313  
FXUS02 KWBC 061500  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1000 AM EST SAT JAN 06 2018  
 
VALID 12Z TUE JAN 09 2018 - 12Z SAT JAN 13 2018  
 
15Z UPDATE...  
 
A SIMILAR APPROACH WAS TAKEN IN THIS FORECAST UPDATE TO OVERNIGHT,  
RESULTING IN A FORECAST KEPT FAIRLY CLOSE TO CONTINUITY. MODELS  
SHOWED SOME RELATIVELY MINOR TIMING DIFFERENCES INITIALLY ON TUE  
WITH UPPER LOW MOVING INTO CA, DIFFERENCES INCREASED BY WED-THU AS  
THE SYSTEM CROSSES THE ROCKIES AND MOVES INTO THE CENTRAL U.S. THE  
GFS HAS CONSISTENTLY BEEN ON THE FASTER SIDE OF THE SPREAD DURING  
THIS TIME PERIOD. A MULTI-MODEL DETERMINISTIC BLEND, INCLUDING THE  
00Z ECMWF/UKMET AND 06Z GFS WAS THE PREFERRED APPROACH FOR DAYS  
3-5, WITH MORE WEIGHT PLACED TOWARD THE SLOWER ECMWF/UKMET  
SOLUTIONS WHICH ALIGN A BIT BETTER WITH THE ENSEMBLE MEANS.  
 
DURING DAYS 6-7 (FRI-SAT), DIFFERENCES EMERGE AMONG THE GUIDANCE  
WITH RESPECT TO SHORTWAVE ENERGY INITIALLY MOVING INTO THE  
NORTHWEST ON WED, AND HOW MUCH AMPLIFICATION OCCURS AS IT REACHES  
THE BASE OF THE CENTRAL U.S. TROUGH FROM LATE THU ONWARD. THE GFS  
HAS CONSISTENTLY BEEN SLIGHTLY MORE AGGRESSIVE WITH THIS  
AMPLIFICATION RESULTING IN A SOMEWHAT SLOWER SOLUTION RELATIVE TO  
THE ECMWF. AT THIS TIME, A COMPROMISE APPROACH WAS TAKEN DURING  
THIS TIME PERIOD. 00Z ECENS/06Z GEFS WEIGHTING WAS BOOSTED TO  
ACCOUNT FOR INCREASED UNCERTAINTY, WITH MINORITY COMPONENTS OF THE  
00Z ECMWF/06Z GFS INCLUDED THROUGH DAY 7. THIS SOLUTION DEPICTS A  
SURFACE LOW OVER THE MID-ATLANTIC AT 12Z NEXT SAT, SOUTH OF THE  
ECMWF POSITION BUT NORTH OF THE GFS POSITION. THE DETAILS OF THIS  
EVOLUTION WILL HAVE IMPORTANT IMPLICATIONS FOR PRECIP AMOUNT AND  
TYPE ACROSS AREAS FROM THE TN/OH VALLEYS TO THE MID-ATLANTIC AND  
NORTHEAST.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0610Z)...  
   
..OVERVIEW
 
 
UPPER PATTERN WILL TRANSITION BACK TO A WESTERN RIDGE AND EASTERN  
TROUGH AFTER FLIPPING FROM THE OPPOSITE MIDWEEK NEXT WEEK. THIS  
WILL TAKE A PACIFIC SYSTEM THROUGH THE SOUTHWEST AND INTO THE  
PLAINS WHILE A CUTOFF LOW IN THE GULF SLOWLY MOVES INTO FLORIDA  
AND WEAKENS AHEAD OF THE INCOMING TROUGH. ANOTHER SURGE OF ARCTIC  
AIR WILL DIVE INTO THE HIGH PLAINS WEDNESDAY AND PUSH  
SOUTHEASTWARD THU-SAT.  
 
