581  
FXUS02 KWBC 241510  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1010 AM EST WED JAN 24 2018  
 
VALID 12Z SAT JAN 27 2018 - 12Z WED JAN 31 2018  
 
16Z UPDATE...  
 
NO BIG CHANGES WERE MADE TO THE DAY SHIFT UPDATE FROM THE  
OVERNIGHT FORECAST. SAT/SUN FORECAST RELIED ON A WELL-CLUSTERED  
DETERMINISTIC BLEND (GFS/ECMWF/CANADIAN/UKMET) WITH A STRONG TURN  
TOWARD THE ENSEMBLE MEANS (NAEFS AND ECMWF ENSEMBLE MEAN) BY NEXT  
TUE/WED. THERE APPEARS TO BE A SEPARATE CONVERGENCE OF THE  
ENSEMBLE MEANS AND DETERMINISTIC MODELS WITH THE SFC LOW TRACKING  
THROUGH THE NORTHEAST SUNDAY INTO EARLY MONDAY -- THE ENSEMBLE  
MEANS WERE CLOSER TO THE MAINE COAST WHILE THE DETERMINISTIC  
MODELS WERE DISPLACED JUST TO THE EAST. FOR NOW, OPTED TO RELY ON  
THE ENSEMBLE MEAN CONSENSUS WHICH FAVORS CONTINUITY UNTIL/UNLESS A  
CLEARER OFFSHORE TREND IS SEEN. NEXT WEEK, STRONG ENSEMBLE  
CONSENSUS SHOWS A DIGGING WESTERN TO CENTRAL CONUS TROUGH AND  
ASSOCIATED SURFACE CYCLOGENESIS THROUGH THE HIGH PLAINS INTO THE  
UPPER MIDWEST. DETAILS DIFFER AMONG THE GFS/ECMWF/CANADIAN BUT  
ENSEMBLE MEANS HAVE BEEN RATHER STEADY FOR AT LEAST A FEW DAYS.  
 
FRACASSO  
 
PREVIOUS DISCUSSION ISSUED AT 0555Z...  
 
   
..PATTERN OVERVIEW
 
 
A PROGRESSIVE FLOW PATTERN APPEARS LIKELY TO REMAIN IN PLACE  
ACROSS THE CONUS THROUGH THE MEDIUM RANGE. ENSEMBLE MEANS CONTINUE  
TO SHOW A PERSISTENT AND INCREASINGLY BLOCKING RIDGE BUILDING FROM  
THE NORTH CENTRAL PACIFIC/BERING SEA TO THE ARCTIC. DOWNSTREAM OF  
THE RIDGE, AN UPPER LOW WILL PERSIST ACROSS THE GULF OF AK THROUGH  
MUCH OF THE PERIOD, WITH EASTWARD MOVEMENT LIKELY BY EARLY NEXT  
WEEK AS THE HIGH LATITUDE BLOCKING RIDGE BECOMES A BIT MORE  
DISCONNECTED FROM THE MID-LATITUDE FLOW. MEANWHILE, THE  
SUBTROPICAL RIDGE WILL PERIODICALLY ATTEMPT TO BUILD NORTHWARD  
ALONG THE EASTERN SEABOARD, RESULTING IN A FEED OF RELATIVELY WARM  
SUBTROPICAL AIR INTO THE SOUTH CENTRAL U.S., AND ENHANCING THE  
LATITUDINAL THERMAL GRADIENT. IN A CONTINUATION OF WHAT WE HAVE  
SEEN IN RECENT DAYS, UPPER LEVEL ENERGY CROSSING THE WESTERN U.S.  
WILL QUICKLY AMPLIFY UPON REACHING THE CENTRAL U.S. WHERE  
LOW-LEVEL BAROCLINICITY WILL QUICKLY INCREASE. THUS, EXPECT THE  
CENTRAL PLAINS AND UPPER MIDWEST TO REMAIN THE ACTIVE STORM TRACK  
THROUGH THE EXTENDED PERIOD, WITH ONE SUCH SYSTEM EARLY THIS  
WEEKEND, AND ANOTHER BY EARLY NEXT WEEK.  
 
IF THE HEMISPHERIC FLOW REGIME EVOLVES AS DESCRIBED, THIS WOULD  
SUPPORT AN ACCUMULATION OF FRIGID ARCTIC AIR IN NORTH CENTRAL  
CANADA BEGINNING NEXT WEEK, WITH THE POTENTIAL FOR INCURSIONS  
SOUTHWARD INTO THE NORTH CENTRAL CONUS BEYOND THE END OF THIS  
FORECAST PERIOD.  
 
