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
LEADING SHORTWAVE ENERGY THAT IS WEAKENING AS IT REACHES THE WEST  
COAST BY EARLY THU HAS ATTAINED REASONABLE AGREEMENT IN THE  
GUIDANCE. HOWEVER MODELS/ENSEMBLES CONTINUE TO SHOW SIGNIFICANT  
SPREAD AND RUN TO RUN VARIABILITY FOR DETAILS OF THE UPSTREAM  
TROUGH. INDIVIDUAL SOLUTIONS HAVE BEEN INDICATING A SPLITTING OF  
FLOW IN DIFFERENT PLACES LEADING TO DIFFERENT TIMING OF HEIGHT  
FALLS DEPENDING ON LOCATION, AND THEN DIFFERENCES IN HOW MUCH  
PHASING WITH CANADIAN FLOW MAY OCCUR AS THE OVERALL TROUGH REACHES  
THE CENTRAL U.S.. LOOKING AT THE OVERALL EASTWARD PROGRESSION OF  
HEIGHT FALLS, ENSEMBLE MEAN TRENDS OVER THE PAST 1-2 DAYS VALID  
AROUND FRI-SAT HAVE BEEN TOWARD A COMPROMISE CLOSER TO THE GEFS  
MEAN WHILE THE ECMWF MEAN HAS BEEN TRENDING SLOWER FROM SOME EARLY  
RUNS THAT WERE QUITE PROGRESSIVE. FROM THAT POINT ONWARD THERE  
HAS BEEN A FAIR DEGREE OF INCONSISTENCY OVER HOW MUCH TROUGH  
ENERGY CROSSES THE SOUTHWEST. LATEST CYCLE GENERALLY LEANS TOWARD  
KEEPING SOMEWHAT HIGHER HEIGHTS OVER THAT REGION. EVEN WITH THESE  
DIFFERENCES, BY DAY 7 MON THE MEANS END UP BEING MORE  
SIMILAR/CONSISTENT THAN MIGHT BE EXPECTED WITH AN UPPER TROUGH  
OVER THE PLAINS/MS VALLEY AND ASSOCIATED SURFACE LOW PRESSURE OVER  
THE MS VALLEY/GREAT LAKES. BEHIND THIS TROUGH THE MEANS ADVERTISE  
A RIDGE BUILDING INTO AT LEAST THE NORTHERN HALF OF THE WEST BUT  
WITH GEFS MEANS WAFFLING ON TIMING/PHASE OF SOUTHERN STREAM FLOW.  
INCONSISTENCY/DIFFERENCES AMONG RECENT OPERATIONAL MODEL RUNS  
REFLECT THE WIDE ENSEMBLE SPREAD. FOR EXAMPLE THE 12Z ECMWF  
TRENDED MUCH SLOWER THAN ITS PREVIOUS RUNS OR CURRENT MAJORITY  
TIMING. MOST RECENT GFS RUNS HAVE BEEN A LITTLE LESS ERRATIC BUT  
STILL VARY WITH SOME SIGNIFICANT DETAILS. THROUGH THE 12Z/18Z  
CYCLE, THE 12Z-18Z GFS AND 12Z CMC WERE THE CLOSEST SOLUTIONS IN  
PRINCIPLE TO THE ENSEMBLE MEANS ALBEIT WITH QUESTIONABLE DETAILS  
AT SOME PARTICULAR VALID TIMES. NOT SURPRISINGLY SOME DETAILS  
HAVE CHANGED IN THE 00Z GFS/UKMET/CMC BUT THEIR SOLUTIONS STILL  
LOOSELY COMPARE WITH THE MEANS AND REINFORCE THE LOW PROBABILITY  
OF THE 12Z ECMWF SCENARIO MID-LATE PERIOD.  
 
