133  
FXUS02 KWBC 231538  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1038 AM EST TUE JAN 23 2018  
 
VALID 12Z FRI JAN 26 2018 - 12Z TUE JAN 30 2018  
   
..16Z UPDATE
 
 
LATEST 00Z/06Z MODELS/ENSEMBLES REMAIN CLOSE TO THEIR PREVIOUS  
RUNS. UPDATED PROGS REFLECT THE LATEST GUIDANCE BUT REMAINED NEAR  
CONTINUITY. MODEL PREFERENCES STARTED WITH A DETERMINISTIC BLEND  
(GFS/ECMWF/CANADIAN/UKMET) WITH A TREND TOWARD THE SLOWER  
ECMWF/CANADIAN/UKMET CLUSTER IN THE EAST THIS WEEKEND AS OPPOSED  
TO THE QUICKER GFS. BY NEXT WEEK, WEIGHTED THE ENSEMBLE MEANS  
(ECMWF ENSEMBLE MEAN ESPECIALLY) MORE WITH ONLY A SLIGHT WEIGHTING  
OF THE ECMWF/CANADIAN.  
 
FRACASSO  
 
SEE BELOW FOR THE PREVIOUS DISCUSSION ISSUED AT 0615Z:  
   
..PATTERN OVERVIEW
 
 
A PROGRESSIVE FLOW PATTERN APPEARS TO REMAIN IN PLACE ACROSS THE  
CONUS THROUGH THE MEDIUM RANGE. ENSEMBLE MEANS 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 THE POTENTIAL FOR SOME EASTWARD MOVEMENT 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 MAKE MULTIPLE ATTEMPTS TO BUILD NORTHWARD  
ALONG THE EASTERN SEABOARD, RESULTING IN A CONTINUED 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 IN  
THE PERIOD (FRI-SUN), AND ANOTHER BEGINNING TO TAKE SHAPE 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
 
 
UNCERTAINTY SURROUNDING THE TIMING AND AMPLITUDE OF THE UPPER  
TROUGH MOVING FROM THE ROCKIES TO THE CENTRAL/EASTERN U.S. FRI-MON  
CONTINUES TO PLAGUE THE FORECAST, DESPITE BROAD LARGE SCALE  
AGREEMENT. THE GFS CONTINUES TO INSIST ON A WEAKER/FASTER TROUGH  
AND SURFACE COLD FRONTAL PROGRESSION, WHILE THE ECMWF/UKMET HAVE  
MAINTAINED A SLOWER SOLUTION, AND HAVE EVEN SLOWED DOWN A BIT  
COMPARED TO THIS TIME LAST NIGHT. THE THINKING LAST NIGHT,  
ESPECIALLY GIVEN THE PERSISTENCE OF THE SLOWER ENSEMBLE SOLUTIONS,  
WAS TO LEAN AWAY FROM THE QUICKER DETERMINISTIC SOLUTIONS, AND  
THAT REMAINS THE CASE TONIGHT. THE ECENS MEAN SHOWS A PROGRESSION  
SIMILAR TO THE DETERMINISTIC ECMWF, ALBEIT SLIGHTLY FASTER, WHILE  
THE GEFS MEAN CONTINUES TO MAINTAIN A SLOWER AND MORE AMPLIFIED  
SOLUTION RELATIVE TO THE GFS. A SLOWER AND MORE ECMWF-LIKE  
SOLUTION WOULD ALSO RESULT IN THE POTENTIAL FOR A MORE SIGNIFICANT  
SURFACE WAVE OF LOW PRESSURE TO DEVELOP ACROSS THE OH  
VALLEY/NORTHEAST SUN NIGHT/MON, RESULTING IN MORE SIGNIFICANT  
WARM/MOIST ADVECTION INTO THE NORTHEASTERN U.S. ENSEMBLE SPREAD  
SHOWS A RANGE OF SOLUTIONS, HOWEVER, WITH SOME EVEN TRACKING A LOW  
ALONG THE NEW ENGLAND COAST. THIS IS PROBABLY THE AREA OF GREATEST  
UNCERTAINTY WITHIN THIS FORECAST ISSUANCE, AND WOULD HAVE  
SIGNIFICANT P-TYPE IMPACTS FOR INTERIOR NEW ENGLAND. FARTHER WEST,  
THE GFS ALSO DEPICTS A SOLUTION QUICKER THAN CONSENSUS FOR THE  
NEXT SYSTEM REACHING THE PAC NW FRI NIGHT/SAT, WITH THE  
ECMWF/UKMET AS WELL AS THE ENSEMBLE MEANS IN GENERAL AGREEMENT ON  
A SOLUTION SLOWER THAN THE GFS. GIVEN THESE CONSIDERATIONS, LEANED  
HEAVILY TOWARD THE 12Z ECMWF/UKMET ALONG WITH THE 12Z ECENS/18Z  
GEFS THROUGH SUN-MON.  
 
LATER IN THE FORECAST PERIOD, AS THE GULF OF AK UPPER LOW BREAKS  
DOWN AND MOVES INTO THE PAC NW PERHAPS AS AN OPEN TROUGH, MODEL  
SOLUTIONS SHOW VARIABILITY PRIMARILY REVOLVING AROUND HOW QUICKLY  
THIS PROCESS OCCURS. THE TIMING OF THE UPPER WAVE WILL ALSO IMPACT  
WHEN ANOTHER SURFACE LOW BEGINS TO TAKE SHAPE EAST OF THE ROCKIES.  
GIVEN THAT THERE IS GENERAL LARGE SCALE AGREEMENT, WITH PRIMARILY  
VARIATIONS IN TIMING, THE FORECAST WAS TRENDED TOWARD HEAVIER  
ECENS/GEFS MEAN WEIGHTING BY LATE MON-TUE, WITH A MINORITY  
COMPONENT OF THE OF THE 12Z ECMWF STILL INCLUDED AS WELL.  
 
   
..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 IS MUCH MORE AGGRESSIVE WITH HEAVY  
RAINFALL ALONG THE GULF COAST, WHILE THE GFS KEEPS THE DEEPEST  
MOISTURE AND HEAVIEST PRECIP OUT OVER THE GULF OF MEXICO. 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.  
THUS, AREAS OF HEAVY RAIN ARE ALSO POSSIBLE ALONG MUCH OF THE  
EASTERN SEABOARD SUN-MON. SNOW WILL ALSO 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 OH  
VALLEY/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. FRI-SUN.  
 
THE PACIFIC NORTHWEST WILL REMAIN UNSETTLED AND WET THROUGH THE  
EXTENDED PERIOD, WITH PERSISTENT ONSHORE FLOW, ALONG WITH ONE  
UPPER TROUGH CROSSING THE REGION FRI 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 IN THE WAKE OF THE LATE WEEK SYSTEM, 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.  
 
RYAN  
 

 
 
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