   
..GUIDANCE ASSESSMENT AND PREFERENCES
 
 
FAVORED MODELS FOR TUE-THU INCLUDED THE 12Z GFS/ECMWF/UKMET WITH  
SOME MINORITY WEIGHTING OF THE 18Z GFS. THEREAFTER, TRENDED TOWARD  
THE 12Z ECMWF ENSEMBLE MEAN WITH ADDITIONAL WEIGHTS TO THE 18Z  
GEFS MEAN, 12Z GFS, AND TWO PREVIOUS RUNS OF THE ECMWF TO ACCOUNT  
FOR DIFFERENCES WITH THE SYSTEM IN THE EAST. THIS MAINTAINED  
CONTINUITY WELL AS THE ENSEMBLES HAVE TRENDED A BIT SLOWER WITH  
THE CLOSED LOW IN THE GULF (BUT STILL NOT AS SLOW AS THE ECMWF)  
AND TOWARD A BIT MORE EMPHASIS ON A SEPARATION OF THE TROUGH INTO  
THE EAST LATER NEXT WEEK THAT MAY CARRY A SOUTHERN LOW INTO THE  
MID-ATLANTIC NEXT SATURDAY. OPTED TO RELY ON THE ENSEMBLE MEAN  
CONSENSUS HERE AND USE THE DETERMINISTIC SOLUTIONS AS MERE  
"SCENARIOS" AS THE DIFFERENCES BETWEEN A GIVEN GFS/ECMWF/CANADIAN  
RUN WERE LARGE IN BOTH SENSIBLE WEATHER AND IMPACTS (I.E.,  
PRECIPITATION TYPE/AMOUNT). 12Z GFS (AND LATEST 00Z GFS)  
REPRESENTED SOMETHING NEAR THE ENSEMBLE MEAN CONSENSUS IN THE  
EAST.  
 
FOR THE WEST, MODELS WERE STILL TRYING TO LESSEN THE DIFFERENCES  
IN TIMING OF THE INCOMING CLOSED LOW INTO CENTRAL CALIFORNIA ON  
TUESDAY. 12Z CANADIAN SEEMED TO BE MOST REMOVED FROM THE BETTER  
ECMWF/GFS-LED CONSENSUS. AS THE SYSTEM PUSHES INTO THE SOUTHERN  
PLAINS AND THEN LIFTS NORTHEASTWARD, HARD TO BE CONFIDENT IN A  
GIVEN SOLUTION BUT LATEST CONSENSUS SEEMS TO KICK THE LEAD SYSTEM  
NORTHEASTWARD AND RELOAD THE TROUGH WITH UPSTREAM VORTICITY WHICH  
MAY TRANSLATE EASTWARD AS A SHARP TROUGH INTO THE SOUTHEAST (GFS)  
OR PERHAPS LIFT NORTHWARD SOONER ALONG THE APPALACHIANS (ECMWF).  
FOR NOW, SPLIT THE DIFFERENCE WITH A SLIGHT EDGE TO THE ECMWF  
ENSEMBLE MEAN WHICH HAS BEEN MORE STEADY THAN THE GEFS MEAN.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
MODEST TO LOCALLY HEAVY RAIN AND HIGH ELEVATION SNOW WILL EXIT  
CALIFORNIA ON TUESDAY AND SLIDE THROUGH THE CENTRAL/SOUTHERN  
ROCKIES ON WEDNESDAY. LOW PRESSURE IN THE PLAINS ON WEDNESDAY WILL  
MOVE NORTHEASTWARD INTO THE WESTERN GREAT LAKES BY THURSDAY AND  
SPREAD SNOW TO THE NORTH OF ITS TRACK FROM NEBRASKA TO MINNESOTA.  
RAIN WILL INCREASE IN COVERAGE IN THE LOWER MISSISSIPPI VALLEY  
INTO THE SOUTHEAST AS ANOTHER SFC LOW LIFTS NORTHEASTWARD TOWARD  
THE MID-ATLANTIC BY LATE SATURDAY. POTENTIAL EXISTS FOR ICING  
NORTH OF THIS LOW IF COLD AIR AT THE SURFACE SINKS SOUTH ENOUGH AS  
PRECIPITATION PUSHES NORTHWARD. ANOTHER ROUND OF RAIN AND MOUNTAIN  
SNOWS WILL RETURN TO THE PACIFIC NORTHWEST AT THE END OF THE WEEK.  
 
TEMPERATURES WILL BE A BIT VARIABLE OVER THE CONUS, WITH A  
MODERATING TREND IN THE EAST BUT A COOLING TREND OVER MONTANA INTO  
THE PLAINS STARTING WEDNESDAY BEHIND THE ARCTIC FRONT.  
TEMPERATURES WILL BE 10-20 DEGREES BELOW AVERAGE BEHIND THE ARCTIC  
FRONT BUT 10-20 DEGREES ABOVE AVERAGE AHEAD OF THE FRONT.  
 
FRACASSO  
 

 
 
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