401  
FXUS02 KWBC 111555  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1055 AM EST SUN NOV 11 2018  
 
VALID 12Z WED NOV 14 2018 - 12Z SUN NOV 18 2018  
 
16Z UPDATE: THE MAIN FORECAST CHALLENGE DURING THE MEDIUM RANGE  
PERIOD IS THE EVOLVE AND SPEED OF THE SOUTHERN STREAM CLOSED LOW  
OVER THE SOUTHERN PLAINS, BEGINNING WEDNESDAY MORNING. THE 6Z GFS  
AND THE 6Z GEFS MEAN ARE ABOUT 250 MILES EAST OF THE  
UKMET/ECMWF/CMC/EC MEAN SOLUTIONS AT THAT TIME, AND THESE  
DIFFERENCES BECOME GREATER BY FRIDAY MORNING AS THIS FEATURE  
REACHES THE EAST COAST, WITH THE GFS AHEAD OF THE GEFS MEAN BY  
THURSDAY MORNING. THE 00Z GFS, WHILE STILL FASTER THAN THE MODEL  
CONSENSUS, IS NOT QUICK AS PROGRESSIVE AND DEPICTS A MORE  
REASONABLE SOLUTION. GIVEN THE GREATER MODEL AGREEMENT ON A  
SLOWER TRACK, MORE WEIGHTING WAS GIVEN TO THIS SCENARIO IN THE WPC  
FORECAST, WITH ONLY A SMALL PERCENTAGE OF THE 00Z GFS BEING  
INCORPORATED AND NO 6Z GFS. THERE HAS ALSO BEEN A TREND FOR A  
STRONGER EAST COAST SURFACE LOW BY THE END OF THE WEEK COMPARED TO  
EARLIER MODEL GUIDANCE, AND THE LATEST WPC FORECAST REFLECTS THAT.  
 
BY THE END OF THE FORECAST PERIOD NEXT WEEKEND, MODEL SPREAD  
INCREASES NEAR THE WEST COAST AS PACIFIC SHORTWAVE IMPULSES BEGIN  
TO BREAK DOWN THE UPPER LEVEL RIDGE OVER THE WESTERN THIRD OF THE  
NATION. THE ENSEMBLE MEANS PROVIDED A GOOD BASELINE FOR THE  
FORECAST, WITH THE EC MEAN SLIGHTLY FASTER THAN THE GEFS MEAN  
REGARDING TROUGH AXIS PLACEMENT. THE PREVIOUS FORECAST DISCUSSION  
FOLLOWS FOR REFERENCE.  
 
D. HAMRICK  
 
   
..OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
OVER THE PAST DAY A LOT OF THE GUIDANCE HAS BEEN EXHIBITING  
SIGNIFICANT CHANGES/TRENDS WITHIN AN EXPECTED RE-ESTABLISHMENT OF  
A WESTERN RIDGE-EASTERN TROUGH PATTERN AFTER A BRIEF FLATTENING OF  
MEAN FLOW VERY EARLY IN THE FORECAST PERIOD. THE TWO PRIMARY  
TRENDS OF NOTE INVOLVE GREATER SEPARATION AND CONCENTRATION OF  
RESIDUAL SHORTWAVE ENERGY REACHING THE SOUTHERN PLAINS/NORTHERN  
MEXICO DURING THE LATTER HALF OF THE SHORT RANGE PERIOD FOLLOWED  
BY A SHARPER/MORE AMPLIFIED DEPICTION OF THE REBUILDING WEST COAST  
RIDGE AND AMPLIFYING DOWNSTREAM TROUGH. THE COMBINATION OF  
STRONGER SOUTHERN TIER ENERGY AND A SHARPER/SLOWER TROUGH  
AMPLIFYING TO ITS NORTHWEST IS SIGNIFICANTLY INCREASING THE  
POTENTIAL FOR A WIDESPREAD PRECIPITATION EVENT OVER THE EASTERN  
STATES AND LOW PRESSURE DEVELOPMENT ALONG THE EAST COAST. DEEPER  
TRENDS WITH THE AMPLIFYING EASTERN TROUGH LEAD TO COLDER  
TEMPERATURES OVER THE EAST BY NEXT WEEKEND. ON THE OTHER HAND THE  
STRENGTHENING TREND FOR THE WEST COAST RIDGE IS DELAYING THE  
ARRIVAL OF PREVIOUSLY ADVERTISED PRECIPITATION AND PROLONGING THE  
DURATION OF ABOVE AVERAGE TEMPERATURES. IN ADDITION ONCE THE  
MOISTURE ARRIVES INTO THE PACIFIC NORTHWEST, AMOUNTS ARE TRENDING  
LOWER AS SOME SOLUTIONS HINT AT A POSSIBLE SPLIT IN SUPPORTING  
TROUGH ENERGY.  
 
