Prognostic Meteorological Discussion
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000 FXUS02 KWBC 201542 PMDEPD EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1041 AM EST SAT JAN 20 2018 VALID 12Z TUE JAN 23 2018 - 12Z SAT JAN 27 2018 ...OVERVIEW... A PROGRESSIVE PATTERN CONTAINING FEATURES OF VARYING SCALES EARLY IN THE PERIOD-- THE MOST PROMINENT BEING THE STRONG STORM TRACKING NORTHEAST FROM THE GREAT LAKES-- WILL GAIN AMPLITUDE WITH TIME AND BECOME DOMINATED BY INCREASINGLY LARGE SCALE FEATURES THAT TEND TO HAVE GREATER PREDICTABILITY. AFTER DEPARTURE OF THE NORTHEASTERN SYSTEM, THE PRIMARY FOCUS WILL BE ON AN EASTERN PACIFIC UPPER TROUGH THAT WILL LIKELY COME ASHORE INTO THE WEST AROUND THU AND REACH THE ROCKIES/PLAINS BY FRI-SAT. THIS TROUGH WILL BE ACCOMPANIED BY LEADING SURFACE LOW PRESSURE/WAVY FRONTAL SYSTEM. ON EITHER SIDE OF THE UPPER TROUGH, EXPECT UPPER RIDGES TO PREVAIL NEAR THE EAST AND WEST COASTS BY NEXT WEEKEND. ...GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES... RECENT TRENDS IN THE GUIDANCE TOWARD BETTER AGREEMENT FOR SIGNIFICANT FEATURES OVER THE PAST DAY HAVE YIELDED, AT LEAST FOR TIME BEING, PRACTICALLY THE BEST POSSIBLE DEGREE OF CONSENSUS THAT COULD BE DESIRED THROUGH THE DAY 7 TIME FRAME. THE ONE EXCEPTION COMES FROM THE 00Z CMC ENSEMBLES WHICH BECOME CURIOUSLY OUT OF PHASE RELATIVE TO OTHER SOLUTIONS (INCLUDING OPERATIONAL CMC RUNS) FOR PACIFIC FLOW AND FLAT WITH THE PATTERN OVER THE LOWER 48 BY THE LATTER HALF OF THE PERIOD. THE MERE EXISTENCE OF THIS SEEMINGLY UNLIKELY SCENARIO AND TYPICAL GUIDANCE ERROR IN THE EXTENDED TIME FRAME PROVIDE A REMINDER THAT SOME CHANGES IN CONSENSUS ARE VERY POSSIBLE IN UPCOMING DAYS. HOWEVER THE LARGE SCALE OF FEATURES BY THE END OF PERIOD MAY HELP TO TEMPER FUTURE SPREAD/VARIABILITY TO SOME DEGREE. FOR THE SYSTEM TRACKING OUT OF THE GREAT LAKES TUE ONWARD, ENSEMBLE LOW PLOTS FROM RECENT DAYS INDICATE THAT A SLOWER TREND OF GFS RUNS/GEFS MEMBERS CONTRIBUTED THE MOST TO CURRENT CONSENSUS. TOWARD NEW ENGLAND AND THE CANADIAN MARITIMES THE LATEST STORM TRACK LIES BETWEEN PRIOR SOUTHERN ECMWF/NORTHERN GFS RUNS. ON THE OTHER HAND FOR EASTERN PACIFIC HEIGHT FALLS MOVING INTO THE WEST, ECMWF AND ESPECIALLY ECMWF ENSEMBLE GUIDANCE FROM RECENT DAYS HAS SLOWED CONSIDERABLY TOWARD THE GFS/GEFS MEAN SCENARIO. IN LIGHT OF THIS TREND AND CURRENT GUIDANCE COMPARISONS, THE 00Z ECMWF MEAN MAY STILL BE A BIT FAST WITH LEADING HEIGHT FALLS FROM LATE WED ONWARD. AN AVERAGE OF OPERATIONAL GUIDANCE PROVIDES A REASONABLE STARTING POINT FOR LOW PRESSURE OFF THE PACIFIC NORTHWEST/BC COAST TUE INTO THU AS WELL AS THE WAVE FORECAST TO REACH THE NORTHERN TIER STATES BY FRI-SAT ALONG WITH THE TRAILING FRONT. GUIDANCE IS STILL SOMEWHAT IN FLUX FOR DETAILS OF SHORTWAVE ENERGY BETWEEN THE ABOVE TWO SYSTEMS. THE PRIMARY TREND IS TOWARD A MORE PROGRESSIVE AND/OR SHEARED DEPICTION WHICH LEADS TO MUCH LOWER RAINFALL POTENTIAL ACROSS FLORIDA AND EXTREME SOUTHEAST COMPARED TO WHAT SOME GFS RUNS