RECENT D+8 MULTI-DAY MEANS HAVE BEEN VARYING WITH THE LATITUDE OF  
STRONGEST POSITIVE HEIGHT ANOMALIES WITHIN AN AXIS OF ABOVE NORMAL  
VALUES OVER THE EAST-CENTRAL PACIFIC INTO ALASKA. LATITUDE  
DIFFERENCES ARE ENOUGH FOR TELECONNECTIONS TO SUGGEST A FAIRLY  
SENSITIVE REGIME WITH RESPECT TO HOW MUCH SOUTHERN  
STREAM/UNDERCUTTING FLOW ULTIMATELY REACHES THE WEST COAST, AND  
EVENTUALLY SIGNIFICANT IMPLICATIONS FOR TEMPS/PRECIP ACROSS THE  
LOWER 48. THIS BIG-PICTURE ISSUE REINFORCES THE FORECAST  
UNCERTAINTY AS EXHIBITED BY THE FULL ARRAY OF ENSEMBLE  
MEMBERS/MODEL RUNS.  
 
OVER THE WESTERN ATLANTIC/EAST COAST, AT LEAST THROUGH THE 12Z/18Z  
CYCLES THE PAST DAY OF GUIDANCE APPEARED TO HAVE STABILIZED  
SOMEWHAT FOR THE RAPIDLY DEEPENING WESTERN ATLANTIC STORM FORECAST  
TO TRACK INTO THE CANADIAN MARITIMES BY EARLY FRI WITH A POSSIBLE  
DEPTH IN THE 950'S-960'S MB AT THAT TIME. THERE IS STILL  
UNCERTAINTY OVER THE FINER DETAILS OF HOW LEADING SHORTWAVE ENERGY  
AND VIGOROUS UPSTREAM DYNAMICS WILL INTERACT BUT DEEPER ENSEMBLE  
MEAN AND UKMET/CMC TRENDS SEEM TO SUGGEST INCREASING POTENTIAL FOR  
A VERY INTENSE SYSTEM WITH SOME MARGIN FOR ERROR IN EXACT DETAILS  
OF EVOLUTION. ENSEMBLE CLUSTERING FOR TRACK HAS BEEN MORE STABLE  
OVER THE PAST 24 HOURS THAN IN PREVIOUS DAYS BUT TRACK DIFFERENCES  
ARE STILL MEANINGFUL FOR THE EAST COAST/NEW ENGLAND-- ESPECIALLY  
WITH THE NEWLY ARRIVED 00Z UKMET/CMC SHOWING A MORE WESTWARD  
WOBBLE CLOSE TO NEW ENGLAND BY LATE THU COMPARED TO THE GFS/GEFS  
MEAN. 12Z/18Z SOLUTIONS APPEARED MORE TIGHTLY CLUSTERED.  
 
