930  
FXUS02 KWBC 101600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST THU JAN 10 2019  
 
VALID 12Z SUN JAN 13 2019 - 12Z THU JAN 17 2019  
   
..OVERVIEW
 
 
PROGRESSIVE AND AMPLIFIED FLOW ACROSS THE NORTH PACIFIC WILL  
CONTINUE TO BRING SIGNIFICANT STORM SYSTEMS TO THE WEST COAST  
PERIODICALLY DURING THE MEDIUM RANGE PERIOD. A STRENGTHENING UPPER  
RIDGE ACROSS THE NORTHERN ROCKIES/GREAT BASIN WILL RESULT IN SPLIT  
FLOW AS THESE PACIFIC SYSTEMS MOVE EASTWARD, WITH ENERGY  
SEPARATING INTO NORTHERN AND SOUTHERN STREAM COMPONENTS. NORTHERN  
STREAM ENERGY WILL AMPLIFY AS IT ENTERS BROAD CYCLONIC UPPER-LEVEL  
FLOW EAST OF THE UPPER-LEVEL RIDGE, RESULTING IN FREQUENT  
SHORTWAVES AND MEAN UPPER TROUGHING FROM THE UPPER MIDWEST TO THE  
GREAT LAKES. A PARTIALLY PHASED SHORTWAVE WILL CROSS THE THE  
MISSISSIPPI/OHIO VALLEYS ON SUN WITH DEVELOPMENT OF A COASTAL LOW  
PRESSURE SYSTEM ALONG THE NORTH/SOUTH CAROLINA COAST LIKELY, WHICH  
SHOULD THEN TRACK NORTHEASTWARD INTO THE ATLANTIC OCEAN.  
MEANWHILE, PROGRESSIVE FLOW ACROSS THE NORTH PACIFIC WILL KEEP THE  
WEST COAST ACTIVE, WITH AN AMPLIFIED TROUGH/UPPER LOW REACHING  
CALIFORNIA SUN-MON, ANOTHER WEAKENING SYSTEM ON WED, AND YET  
ANOTHER APPROACHING BY THU.  
 
A TREND IS BECOMING EVIDENT AMONG THE GUIDANCE THAT A RATHER  
SIGNIFICANT PATTERN CHANGE COULD BEGIN TO UNFOLD TOWARD THE LATTER  
PORTION OF THE MEDIUM RANGE AND BEYOND. MODEL/ENSEMBLE SOLUTIONS  
CONTINUE TO TREND STRONGER WITH THE UPPER RIDGE ACROSS WESTERN  
NORTH AMERICA BY THE MIDDLE OF NEXT WEEK, WHILE A DOWNSTREAM UPPER  
VORTEX DEEPENS NORTH OF HUDSON BAY, SETTING UP BROAD  
NORTH-NORTHWESTERLY FLOW FROM THE ARCTIC INTO CENTRAL CANADA.  
FARTHER SOUTH ACROSS THE CONUS, THE FLOW APPEARS TO REMAIN  
PROGRESSIVE, ORIGINATING IN THE NORTH PACIFIC, WHICH SHOULD LIMIT  
THE SOUTHWARD INTRUSION OF ARCTIC AIR INTO THE CONUS FOR NOW.  
NONETHELESS, WOULD EXPECT AN AREA OF ARCTIC AIR TO BEGIN POOLING  
ACROSS CENTRAL CANADA BY THE MIDDLE OF NEXT WEEK, WITH THE  
POTENTIAL FOR INCURSIONS SOUTHWARD, ESPECIALLY INTO THE  
MIDWEST/GREAT LAKES.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
A MULTI-MODEL BLEND SERVED AS A STARTING POINT FOR THE FORECAST  
DURING DAYS 3-5 (SUN-TUE), INCLUDING THE 06Z GFS AND THE 00Z GEFS,  
UKMET, OPERATIONAL ECMWF, AND ECMWF ENSEMBLE MEAN. MODELS STILL  
HAVE YET TO FULLY RESOLVE THE SPECIFIC STRUCTURE OF THE LEADING  
SHORTWAVE ENERGY CROSSING THE OHIO AND TENNESSEE VALLEYS ON SUN AS  
WELL AS THE POSSIBILITY OF A SECONDARY AMPLIFYING SHORTWAVE ON  
MON, BOTH OF WHICH PLAY A ROLE IN THE TIMING/TRACK OF THE LOW  
PRESSURE SYSTEM OFFSHORE. IN TERMS OF THE COASTAL LOW TRACK,  
GUIDANCE SEEMED VERY WELL CLUSTERED AROUND THE ECENS MEAN, WHICH  
IS VERY CLOSE TO THE PREFERRED WPC SOLUTION. THE STRENGTH OF THE  
LOW HAS BEEN TRENDING WEAKER IN THE GFS, CLOSER TO THE EC.  
SOLUTIONS WERE REASONABLY WELL-CLUSTERED WITH THE AMPLIFIED SYSTEM  
APPROACHING CALIFORNIA DURING THE DAY 3-5 PERIOD, THOUGH THERE ARE  
DIFFERENCES IN THE DEGREE TO WHICH THE UPPER-LEVEL LOW IS CUT OFF  
FROM THE MAIN FLOW. A CONSENSUS/MULTI-MODEL SOLUTION WAS PREFERRED  
HERE AS WELL.  
 
