968  
FXUS02 KWBC 241516  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1015 AM EST THU JAN 24 2019  
 
VALID 12Z SUN JAN 27 2019 - 12Z THU JAN 31 2019  
 
15 UTC UPDATE...  
 
MODEL CONSENSUS HAS IMPROVED OVER THE PAST DAY, ESPECIALLY DURING  
THE LATTER PORTION OF THE MEDIUM RANGE PERIOD. A HIGHLY AMPLIFIED  
LARGE SCALE FLOW REGIME WILL PERSIST WITH RIDGING OFF THE WEST  
COAST AND A BROAD TROUGH/CLOSED UPPER LOW FROM HUDSON BAY SOUTH  
INTO THE GREAT LAKES/MIDWEST. SHORTWAVE ENERGY ORIGINATING IN THE  
ARCTIC WILL TRAVERSE THE EASTERN SIDE OF THE RIDGE AND MOVE INTO A  
BROAD REGION OF CYCLONIC UPPER FLOW ACROSS THE EASTERN HALF OF THE  
CONUS, REINFORCING THE UPPER-LEVEL TROUGH. WHILE MODELS CONTINUE  
TO SHOW SOME TIMING/AMPLITUDE DIFFERENCES WITH RESPECT TO THESE  
INDIVIDUAL SHORTWAVES, CONSENSUS HAS GENERALLY IMPROVED OVER  
RECENT MODEL RUNS. THE REGION FROM THE CENTRAL PLAINS TO THE GREAT  
LAKES AND NORTHEAST WILL BE THE DOMINANT STORM TRACK DURING THE  
PERIOD, WITH A COUPLE LOW PRESSURE SYSTEMS TRAVERSING. MODEL  
SOLUTIONS SHOWED REASONABLY GOOD CLUSTERING WITH EACH OF THESE  
SYSTEMS. BY DAYS 6-7 (WED-THU) THE MAIN QUESTION BECOMES TO WHAT  
DEGREE NORTH PACIFIC SHORTWAVE ENERGY WILL BE ABLE TO FLATTEN THE  
RIDGE, AND WHETHER MORE BLOCKED FLOW BEGINS TO EVOLVE ACROSS THE  
ARCTIC, WITH A NUMBER OF SOLUTIONS SUGGESTING AN ANOMALOUS CLOSED  
RIDGE DEVELOPING NEAR THE BERING STRAIT (WHICH WOULD REINFORCE THE  
EXISTING AMPLIFIED FLOW PATTERN ACROSS THE CONUS. GIVEN THESE  
CONSIDERATIONS, A MULTI-MODEL BLEND OF THE LATEST DETERMINISTIC  
GUIDANCE SERVED AS A FORECAST STARTING POINT FOR DAYS 3-5  
(SUN-TUE), WITH AN INCREASE IN WEIGHTING OF ENSEMBLE MEANS (ECENS  
AND GEFS) DURING DAYS 6-7. THIS YIELDED A FORECAST VERY CLOSE TO  
CONTINUITY.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0659 UTC)  
 
   
..OVERVIEW
 
 
DOWNSTREAM FROM A MEAN RIDGE TENDING TO BE ALIGNED JUST OFF THE  
WEST COAST, VERY DEEP AND AMPLIFIED MEAN TROUGHING WILL PERSIST  
OVER THE EASTERN HALF TO TWO-THIRDS OF NORTH AMERICA. THE EXACT  
SHAPE OF THE OVERALL TROUGH WILL VARY FROM DAY TO DAY IN RESPONSE  
TO EMBEDDED SHORTWAVES. THIS PATTERN WILL MAINTAIN COLD WEATHER  
OVER MANY AREAS EAST OF THE ROCKIES, WITH THE WEST-CENTRAL U.S.  
RECEIVING A GLANCING BLOW OF CHILLY AIR FOR A TIME AS WELL. THE  
MOST NOTABLE CHANGE IN THE GUIDANCE OVER THE PAST DAY APPEARS OVER  
AND NORTH OF THE EAST-CENTRAL PACIFIC. LATEST SOLUTIONS ARE  
SHOWING A MORE PRONOUNCED UPPER HIGH BUILDING OVER OR WEST OF THE  
BERING STRAIT WHILE TROUGHING DIGS OVER THE EAST-CENTRAL PACIFIC.  
THIS EVOLUTION ULTIMATELY KEEPS THE MEAN RIDGE IN PLACE CLOSER TO  
THE WEST COAST. AT LEAST FOR THE MOMENT THE GUIDANCE IS SHOWING  
BETTER AGREEMENT WITH THE MOST SIGNIFICANT FEATURES BUT THERE ARE  
STILL STRAY OPERATIONAL SOLUTIONS AND SOME ENSEMBLES THAT TEMPER  
CONFIDENCE TO SOME DEGREE.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
WITHIN AND AROUND THE CORE OF THE MEAN TROUGH, THE LEADING FEATURE  
OF INTEREST WILL BE AN UPPER LOW FORECAST TO BE OVER ONTARIO AT  
THE START OF THE PERIOD EARLY SUN. THIS UPPER LOW WILL SUPPORT  
LOW PRESSURE THAT SHOULD TRACK ACROSS SOUTHEASTERN CANADA WITH  
ASSOCIATED FRONTAL SYSTEM CROSSING THE GREAT LAKES/NORTHEAST.  
OVER THE PAST 24-36 HOURS THE OPERATIONAL GFS/ECMWF HAVE GENERALLY  
LED THE ENSEMBLE MEANS FOR DEPICTING THIS SYSTEM'S DEFINITION  
WHILE RECENT TRENDS HAVE BEEN SOMEWHAT FASTER. BEHIND THIS SYSTEM  
A VIGOROUS SHORTWAVE DROPPING SOUTHEAST FROM WESTERN CANADA WILL  
BRING A RATHER STRONG CLIPPER TYPE SYSTEM ACROSS THE NORTHERN  
PLAINS AND MIDWEST, THEN MOST LIKELY THE LOWER/EASTERN GREAT LAKES  
INTO NEW ENGLAND AND EASTERN CANADA. A STRONG COLD FRONT WILL  
EXTEND SOUTHWEST FROM THE SURFACE LOW. GUIDANCE HAS PROVIDED A  
SIGNAL FOR THIS SYSTEM OVER MULTIPLE DAYS BUT SO FAR THERE HAS  
BEEN A FAIR AMOUNT OF WAFFLING ON THE EXACT LATITUDE OF SURFACE  
LOW TRACK. THUS THE PREFERRED CONSENSUS/AVERAGE APPROACH COULD  
WELL BE EXTENDED TO A MULTI-RUN MEAN.  
 
