731  
FXUS02 KWBC 111513  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1112 AM EDT THU OCT 11 2018  
 
VALID 12Z SUN OCT 14 2018 - 12Z THU OCT 18 2018  
   
..15 UTC UPDATE
 
 
GIVEN CONTINUED MID/UPPER-LEVEL BLOCKING OFF THE WEST COAST OF  
NORTH AMERICA, MODELS SPREAD REMAINS SLIGHTLY BELOW AVERAGE, WITH  
FORECAST CONFIDENCE SLIGHTLY ABOVE AVERAGE THROUGH THE MEDIUM  
RANGE. A MULTI-MODEL DETERMINISTIC BLEND (ECMWF/UKMET/GFS) SERVED  
AS A REASONABLE STARTING POINT FOR THE FORECAST DURING DAYS 3-5  
(SUN-TUE). SOME TIMING DIFFERENCES WERE NOTED BETWEEN THE GFS AND  
THE ECMWF/UKMET WITH SHORTWAVE ENERGY CROSSING THE NORTHERN  
PLAINS/GREAT LAKES DURING THIS TIME PERIOD, WITH THE GFS PLACING  
MORE EMPHASIS'S ON A LEADING SHORTWAVE RESULTING IN A FASTER  
PROGRESSION OF THE TROUGH, WHILE THE ECMWF/UKMET PLACED MORE  
EMPHASIS ON A SECOND SHORTWAVE RESULTING IN A SLOWER OVERALL  
PROGRESSION. GIVEN THE OVERALL AMPLIFIED STATE OF THE FLOW, OPTED  
TO LEAN A BIT MORE TOWARD THE ECMWF/UKMET IDEA DURING THIS TIME  
PERIOD. DURING DAYS 6-7 (WED-THU) MODELS/ENSEMBLE SHOW GOOD  
CONSENSUS THAT A BROAD UPPER TROUGH AXIS WILL CROSS THE EASTERN  
U.S. WITH ANOTHER SURFACE FRONT USHERING IN A REINFORCING ROUND OF  
POLAR AIR FROM THE MIDWEST TO THE EAST COAST. MEANWHILE,  
CONFIDENCE IS HIGH THAT THE BLOCKING UPPER RIDGE OFF THE WEST  
COAST WILL HOLD STRONG, WITH PERHAPS A SLOW EASTWARD MIGRATION TO  
RIGHT ALONG THE WEST COAST BY THE MIDDLE OF NEXT WEEK. INCREASED  
WEIGHTING OF ENSEMBLE MEANS (ECENS/NAEFS) DURING DAYS 6-7, BUT  
CONFIDENCE WAS SUFFICIENT TO CONTINUE A SIZABLE MINORITY OF  
ECMWF/GFS DETERMINISTIC GUIDANCE IN THE FORECAST THROUGH THE  
PERIOD. THE PREVIOUS DISCUSSION FROM OVERNIGHT FOLLOWS BELOW.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0645 UTC)  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
GUIDANCE CONSENSUS SHOWS GOOD CONTINUITY IN PRINCIPLE WITH THE  
IDEA OF A STRONG EASTERN PACIFIC RIDGE ALOFT BUILDING INTO THE  
NORTHWESTERN U.S. AND WESTERN CANADA, WITH SHORTWAVES IN THE  
DOWNSTREAM MEAN TROUGH SUPPORTING A SERIES OF SURFACE  
WAVES/FRONTAL SYSTEMS. THE AXIS OF THE OVERALL MEAN TROUGH SHOULD  
REACH EASTERN NORTH AMERICA LATE IN THE PERIOD AS THE UPSTREAM  
RIDGE MOVES INLAND. THERE IS ALSO A PERSISTENT SIGNAL TOWARD A  
WEAKNESS/UPPER LOW OVER THE SOUTHWESTERN U.S./EXTREME NORTHWESTERN  
MEXICO BUT SOME QUESTION OVER EXACTLY HOW THIS FEATURE EVOLVES.  
 
