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