DURING DAYS 6-7 (WED-THU) UNCERTAINTIES BEGIN TO QUICKLY CROP UP  
IN BOTH THE NORTHERN AND SOUTHERN STREAMS. SHORTWAVE ENERGY  
CROSSING THE ROCKIES ON DAY 5 AND REACHING THE CENTRAL U.S. BY DAY  
6 HAS SHOWN QUITE A BIT OF RUN-TO-RUN VARIABILITY AMONG THE  
GUIDANCE AS TO HOW AMPLIFIED THE FEATURE WILL BECOME. ADDITIONAL  
SHORTWAVE ENERGY REACHING THE WEST COAST BY WED NIGHT/THU SHOWS  
SOMEWHAT BETTER CONSENSUS AMONG THE GUIDANCE, ESPECIALLY WITH  
RESPECT TO TIMING OF THE FEATURE. FARTHER NORTH, THE  
AFOREMENTIONED HIGHLY AMPLIFIED FLOW PATTERN DEVELOPING ACROSS  
CANADA WILL OPEN THE DOOR FOR SOUTHWARD TRANSPORT OF SHORTWAVE  
ENERGY ORIGINATING IN THE ARCTIC, WHICH MODELS GENERALLY DO A  
PARTICULARLY POOR JOB HANDLING. THE ECMWF HAS SHOWN A BIT MORE  
CONSISTENCY WITH THE NORTHERN STREAM FLOW DURING THIS TIME FRAME  
RELATIVE TO THE GFS. INCREASING WEIGHT WAS GIVEN TO ENSEMBLE MEANS  
OF THE NAEFS AND ECENS BY DAYS 6-7 DUE TO THE UNCERTAINTY ON  
TIMING AND POSITION OF SHORTWAVES WITHIN THE BROAD SCALE FLOW,  
WITH COMPONENTS OF THE OPERATIONAL EC FOR ADDED DETAIL AND SOME  
CONTINUITY FROM THE WPC FORECAST.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE LOW PRESSURE SYSTEM MOVING THROUGH THE SOUTHEAST U.S. AND OFF  
THE EASTERN SEABOARD SUN-MON IS EXPECTED TO PRODUCE A BAND OF  
WINTRY PRECIPITATION FROM THE OHIO VALLEY TO THE SOUTHERN  
MID-ATLANTIC, WITH HEAVY SNOW POSSIBLE FOR SOME AREAS. GUSTY WINDS  
MAY ALSO BE POSSIBLE ALONG THE MID-ATLANTIC COAST SUN NIGHT INTO  
MON AS THE SYSTEM BEGINS TO DEEPEN OFFSHORE. THE WEST COAST WILL  
REMAIN ACTIVE GIVEN THE RAPID FREQUENCY OF SHORTWAVE ENERGY AND  
FRONTAL SYSTEMS. THE HEAVIEST PRECIPITATION (LOW ELEVATION RAIN  
AND MOUNTAIN SNOW) WILL BE FOCUSED ACROSS CALIFORNIA, WITH  
MULTIPLE ROUNDS OF PRECIPITATION POSSIBLE THROUGH THE FORECAST  
PERIOD. TOTAL PRECIPITATION AMOUNTS OVER THE PERIOD COULD BE  
HEAVY, WITH UPSLOPE FLOW ENHANCING QPF TO 4 TO 8 INCHES IN THE  
SIERRA AND COASTAL RANGES.  
 
ABOVE AVERAGE TEMPERATURES ARE EXPECTED SUN-TUE ACROSS MUCH OF THE  
CENTRAL AND NORTHERN PLAINS TO THE UPPER AND MIDDLE MISSISSIPPI  
VALLEY REGIONS, WITH HIGH TEMPERATURES GENERALLY 10-15 DEG F ABOVE  
AVERAGE. THEN ON WED TO THURS, ARCTIC AIR MAY APPROACH THE  
NORTHERN PLAINS AND UPPER MIDWEST REGIONS, COOLING DOWN  
TEMPERATURES. THE SOUTHEAST AND EASTERN SEABOARD WILL INITIALLY  
SEE BELOW AVERAGE TEMPERATURES IN THE WAKE OF THE UPPER TROUGH AND  
LOW PRESSURE SYSTEM, WITH HIGHS 5-10 DEG BELOW AVERAGE SUN-MON,  
AND MODERATING TEMPERATURES FROM TUE ONWARD.  
 
RYAN/TATE  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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