HAD BEEN ANTICIPATING IN RECENT DAYS. IN LIGHT OF THE REMARKABLY GOOD AGREEMENT IN MOST ASPECTS OF GUIDANCE THROUGH THE 06Z CYCLE, THE DAYS 3-5 TUE-THU FORECAST USED EXCLUSIVELY OPERATIONAL MODEL INPUT FROM THE 00Z ECMWF/06Z GFS AND 00Z UKMET/CMC WHILE DAYS 6-7 FRI-SAT MAINTAINED MAJORITY WEIGHT OF THE ECMWF/GFS WITH LESSER CONTRIBUTION FROM THE 06Z GEFS/00Z ECMWF MEANS AND 00Z CMC. ...SENSIBLE WEATHER HIGHLIGHTS/THREATS... EXPECT SNOW AND SOME GUSTY WINDS OVER PARTS OF THE GREAT LAKES REGION ON TUE AS STRONG LOW PRESSURE TRACKS NORTHEASTWARD. TRAILING COLD CYCLONIC FLOW MAY PRODUCE AREAS OF LAKE EFFECT SNOW INTO WED. RAIN AND PERHAPS A FEW THUNDERSTORMS WILL BE POSSIBLE ALONG/AHEAD OF THE SURFACE COLD FRONT ON TUE ALONG MOST OF THE EAST COAST. STRONG DYNAMICS ALOFT AND LOW LEVEL ATLANTIC INFLOW MAY GENERATE AREAS OF HEAVY RAIN OVER SOUTHERN NEW ENGLAND WHILE A WINTRY MIX OF PRECIPITATION IS POSSIBLE FOR NORTHERN NEW ENGLAND. THE EASTERN PACIFIC UPPER TROUGH EVENTUALLY MOVING INTO THE WESTERN/CENTRAL U.S. WILL SPREAD A BROAD AREA OF PRECIP FROM THE WEST COAST INTO THE EAST-CENTRAL STATES OVER THE COURSE OF THE PERIOD. LOCALLY HEAVY RAIN AND HIGHER ELEVATION SNOW SHOULD BE CONFINED TO THE CENTRAL/NORTHERN WEST COAST TUE-WED. THEN EXPECT SOME OF THIS MOISTURE TO EXTEND SOUTH INTO AT LEAST CENTRAL CALIFORNIA AND EASTWARD ACROSS THE NORTHERN 2/3 TO 3/4 OF THE WEST THROUGH THE ROCKIES BY THU-FRI. SNOW LEVELS SHOULD TREND LOWER BY MID-LATE WEEK AS THE UPPER TROUGH MOVES INTO THE WEST. LEADING LOW PRESSURE REACHING THE NORTHERN TIER BY FRI-SAT MAY BE ACCOMPANIED BY AREAS OF COLD SECTOR SNOW. MEANWHILE AHEAD OF THE TRAILING COLD FRONT, SOUTHERLY FLOW WILL BEGIN TO SPREAD MOISTURE AND INSTABILITY FROM THE GULF OF MEXICO NORTHWARD TOWARD THE END OF THE WEEK INTO THE WEEKEND. SPECIFICS OF RAINFALL INITIATION AND COVERAGE/INTENSITY WILL BE SENSITIVE TO DETAILS THAT WILL TAKE TIME TO RESOLVE, BUT THE CONSENSUS UPPER PATTERN BY NEXT SAT (CENTRAL U.S. TROUGH, EAST COAST/WESTERN ATLANTIC RIDGE) SUGGESTS A HIGH PROBABILITY OF SIGNIFICANT RAINFALL OVER AT LEAST A PORTION OF THE EASTERN HALF OF THE COUNTRY BY NEXT WEEKEND. CURRENT GUIDANCE PROVIDES THE BEST SIGNAL FOR ENHANCED RAINFALL FROM THE LOWER HALF OF THE MS VALLEY INTO THE SOUTHEAST. THE PERIOD WILL START WITH VERY WARM TEMPS ALONG THE EAST COAST AHEAD OF THE APPROACHING COLD FRONT ON TUE, WITH SOME PLUS 10-20F ANOMALIES FOR HIGHS AFTER EVEN MORE EXTREME DEPARTURES FROM NORMAL FOR THE MORNING LOWS. BEHIND THE FRONT A MAJORITY OF THE LOWER 48 SHOULD SEE HIGHS WITHIN 10F EITHER SIDE OF NORMAL FOR TUE-WED. AMPLIFICATION AND MODERATE PROGRESSION OF THE UPPER PATTERN WILL BRING ANOTHER SURGE OF WARMTH INTO THE CENTRAL/EASTERN STATES THU-SAT WITH TEMPS AT SOME LOCATIONS REACHING 15-30F ABOVE NORMAL. AT THE SAME TIME COLDER AIR WILL SPREAD ACROSS THE WESTERN HALF OF THE COUNTRY WITH MANY AREAS SEEING AT LEAST A DAY OR TWO WITH TEMPS 5-15F BELOW NORMAL. RAUSCH $$

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