WHAT APPEARS TO BE THE MOST EXTREME ASPECT OF THE FORECAST IS A  
DEEP UPPER LOW WHICH A NUMBER OF MODELS (MINUS THE 12Z CMC) BRING  
DOWN TOWARD LAKE SUPERIOR BY DAY 6 WED. THIS LOW HAS ITS ORIGIN  
OVER THE ARCTIC BEYOND THE CANADIAN ARCHIPELAGO. INTERESTINGLY  
THE ECWMF MEAN HAS BEEN LEADING THE GEFS MEAN FOR THE SOUTHWARD  
EXTENT OF THIS UPPER LOW--WITH THE 18Z GEFS MEAN FINALLY MATCHING  
THE ECMWF MEAN POSITION OVER LAKE SUPERIOR EARLY WED--BUT GFS RUNS  
HAVE GENERALLY BEEN A LITTLE LESS ERRATIC THAN THE ECMWF WITH  
SPECIFICS OF THE UPPER LOW AND SURROUNDING FLOW. THUS NOT YET  
CONFIDENT IN THE 12Z ECMWF THAT BRINGS THE UPPER LOW FARTHER SOUTH  
THAN THE AFOREMENTIONED MEANS. DEPENDING ON EXACT UPPER LOW  
DEPTH, 500MB HEIGHT ANOMALIES MAY REACH AT LEAST 3-4 STANDARD  
DEVIATIONS BELOW NORMAL ALONG/SOUTH OF THE LOW'S PATH. THIS  
FEATURE MAY PROMOTE FURTHER STRENGTHENING OF THE MIDWEST/GREAT  
LAKES SYSTEM THROUGH NEW ENGLAND AND EASTERN CANADA. DEEPER  
TRENDS IN THE MEANS FOR THE UPPER LOW BUT SOME LINGERING TRACK  
SPREAD IN THE GUIDANCE FAVOR A COMBINATION OF MODEL/MEAN IDEAS.  
 
AROUND THE SOUTHERN/EASTERN PERIPHERY OF THE OVERALL TROUGH THERE  
CONTINUES TO BE A MORE SUPPRESSED TREND FOR ANY WAVINESS ACROSS  
THE GULF OF MEXICO AND INTO THE WESTERN ATLANTIC. MODEL  
INCONSISTENCY AND SPREAD KEEP CONFIDENCE BELOW AVERAGE FOR  
SPECIFICS THAT AFFECT INTENSITY OF RAINFALL OVER THE FLORIDA  
PENINSULA. GUIDANCE AS A WHOLE IS MAINTAINING RECENT TRENDS  
TOWARD LOWER POTENTIAL FOR ANY WAVES TO HAVE A SIGNIFICANT EFFECT  
ON THE EAST COAST.  
 
ALONG THE WEST COAST THE AFOREMENTIONED ADJUSTMENTS WITH THE  
UPSTREAM PATTERN LEAD TO FAIRLY GOOD AGREEMENT IN THE MODELS/MEANS  
TOWARD A MORE PERSISTENT RIDGE ALOFT WITH GFS/ECMWF RUNS GENERALLY  
A LITTLE STRONGER THAN THEIR MEANS. THE 00Z CMC HAS STRENGTHENED  
ITS RIDGE COMPARED TO THE 12Z RUN THAT WAS MORE ROUNDED SO THERE  
IS SUPPORT FOR OPERATIONAL MODEL INPUT TO ENHANCE THE MEANS LATE  
IN THE PERIOD.  
 