BASED ON RELATIVE AGREEMENT FOR SIGNIFICANT FEATURES, THE LATEST  
FORECAST INCORPORATED OPERATIONAL GUIDANCE DAYS 3-4 THU-FRI WITH  
THE 12Z-18Z GFS AND 12Z ECMWF GIVEN EQUAL WEIGHT AND LESSER INPUT  
FROM THE 12Z CMC/UKMET. DAY 5 SAT SERVED AS A TRANSITION WITH  
INCREASING 18Z GEFS/12Z ECMWF MEAN INCLUSION AND TAPERING OF THE  
12Z ECMWF, FOLLOWED BY DAYS 6-7 SUN-MON THAT FOLLOWED A GFS/CMC  
AND ENSEMBLE MEAN BLEND.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
CONFIDENCE IS INCREASING THAT PORTIONS OF NEW ENGLAND SHOULD  
RECEIVE A SIGNIFICANT SNOWFALL FROM THE POWERFUL STORM SYSTEM  
EXPECTED TO TRACK FROM THE WESTERN ATLANTIC INTO THE CANADIAN  
MARITIMES THU-FRI, WITH MUCH OF THE NORTHEAST ALSO LIKELY TO SEE A  
PERIOD OF STRONG WINDS THAT WILL CAUSE BLOWING OF SNOW AND  
DANGEROUSLY COLD WIND CHILLS. THERE IS POTENTIAL FOR SOME SNOW  
ALONG THE MID ATLANTIC COAST AS WELL WITH AMOUNTS VERY SENSITIVE  
TO EXACT TRACK OF THE LOW. BEHIND THIS SYSTEM, MUCH OF THE  
EASTERN HALF OF THE COUNTRY WILL SEE VERY COLD TEMPS THU INTO SAT  
WITH ONE OR MORE DAYS OF READINGS 20-30F BELOW NORMAL OVER  
NORTHERN/CENTRAL AREAS. PARTS OF THE NORTHEAST MAY BE MORE THAN  
30F BELOW NORMAL UNDER THE CORE OF UPPER TROUGHING. DAILY RECORDS  
SHOULD BE MOST PLENTIFUL FOR LOW MAX VALUES ON FRI AND SAT WITH  
SOME SCATTERED RECORD LOWS AS WELL. A FEW COLD RECORDS ARE  
POSSIBLE THU AND TEMPS MAY STILL BE COLD ENOUGH TO PRODUCE A FEW  
RECORDS OVER NEW ENGLAND ON SUN. TEMPS WILL MODERATE FROM WEST TO  
EAST WITH TIME AS PACIFIC FLOW EVENTUALLY PUSHES ACROSS THE  
WEST/CENTRAL U.S.. THIS PROGRESSION WILL ALSO SUPPORT A GRADUAL  
DECREASE OF LAKE EFFECT SNOW THAT WILL BE IN PROGRESS LATE THIS  
WEEK INTO THE WEEKEND.  
 
THE WEST WILL SEE A MORE UNSETTLED PERIOD COMPARED TO RECENT DAYS,  
WITH WEAKENING SHORTWAVE ENERGY SUPPORTING ONE AREA OF RAIN/HIGH  
ELEVATION SNOW TO THE CENTRAL WEST COAST AT THE START OF THE  
FORECAST PERIOD AND UPSTREAM TROUGH ENERGY BRINGING FURTHER PRECIP  
INTO AND ACROSS THE WEST. DIFFICULTY THAT MODELS ARE HAVING IN  
RESOLVING DETAILS OF FLOW WITHIN THE OVERALL TROUGH LOWERS  
CONFIDENCE IN DETERMINING WHERE BEST ENHANCEMENT MAY OCCUR.  
CURRENTLY MODEL/ENSEMBLE GUIDANCE SHOWS NORTHERN CALIFORNIA AND  
PACIFIC NORTHWEST AS HAVING THE BEST PROBABILITY OF SEEING THE  
MOST PRECIP FOR THE 5-DAY PERIOD. SOME OF THIS MOISTURE WILL  
EXTEND INLAND THROUGH THE ROCKIES. PROGRESSION OF FLOW ALOFT  
SHOULD BRING A LOW/FRONTAL SYSTEM INTO THE EAST-CENTRAL U.S. BY  
THE START OF NEXT WEEK, WITH ACCOMPANYING INCREASE IN PRECIP  
POTENTIAL FROM THE EASTERN PLAINS/MS VALLEY EASTWARD. EXPECT SNOW  
OVER NORTHERN AREAS AND RAIN TO THE SOUTH WITH A TRANSITION OVER  
CENTRAL LATITUDES. RAIN COULD BE ON THE HEAVY SIDE FROM THE LOWER  
HALF OF THE MS VALLEY EASTWARD. MUCH OF THE WEST WILL LIKELY SEE  
ABOVE NORMAL TEMPS WITH SOME LOCATIONS SEEING MULTIPLE DAYS OF  
PLUS 10-20F ANOMALIES FOR MIN AND/OR MAX READINGS. WITH LESSER  
ANOMALIES, SOME OF THIS WARMTH WILL EXTEND INTO THE PLAINS AND  
BEYOND BY THE LATTER HALF OF THE PERIOD.  
 
RAUSCH  
 

 
 
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