WITHIN THE AGREEABLE MEAN PATTERN THERE ARE STILL DETAILS FOR  
WHICH INDIVIDUAL MODELS/ENSEMBLES ARE HAVING DIFFICULTY RESOLVING.  
THE GREATEST OVERALL UNCERTAINTY INVOLVES THE DISTRIBUTION OF  
ENERGY WITHIN THE INITIAL TROUGH EXTENDING FROM THE NORTHERN TIER  
STATES INTO THE WEST AS OF EARLY DAY 3 SUN. SOLUTIONS CONTINUE TO  
VARY SIGNIFICANTLY WITH WHAT PROPORTION OF ENERGY IN THE WESTERN  
U.S. PART OF THE TROUGH MIGHT PEEL OFF INTO THE MEAN LOW NEAR THE  
SOUTHWEST VERSUS CONTINUING ONWARD IN THE WESTERLIES. GFS RUNS  
HAD BEEN FAIRLY CONSISTENT WITH KEEPING MOST OF THE ENERGY IN THE  
PROGRESSIVE NORTHERN STREAM BUT THE 18Z AND NEW 00Z RUNS HAVE  
ADJUSTED TO AN IN-BETWEEN SCENARIO THAT TAKES LONGER FOR THE  
SOUTHERN PART OF THE SHORTWAVE TO EJECT THROUGH THE  
CENTRAL-EASTERN STATES. ECMWF RUNS HAVE BEEN ERRATIC, THE CMC HAS  
BEEN ON THE PROGRESSIVE SIDE, AND THE UKMET HAS GENERALLY WANTED  
TO HOLD BACK MORE SOUTHERN ENERGY IN SOME FASHION. THUS  
CONFIDENCE IS REALLY NOT MUCH IMPROVED VERSUS THE PAST 1-2 DAYS.  
THROUGH THE 12Z/18Z CYCLES THE OVERALL SIGNAL FROM THE GUIDANCE  
WAS TO HOLD BACK THE SHORTWAVE SOMEWHAT COMPARED TO THE PRIOR  
12-HOURLY RUNS, BUT GUIDANCE VOLATILITY SUGGESTS NOT GOING  
COMPLETELY THAT WAY. AS A RESULT BY DAY 4 MON PREFER TO INCLUDE  
SOMEWHAT MORE 18Z GEFS/12Z ECMWF MEAN INPUT (40 PERCENT) THAN  
TYPICALLY DESIRED THAT EARLY IN THE PERIOD TO PROVIDE A SLIGHT  
TONING DOWN OF THE ADJUSTMENTS MADE BY A GENERAL BLEND OF THE 18Z  
GFS AND 12Z ECMWF/UKMET/CMC. THE RESULT IS A SLOWER FRONTAL  
PROGRESSION OVER PARTS OF THE EAST BY TUE BUT NOT QUITE TO THE  
EXTENT OF AN OPERATIONAL MODEL AVERAGE.  
 
MEANWHILE ENERGY ALONG AND JUST SOUTH OF THE CANADIAN BORDER  
REMAINS ANOTHER FORECAST PROBLEM, WITH DIFFERENCES ULTIMATELY  
AFFECTING THE SURFACE PATTERN OVER CENTRAL/NORTHERN PARTS OF THE  
EASTERN U.S. AROUND EARLY NEXT WEEK. SPORADIC SOLUTIONS (NEW 00Z  
UKMET/CMC, OLD 00Z/10 ECMWF) LEAD TO FAIRLY STRONG SURFACE  
DEVELOPMENT AFFECTING THE NORTHEAST WHILE THE CURRENTLY PREFERRED  
REMAINING MODEL/ENSEMBLE MEAN CONSENSUS IS MUCH WEAKER WITH ANY  
MEANINGFUL SURFACE DEVELOPMENT WAITING UNTIL REACHING THE CANADIAN  
MARITIMES.  
 
WITH TYPICAL DETAIL DIFFERENCES/VARIABILITY, GUIDANCE SHOWS BETTER  
THAN AVERAGE CLUSTERING AND CONTINUITY FOR UPSTREAM ENERGY THAT  
SHOULD ENTER NORTHWESTERN CANADA ON SUN AND THEN DROP INTO THE  
MIDWEST/GREAT LAKES TUE-WED WITH ACCOMPANYING LOW PRESSURE AND  
FRONTAL SYSTEM. OVER THE PAST COUPLE DAYS THERE HAVE BEEN SOME  
INTERMITTENT SIGNALS FROM GUIDANCE THAT THIS SYSTEM MAY INTERACT  
WITH WAVINESS JUST OFF THE MID-ATLANTIC/NORTHEAST COAST AROUND  
MIDWEEK. ANOTHER SYSTEM MAY APPROACH THE UPPER GREAT LAKES BY DAY  
7 THU BUT WITH MUCH GREATER SPREAD FOR DETAILS AND THUS LOW  
CONFIDENCE AT THIS TIME.  
 