   
..MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES
 
 
MODEL SOLUTIONS CONTINUE TO DIFFER SURROUNDING THE TIMING AND  
AMPLITUDE OF THE UPPER TROUGH AND ASSOCIATED SURFACE FRONT  
CROSSING THE CENTRAL U.S. SAT AND THE EAST ON SUN. THE GFS REMAINS  
A FASTER AND LESS AMPLIFIED SOLUTION AT 500 HPA, WHILE THE ECMWF  
AND ESPECIALLY THE CMC ARE ON THE SLOWER/AMPLIFIED SIDE OF THE  
SPREAD. A COMPARISON OF SPAGHETTI PLOTS OF ALL ECENS/GEFS/CMCE  
MEMBERS DOES SHOW A DEFINITE CONVERGENCE OF SOLUTIONS COMPARED TO  
PREVIOUS RUNS, TOWARD A MIDDLE GROUND SOLUTION. THE 12Z ECMWF WAS  
VERY CLOSE TO THIS CONSENSUS OF ENSEMBLE MEANS. AN AREA OF EVEN  
GREATER UNCERTAINTY DEVELOPS BY SUN-MON, AS SURFACE LOW  
DEVELOPMENT OCCURS IN THE VICINITY OF THE MID-ATLANTIC AND EITHER  
TRACKS OFFSHORE OR ACROSS NEW ENGLAND. ENSEMBLES CONTINUE TO SHOW  
A LARGE RANGE OF SOLUTIONS AMONG THEIR MEMBERS, RANGING FROM A  
SURFACE LOW THAT TRACKS FROM THE OH VALLEY INTO QUEBEC, TO ONE  
THAT DEVELOPS NEAR THE MID-ATLANTIC OR SOUTHEAST COAST AND REMAINS  
ALMOST ENTIRELY OFFSHORE. ENSEMBLE MEANS CONTINUE TO SHOW MODEST  
CONSENSUS ON A SURFACE LOW DEVELOPING ACROSS THE MID-ATLANTIC  
REGION ON SUN AND THEN TRACKING NEAR THE NEW ENGLAND COASTLINE SUN  
NIGHT/MON. THIS IDEA WAS GENERALLY CLOSE TO CONTINUITY AS WELL,  
AND CONTINUES TO BE THE FAVORED SOLUTION AT THIS TIME. AS  
DISCUSSED LAST NIGHT, THE EXACT TRACK OF THE SURFACE LOW WILL HAVE  
SIGNIFICANT IMPACTS ON P-TYPE AND AMOUNT FOR THE NORTHEASTERN U.S.  
 
IN THE WESTERN U.S. MODELS SHOWED SOMEWHAT BETTER AGREEMENT WITH  
RESPECT TO UPPER SHORTWAVE AND SURFACE LOW REACHING THE PAC NW ON  
SAT AND ANOTHER SUN-MON. MODEL SOLUTIONS HERE HAVE BEEN A BIT MORE  
CONSISTENT FROM RUN-TO-RUN COMPARED TO THE CENTRAL/EASTERN U.S.  
SYSTEM. SOLUTIONS BEGIN TO DIFFER A BIT BY TUE-WED AS SIGNIFICANT  
AMPLIFICATION OF THE UPPER TROUGH OCCURS ACROSS THE FOUR  
CORNERS/ROCKIES, BUT AGREEMENT SEEMS ABOVE AVERAGE GIVEN THE DAY  
6-7 TIME FRAME. ANOTHER SURFACE CYCLONE WILL LIKELY TAKE SHAPE BY  
THE MIDDLE OF NEXT WEEK ACROSS THE CENTRAL U.S. AS THE UPPER  
TROUGH APPROACHES. SOLUTIONS VARY WITH RESPECT TO THE TIMING OF  
CYCLOGENESIS, BUT GENERAL AGREEMENT EXISTS THAT A LOW SHOULD  
DEVELOP SOMEWHERE ACROSS THE NORTHERN OR CENTRAL PLAINS BY LATE  
TUE OR WED. AN ENSEMBLE MEAN APPROACH WAS FAVORED HERE GIVEN THE  
TIME SCALE UNDER CONSIDERATION.  
 