LOOKING AT GUIDANCE COMPARISONS THROUGH THE 12Z/18Z CYCLES, THERE  
WAS DECENT CLUSTERING FROM DAY 3 WED INTO EARLY DAY 4 THU. AFTER  
THAT TIME SPECIFICS DIVERGED REGARDING THE SOUTHERN/EASTERN ENERGY  
AND UPSTREAM AMPLIFYING TROUGH ALONG WITH CORRESPONDING SURFACE  
EVOLUTION. THE SCENARIO THAT SEEMS TO BE IN THE PROCESS OF BEING  
REINFORCED IN THE 00Z GUIDANCE HAD GENERALLY BEEN A MINORITY  
SOLUTION SEEN IN A SMALL PERCENTAGE OF ENSEMBLE MEMBERS ALONG WITH  
A RUN OR TWO OF THE ECMWF A NUMBER OF DAYS AGO. THE SIGNAL  
STARTED TO REAPPEAR A BIT YESTERDAY AND THE PAST COUPLE ECMWF RUNS  
PLUS THE 12Z CMC AND 18Z GFS/GEFS MEAN ADDED TO IT. AT THE TIME  
OF FORECAST PREPARATION IT APPEARED BEST FOR A SINGLE  
DETERMINISTIC FORECAST TO INCORPORATE BOTH OPERATIONAL MODEL AND  
ENSEMBLE MEAN IDEAS WHICH WOULD PROVIDE SOME DEGREE OF TRENDING  
WHILE AWAITING AGREEMENT ON THE SPECIFICS.  
 
REGARDING NORTHERN U.S./SOUTHERN CANADA FLOW, GFS RUNS HAVE  
GENERALLY BEEN AT ODDS WITH THE MAJORITY CLUSTER OF GUIDANCE OVER  
THE PAST COUPLE DAYS. CONSENSUS HAS HELD FAIRLY STEADY WITH THE  
SYSTEM FORECAST TO TRACK OVER/NEAR THE UPPER GREAT LAKES LATE IN  
THE WEEK ASIDE FROM PERHAPS SOME MODEST TIMING ADJUSTMENTS.  
HOWEVER LATEST GFS RUNS HAVE BEEN STRAYING TO THE NORTH OF  
ESTABLISHED CONSENSUS WHILE 24 HOURS AGO THEY WERE ACCELERATING  
AHEAD OF CONSENSUS. AT THE SAME TIME EARLIER GFS RUNS HAD BEEN  
AMONG THE MOST EAGER TO BRING UPSTREAM PACIFIC ENERGY INTO THE  
WESTERN NORTH AMERICA RIDGE LATE IN THE WEEK. MORE RECENT GFS  
RUNS ARE MUCH CLOSER TO THE GUIDANCE MAJORITY.  
 
REGARDING THE EASTERN PACIFIC TROUGH NEARING THE WEST COAST LATE  
IN THE PERIOD, ENSEMBLE SPREAD HAS IMPROVED OUT TO ABOUT DAYS 5-6  
FRI-SAT. DURING THE WEEKEND THERE IS A PRONOUNCED DIVERGENCE OF  
MODELS/ENSEMBLE MEMBERS REGARDING HOW MUCH FLOW SEPARATION MAY  
OCCUR AND WHERE. PREFERENCE LEANS TOWARD THE MEANS LATE IN THE  
PERIOD GIVEN THE UNCERTAINTY IN SPECIFICS BUT WITH THE  
UNDERSTANDING THAT THE TROUGH MAY ULTIMATELY BE LESS PHASED.  
 
