166  
FXUS02 KWBC 160700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST SUN DEC 16 2018  
 
VALID 12Z WED DEC 19 2018 - 12Z SUN DEC 23 2018  
   
..SIGNIFICANT PRECIPITATION OVER THE NORTHWEST MID-LATE WEEK
 
 
   
..CONFIDENCE INCREASING FOR A LATE WEEK STORM OVER THE EAST
 
 
   
..OVERVIEW
 
 
SOME DETAILS REMAIN IN QUESTION BUT THERE IS GOOD  
AGREEMENT/CONTINUITY WITH THE GENERAL IDEA THAT A RAPIDLY  
BUILDING/SHARPENING RIDGE OVER THE WEST WED-THU WILL SUPPORT  
DOWNSTREAM DIGGING OF SHORTWAVE ENERGY INTO AN AMPLIFIED TROUGH  
OVER THE MISSISSIPPI VALLEY BY THU. EASTERN TROUGH ENERGY,  
POSSIBLY CLOSING OFF AN UPPER LOW FOR A TIME, SHOULD SUPPORT A  
FAIRLY STRONG SURFACE SYSTEM THAT PROGRESSES NORTH-NORTHEAST OVER  
THE EASTERN U.S. INTO CANADA FRI ONWARD AS INCOMING PACIFIC  
SHORTWAVE ENERGY DAMPENS THE WESTERN RIDGE. BY NEXT WEEKEND THE  
OVERALL MEAN FLOW BECOMES NOTICEABLY LESS AMPLIFIED WITH THE HINT  
OF A SPLIT FLOW PATTERN BECOMING EVIDENT ALONG/JUST INLAND FROM  
THE WEST COAST BY NEXT SUN. THE PROGRESSIVE/LOW AMPLITUDE NATURE  
OF FLOW IN THIS TIME FRAME WILL LIKELY CREATE DIFFICULTIES IN  
RESOLVING SHORTWAVE SPECIFICS.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
THE 12Z GFS/ECMWF AND THE 18Z GEFS/12Z ECMWF MEANS PROVIDED THE  
BEST CLUSTERING FOR THE OVERALL DAYS 3-7 FORECAST. THUS THEY WERE  
USED EXCLUSIVELY AS THE INPUT FOR THE SURFACE/500MB PROGS, WITH  
PRIMARY EMPHASIS ON THE OPERATIONAL RUNS EARLY AND THE ENSEMBLE  
MEANS LATE. THE 18Z GFS WAS COMPARABLE FOR THE EASTERN U.S. STORM  
BUT POSED QUESTIONS FOR THE SHORTWAVE ENERGY REACHING WESTERN  
NORTH AMERICA AROUND MID-PERIOD.  
 
UPPER TROUGH ENERGY OVER THE PACIFIC NEAR 150W LONGITUDE AS OF  
EARLY DAY 3 WED IS CONTINUING TO POSE SIGNIFICANT FORECAST  
DIFFICULTIES BEYOND THAT TIME. INDIVIDUAL MODELS AND ENSEMBLE  
MEMBERS STILL DISPLAY MUCH WIDER THAN DESIRED SPREAD FOR HOW THE  
TROUGH ENERGY SEPARATES, WITH THE NORTHERN PORTION SERVING TO  
FLATTEN THE WESTERN RIDGE AND THE SOUTHERN REMAINDER LINGERING  
BACK FARTHER WESTWARD. UKMET RUNS ARE VERY PERSISTENT IN BEING  
THE OPERATIONAL MODEL EXTREME IN BEING MORE PROGRESSIVE WITH THE  
NORTHERN SHORTWAVE. ON THE OTHER HAND GFS/GEFS RUNS HAD BEEN  
SLOWEST WITH THE NORTHERN SHORTWAVE BUT RECENT GEFS RUNS HAVE BEEN  
GRADUALLY TRENDING TOWARD A MORE SUBDUED FORM OF ECMWF-BASED  
SOLUTIONS, AND GFS RUNS SEEM TO BE HEADING INTO THAT INTERMEDIATE  
SCENARIO AS WELL. THE 18Z GFS HAD REVERTED BACK TO THE SLOWER  
SCENARIO BUT THE NEW 00Z VERSION IS FASTER ONCE AGAIN. THIS  
SHORTWAVE ENERGY WILL DETERMINE THE STRENGTH/TRACK OF LOW PRESSURE  
EXPECTED TO TRACK OVER OR JUST NORTHWEST OF VANCOUVER ISLAND  
AROUND THU. THUS FAR THERE IS POOR CLUSTERING FOR THIS WAVE SO  
CONFIDENCE IS BELOW AVERAGE FOR A DAY 4 FORECAST. THEN THE WAVE  
WILL PROGRESS INTO WESTERN CANADA AND LIKELY TRACK NEAR THE  
U.S.-CANADIAN BORDER.  
 