GIVEN THESE CONSIDERATIONS, A BLEND OF THE ECMWF/UKMET/GFS (MOST  
HEAVILY WEIGHTED TOWARD THE ECMWF) WAS USED DURING DAYS 3-4. AFTER  
THE DEVELOPMENT OF THE NORTHEASTERN U.S. SURFACE LOW LATE DAY 4  
INTO DAY 5, THE GFS WAS ALMOST ENTIRELY REMOVED FROM THE BLEND,  
WHILE ECENS/NAEFS WEIGHTING WAS INCREASED.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
MODELS CONTINUE TO SUGGEST THE POTENTIAL FOR HEAVY RAINFALL ACROSS  
PORTIONS OF THE GULF COAST STATES OVER THE WEEKEND. THE DEGREE TO  
WHICH MOISTURE/INSTABILITY ARE ABLE TO BE TRANSPORTED NORTH SHOULD  
BE INVERSELY PROPORTIONAL TO THE AMPLITUDE AND SPEED OF THE UPPER  
TROUGH. THUS, THE ECMWF/UKMET/CMC ARE MUCH MORE AGGRESSIVE WITH  
HEAVY RAINFALL ALONG THE GULF COAST, WHILE THE GFS KEEPS THE  
DEEPEST MOISTURE AND HEAVIEST PRECIP MOSTLY OUT OVER THE GULF OF  
MEXICO AND PERHAPS PORTIONS OF THE FL PENINSULA. GIVEN THAT THE  
FORECAST IS LEANING TOWARD A SLOWER AND MORE AMPLIFIED SOLUTION,  
WOULD EXPECT THE POTENTIAL FOR RAIN AND THUNDERSTORMS TO PRODUCE  
HEAVY RAINFALL ACROSS THE GULF COAST STATES. A WEAK STATIONARY  
FRONTAL BOUNDARY FORECAST TO LINGER ALONG THE GULF COAST AHEAD OF  
THE COLD FRONT WOULD LIKELY SERVE TO FOCUS THE HEAVIEST RAINFALL  
AMOUNTS, WITH MULTI-INCH RAINFALL TOTALS POSSIBLE. FARTHER NORTH,  
LOCALLY HEAVY RAINFALL AMOUNTS WILL ALSO BE POSSIBLE IN  
ASSOCIATION WITH THE COLD FRONTAL PASSAGE, FROM THE INTERIOR  
SOUTHEAST STATES TO THE TN/OH VALLEYS. A SLOWER AND DEEPER TROUGH  
PROGRESSION WOULD ALSO ALLOW FOR THE POTENTIAL TRANSPORT OF DEEPER  
MOISTURE NORTHWARD EAST OF THE APPALACHIANS, ALTHOUGH THE DEGREE  
TO WHICH THIS OCCURS WILL ALSO DEPEND ON THE TRACK OF ANY SURFACE  
LOW THAT DEVELOPS ALONG THE FRONT SUN-MON. THUS, AREAS OF AT LEAST  
LOCALLY HEAVY RAIN ARE ALSO POSSIBLE ALONG MUCH OF THE EASTERN  
SEABOARD SUN-MON. SNOW WILL BE POSSIBLE ON THE NORTHERN SIDE OF  
THE SYSTEM AS WELL AS PERHAPS BRIEFLY BEHIND THE COLD FRONT FOR  
SOME AREAS. AS DESCRIBED ABOVE, THE DEGREE TO WHICH WINTER WEATHER  
IS A THREAT WILL DEPEND GREATLY ON THE DEVELOPMENT AND TRACK OF A  
POTENTIAL SURFACE LOW ACROSS THE MID-ATLANTIC/NORTHEAST SUN-MON.  
TEMPERATURES WILL BE WELL ABOVE AVERAGE IN THE WARM SECTOR AHEAD  
OF THIS LOW PRESSURE SYSTEM AS WARM AND HUMID SUBTROPICAL AIR  
STREAMS NORTHWARD. HIGH TEMPS AT LEAST 10-20 DEG F ABOVE AVERAGE  
ARE LIKELY ACROSS MUCH OF THE CENTRAL/EASTERN U.S. SAT-SUN.  
 
THE PACIFIC NORTHWEST WILL REMAIN UNSETTLED AND WET THROUGH MUCH  
OF THE EXTENDED PERIOD, WITH PERSISTENT ONSHORE FLOW, ALONG WITH  
ONE LOW PRESSURE SYSTEM AFFECTING THE REGION SAT AND THE NEXT ONE  
BY MON-TUE. THUS, HEAVY RAIN AND MOUNTAIN SNOWS ARE LIKELY  
THROUGHOUT THE PERIOD, WITH THE HEAVIEST PRECIPITATION FALLING  
ACROSS THE WA/OR COASTAL RANGES AND THE CASCADES. THE INTERIOR  
WEST WILL REMAIN RELATIVELY DRY INITIALLY, AND WARMING  
TEMPERATURES ARE EXPECTED OVER THE WEEKEND AS UPPER RIDGING  
BRIEFLY BUILDS OVERHEAD IN THE WAKE OF THE FIRST UPPER TROUGH, AND  
AHEAD OF THE SECOND ONE. AS THE TROUGH AMPLIFIES ACROSS THE  
ROCKIES TUE-WED, HOWEVER, COLDER AIR WILL PLUNGE SOUTHWARD INTO  
MUCH OF THE INTERIOR WEST AS WELL AS THE NORTHERN HIGH PLAINS, AND  
SNOW WILL BE POSSIBLE FOR MUCH OF THE ROCKIES AS WELL AS PERHAPS  
INTO THE NORTHERN PLAINS BY TUE/TUE NIGHT.  
 
RYAN  
 

 
 
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