BASED ON THE COMBINATION OF FORECAST CONSIDERATIONS, THE FORECAST  
STARTED WITH A BLEND OF 12Z OPERATIONAL MODELS (MORE GFS/ECMWF  
THAN CMC/UKMET) EARLY IN THE PERIOD AND THEN INCREASED 18Z  
GEFS/12Z ECMWF MEAN INPUT SO THAT THE MEANS COMPRISED 70 PERCENT  
OF THE BLEND BY DAY 7 SUN.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
VERY DEEP STORM SYSTEM REACHING EASTERN CANADA BY EARLY WED MAY  
BRING STRONG WINDS TO PARTS OF NEW ENGLAND AROUND MIDWEEK. ALSO  
EXPECT SOME LINGERING LAKE EFFECT SNOW ON WED, MAINLY OVER THE  
NORTHEASTERN HALF OF THE GREAT LAKES REGION. THEN DURING THE  
LATTER HALF OF THE WEEK GUIDANCE IS SHOWING GREATER POTENTIAL FOR  
AN AREA OF MOISTURE TO OVERSPREAD PORTIONS OF THE EASTERN U.S. IN  
ASSOCIATION WITH A MID LEVEL FEATURE LIFTING OUT OF THE SOUTHERN  
PLAINS AND SURFACE LOW PRESSURE NEAR THE EAST COAST. SOME WINTRY  
PRECIPITATION MAY BE POSSIBLE IN WESTERN/NORTHERN PORTIONS OF THE  
MOISTURE SHIELD. CONFIDENCE IS CURRENTLY BELOW AVERAGE REGARDING  
SPECIFICS AS GUIDANCE HAS BEEN CHANGING CONSIDERABLY OVER RECENT  
RUNS. THERE IS SOMEWHAT BETTER CONFIDENCE IN PERSISTENCE OF SOME  
RAINFALL OVER/NEAR FLORIDA UNTIL LOW PRESSURE TO THE NORTH PUSHES  
THE TRAILING FRONT EASTWARD. LATE THIS WEEK INTO THE WEEKEND LOW  
PRESSURE TRACKING NEAR THE CANADIAN BORDER WILL BRING AN EPISODE  
OF MOSTLY LIGHT PRECIPITATION TO THE GREAT LAKES, FOLLOWED BY  
ANOTHER PERIOD OF LAKE EFFECT SNOW. SHORTWAVE ENERGY REACHING THE  
WEST COAST BY EARLY WED WILL PRODUCE A BRIEF PERIOD OF RAIN AND  
HIGHER ELEVATION SNOW OVER THE PACIFIC NORTHWEST AND NORTHERN  
ROCKIES. SOME MOISTURE MAY REACH THE PACIFIC NORTHWEST COAST THIS  
WEEKEND BUT IT IS LIKELY THAT PRECIPITATION AMOUNTS WILL BE ON THE  
LIGHT SIDE.  
 
THE MIDWEEK TIME FRAME WILL SEE THE MOST EXTREME COLD ANOMALIES  
OVER THE EAST-CENTRAL U.S. WITH SOME HIGHS 20-25F BELOW NORMAL AND  
POSSIBLY COLD ENOUGH FOR DAILY RECORDS. RECORDS FOR MORNING LOWS  
APPEAR TO BE SOMEWHAT LESS LIKELY/WIDESPREAD. THERE SHOULD BE  
SOME MODERATION LATE IN THE WEEK BUT THE EAST MAY TREND COLDER  
AGAIN NEXT WEEKEND WITH EXPANDING COVERAGE OF HIGHS 10-15F BELOW  
NORMAL. WESTERN WARMTH WILL COVER A DECENT PROPORTION OF THE AREA  
FROM THE WEST COAST STATES INTO THE NORTHERN HIGH PLAINS WITH SOME  
POCKETS OF PLUS 10-15F ANOMALIES FOR MIN/MAX READINGS.  
 
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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