TOWARD THE END OF THE PERIOD SHORTWAVE DETAILS BECOME VERY  
AMBIGUOUS DUE TO THE PROGRESSIVE AND LOW AMPLITUDE NATURE OF FLOW.  
D+8 MULTI-DAY MEANS SHOW A DEVELOPING AREA OF POSITIVE HEIGHT  
ANOMALIES OVER THE PACIFIC NEAR 150W LONGITUDE, WITH  
TELECONNECTIONS SUPPORTING A DECENT POTENTIAL FOR WEST-CENTRAL  
U.S. MEAN TROUGHING. SMOOTHING OUT THE SMALLER SCALE FEATURES,  
THERE IS SOME COMFORT IN THAT GUIDANCE SEEMS TO BE EVOLVING TOWARD  
SUCH A FEATURE IN THE DAYS 7-8 TIME FRAME.  
 
FARTHER EAST THE GUIDANCE HAS SHOWN CONSIDERABLE CONSOLIDATION  
RELATIVE TO THE PAST 1-2 DAYS, WITH SEEMINGLY LESS SENSITIVITY TO  
EXACT DETAILS OF UPSTREAM FLOW PROVIDING SOME IMPROVEMENT TO  
CONFIDENCE AS WELL. THE LATEST MAJORITY CLUSTER SHOWS AMPLIFYING  
NORTHERN PLAINS ENERGY ON DAY 3 WED RAPIDLY AMPLIFYING AND  
OVERWHELMING AN INITIAL SOUTHERN STREAM TROUGH, WITH POTENTIAL FOR  
AN UPPER LOW TO CLOSE OFF OVER OR A LITTLE SOUTH OF THE TENNESSEE  
VALLEY BY THU-FRI, FOLLOWED BY NORTH-NORTHEAST PROGRESSION. AS  
THIS OCCURS, CONSENSUS SHOWS DEEPENING SURFACE LOW PRESSURE  
TRACKING INTO THE UPPER OHIO VALLEY/CENTRAL APPALACHIANS BY EARLY  
FRI AND THEN THE EASTERN GREAT LAKES LATER IN THE DAY BEFORE  
CONTINUING INTO EASTERN CANADA. RECENT ECMWF RUNS WERE SOMEWHAT  
EARLIER TO CATCH ONTO THE DEGREE OF TROUGH  
AMPLIFICATION/SHARPENING VERSUS THE GFS. SO FAR IT APPEARS THAT  
THE MIDDLE OF EARLIER GUIDANCE SPREAD IS WORKING OUT WELL FOR  
TIMING, WITH CMC MEAN RUNS TRENDING SLOWER (BUT STILL LIKELY TOO  
FAST) WHILE LATEST GEFS MEAN RUNS ARE A LITTLE FASTER THAN 12Z/18Z  
VERSIONS FROM A COUPLE DAYS AGO.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
ONE WAVE OF HEAVY RAIN/HIGHER ELEVATION SNOW OVER THE NORTHWEST  
DURING THE SHORT RANGE PERIOD SHOULD BE TAPERING OFF BY NEXT WED.  
THEN LOW PRESSURE LIKELY TO TRACK JUST OFFSHORE THE PACIFIC  
NORTHWEST INTO WESTERN CANADA THU-THU NIGHT SHOULD BRING ANOTHER  
EPISODE OF SIGNIFICANT PRECIPITATION. THIS EVENT MAY HAVE  
SOMEWHAT MORE LIMITED EXTENT OF HEAVIEST ACTIVITY, MOST LIKELY  
FOCUSED OVER THE OLYMPICS AND NORTHERN CASCADES, BUT STILL EXPECT  
SOME MOISTURE TO REACH INTO EXTREME NORTHERN CALIFORNIA. THERE  
MAY BE A PERIOD OF FAIRLY STRONG WINDS IF THE SURFACE LOW IS ON  
THE DEEPER SIDE OF THE FULL GUIDANCE SPREAD. EXPECT LIGHTER AND  
MORE SCATTERED PRECIPITATION FRI FOLLOWED BY PERIODS OF MOSTLY  
LIGHT-MODERATE RAIN/SNOW DURING THE WEEKEND.  
 