FOR THE UPPER LOW OVER OR NEAR THE SOUTHWEST, INDIVIDUAL SOLUTION  
SPREAD TEMPERS CONFIDENCE IN SPECIFICS OF WHICH PIECE(S) OF ENERGY  
MAY CONTRIBUTE TO THE FEATURE BUT AT LEAST THERE IS BETTER  
CONFIDENCE IN THE GENERAL EXISTENCE OF AN UPPER LOW/TROUGH.  
 
FOLLOWING AN OPERATIONAL MODEL BLEND ON DAY 3 SUN AND THE  
AFOREMENTIONED MODEL/MEAN BLEND THE FOLLOWING COUPLE DAYS, THE 18Z  
GFS AND 12Z ECMWF/CMC WERE SUFFICIENTLY COMPARABLE TO THE ENSEMBLE  
MEANS FOR THE OVERALL PATTERN THAT THE FORECAST WAS ABLE TO  
MAINTAIN ABOUT AN EVEN WEIGHT OF MODELS AND ENSEMBLES THROUGH DAY  
7 THU.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE SERIES OF CLIPPER SYSTEMS WILL BRING PERIODS OF MOSTLY LIGHT  
RAIN AND POSSIBLY COLD SECTOR SNOW TO PARTS OF THE GREAT LAKES AND  
NORTHEAST. THE COLD FRONT TRAILING FROM THE LEADING SYSTEM WILL  
CROSS THE EASTERN STATES EARLY-MID PERIOD WITH SOME ACCOMPANYING  
RAINFALL, POSSIBLY AIDED IN PART BY MOISTURE ORIGINALLY ASSOCIATED  
WITH SERGIO. RAINFALL DISTRIBUTION/TIMING IS STILL FAIRLY  
UNCERTAIN HOWEVER DUE TO QUESTION MARKS OVER SPECIFICS OF  
SHORTWAVE ENERGY ALOFT/SURFACE WAVE(S). EARLY IN THE PERIOD AN  
AREA OF SNOW SHOULD PROGRESS SOUTHWARD OVER THE CENTRAL ROCKIES  
WITH RAIN OVER THE SOUTHERN ROCKIES/PLAINS. DEPENDING ON  
SPECIFICS OF THE UPPER PATTERN, RAIN MAY PERSIST OVER THE SOUTHERN  
PLAINS WITH SOME SCATTERED RAINFALL POTENTIALLY EXTENDING BACK  
THROUGH THE SOUTHERN ROCKIES/SOUTHWEST.  
 
STRONG HIGH PRESSURE BUILDING SOUTH/SOUTHEAST FROM THE NORTHERN  
ROCKIES WILL BRING MULTIPLE DAYS OF BELOW NORMAL TEMPERATURES TO  
THE CENTRAL U.S., THOUGH NORTHERN AREAS WILL SEE STEADY MODERATION  
WITH TIME AS THE UPPER PATTERN SHIFTS EASTWARD. EXPECT SUN-MON TO  
FEATURE THE GREATEST NEGATIVE ANOMALIES (SOME HIGHS 20-35F BELOW  
NORMAL) AND BEST POTENTIAL FOR DAILY RECORDS TO BE  
APPROACHED/BROKEN. CURRENTLY EXPECT RECORD COOL HIGHS TO BE MORE  
NUMEROUS THAN RECORD LOWS. TO A LESSER DEGREE THIS COOL AIR WILL  
EXTEND EASTWARD TO KEEP ALL BUT THE SOUTHEAST BELOW NORMAL FOR  
HIGHS MOST OF THE PERIOD. THE UPPER RIDGE BUILDING INTO THE  
NORTHWEST WILL PROMOTE HIGHS UP TO 10F OR SO ABOVE NORMAL.  
 
RAUSCH  
 
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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