BASED ON THE ABOVE ASSESSMENT THE UPDATED FORECAST USED MOSTLY  
OPERATIONAL GUIDANCE DURING THE FIRST HALF OF THE PERIOD AND THEN  
TRENDED TOWARD AN EVEN MODEL/ENSEMBLE MEAN BLEND EMPLOYING THE  
12Z-18Z GFS/12Z ECMWF AND THE 18Z GEFS/12Z ECMWF MEANS.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE FIRST SYSTEM OF NOTE WILL AFFECT THE GREAT LAKES/NEW ENGLAND  
EARLY IN THE PERIOD WITH SOME LEADING SNOW OVER NEW ENGLAND AND A  
PERIOD OF LAKE EFFECT SNOW BEHIND THE LOW. THE NEXT SYSTEM  
DROPPING SOUTHEAST FROM WESTERN CANADA WILL BRING A BAND OF  
POTENTIALLY SIGNIFICANT SNOW TO LOCATIONS JUST NORTH OF THE LOW  
TRACK AND THEN IN LEADING WARM ADVECTION OVER NEW ENGLAND.  
CURRENTLY THE HIGHEST SNOWFALL PROBABILITIES EXTEND FROM THE  
NORTHERN PLAINS THROUGH THE MIDWEST AND LOWER GREAT LAKES INTO NEW  
ENGLAND. THIS IS STILL SUBJECT TO SOME CHANGE DUE TO TRACK  
VARIABILITY IN THE GUIDANCE THUS FAR. HOW MUCH MOISTURE THIS  
SYSTEM PRODUCES IS ANOTHER UNCERTAINTY, WITH MOST OPERATIONAL  
MODELS GENERATING SIGNIFICANTLY MORE QPF THAN THE ENSEMBLE MEANS.  
THE MEANS HAVE TRENDED WETTER OVER THE PAST DAY WHILE THE HIGHEST  
SOLUTIONS MAY BE OVERDONE FOR BEING IN COLD AIR--FAVORING AN  
INTERMEDIATE FORECAST PERHAPS A LITTLE MORE THAN HALFWAY TOWARD AN  
OPERATIONAL MODEL AVERAGE AS A STARTING POINT. ONCE THE FRONT  
TRAILING FROM THE LOW REACHES THE MISSISSIPPI VALLEY AND POINTS  
EAST, SOME RAIN MAY DEVELOP IN THE WARM SECTOR AND THEN POSSIBLY  
END AS A LITTLE SNOW WITH FRONTAL PASSAGE. AFTER THE SYSTEM  
DEPARTS EXPECT ANOTHER PERIOD OF LAKE EFFECT SNOW. ELSEWHERE, THE  
DETAILS REMAIN QUITE UNCERTAIN BUT ONE OR MORE LOW LATITUDE  
FRONTAL WAVES MAY PRODUCE LOCALLY HEAVY RAINFALL OVER THE SOUTHERN  
FLORIDA PENINSULA IN THE LATE WEEKEND/EARLY WEEK TIME FRAME. AN  
AREA OF LOCALLY ENHANCED SNOW WILL DROP SOUTHWARD ALONG THE  
ROCKIES SUN-MON BEHIND A STRONG COLD FRONT.  
 
THE MOST PROMINENT FOCUS FOR COLDEST AIR REMAINS OVER THE UPPER  
MISSISSIPPI VALLEY/MIDWEST/GREAT LAKES. SOME TEMPERATURES MAY BE  
LOCALLY 10-25F BELOW NORMAL SUN-MON BUT OVER A MUCH MORE LIMITED  
AREA THAN THE COLD OUTBREAK EXPECTED TO OCCUR BEHIND THE STORM  
TRACKING THROUGH THE MIDWEST EARLY NEXT WEEK--ENHANCED BY A  
COMPACT ARCTIC UPPER LOW WHOSE TRACK COULD TRACK CLOSE TO OR OVER  
LAKE SUPERIOR. BY THE MIDDLE OF NEXT WEEK EXPECT BROAD COVERAGE  
OF READINGS 10F OR GREATER BELOW NORMAL OVER THE EASTERN HALF OF  
THE COUNTRY WITH THE UPPER MISSISSIPPI VALLEY AND VICINITY  
POTENTIALLY SEEING TEMPERATURES EXCEED 30F BELOW NORMAL FOR AT  
LEAST ONE DAY. THE HIGH PLAINS WILL BE QUITE WARM ON SUN BEFORE A  
STRONG COLD FRONT BRINGS TEMPERATURES BELOW NORMAL OVER THE  
ROCKIES/PLAINS EARLY NEXT WEEK. THE WEST COAST STATES WILL REMAIN  
ABOVE NORMAL THROUGH THE PERIOD WITH LOCALIZED PLUS 10F OR GREATER  
ANOMALIES MOST LIKELY ON SUN.  
 
RAUSCH  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION AND WINTER WEATHER OUTLOOK  
PROBABILITIES 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  
 

 
 
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