THE STORM SYSTEM EXPECTED TO AFFECT THE EAST SHOULD SPREAD A BROAD  
AREA OF MOISTURE OVER MOST OF THE EASTERN HALF OF THE LOWER 48  
DURING THE LATTER HALF OF THE WEEK, WITH CHILLY CYCLONIC FLOW  
SUPPORTING LAKE EFFECT/TERRAIN-ENHANCED PRECIPITATION DURING THE  
WEEKEND. WHILE SOME DETAILS OF SYSTEM EVOLUTION HAVE CHANGED OVER  
THE PAST DAY, BEST GUIDANCE SIGNALS FOR HIGHEST RAINFALL TOTALS  
FOR THIS EVENT CONTINUE TO EXIST OVER THE FLORIDA PENINSULA AND  
NORTH CAROLINA COAST. OTHER RELATIVE MAXIMA MAY EXIST WITH  
ATLANTIC INFLOW ACROSS THE NORTHERN HALF OF THE EAST COAST AND TO  
THE NORTHWEST OF THE STORM TRACK. THIS SHOULD BE A FAIRLY WARM  
SYSTEM FOR THE TIME OF YEAR, WITH MOST SNOW CONFINED TO THE  
WESTERN PERIPHERY AND THEN OVER THE GREAT LAKES AND VICINITY AFTER  
SYSTEM PASSAGE.  
 
EXPECT THE GREATEST TEMPERATURE ANOMALIES TO BE OF THE WARM  
VARIETY OVER THE CENTRAL U.S. DURING WED-FRI, WITH SOME AREAS IN  
THE NORTHERN PLAINS SEEING MIN TEMPERATURES IN PARTICULAR  
EXCEEDING 20F ABOVE NORMAL. SUCH ANOMALIES MAY REACH DAILY  
RECORDS FOR WARM MINS. MEANWHILE THE EASTERN U.S. STORM MAY PULL  
IN A BRIEF SURGE OF WARMTH TO THE MID-ATLANTIC/NORTHEAST LATE IN  
THE WEEK. THE PACIFIC ORIGIN OF SUPPORTING ENERGY FOR THIS STORM  
SHOULD KEEP COLD SECTOR TEMPERATURES FROM REACHING MUCH LOWER THAN  
NORMAL, THOUGH SOME COOLER READINGS MAY BE POSSIBLE OVER THE  
SOUTHEAST AROUND FRI-SAT. THE OVERALL FLATTER TREND FOR FLOW  
ALOFT SHOULD PROMOTE MODERATION OF TEMPERATURES TOWARD NORMAL OVER  
MANY AREAS OF THE COUNTRY BY NEXT SUN.  
 
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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