Prognostic Meteorological Discussion
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000 FXUS02 KWBC 270651 PMDEPD EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 251 AM EDT MON APR 27 2015 VALID 12Z THU APR 30 2015 - 12Z MON MAY 04 2015 ...OVERVIEW... AFTER AN ERN CONUS-WRN ATLC EVOLUTION SFC/ALOFT MORE TYPICAL OF WINTER... THE PATTERN UPSTREAM QUICKLY TAKES ON MORE OF A SUMMERTIME LOOK WITH PROGRESSIVE LOW AMPLITUDE MEAN FLOW ACROSS THE NRN CONUS/SRN CANADA. MODEST MEAN TROUGHING IS EXPECTED TO PERSIST ALONG THE SRN HALF OF THE WEST COAST. GUIDANCE CONTINUES TO HAVE DIFFICULTIES IN RESOLVING IMPORTANT MID LVL DETAILS OVER THE EAST KEEPING CONFIDENCE LOWER THAN DESIRED FOR ASSOC WRN ATLC LOW PRES. AT THE SAME TIME AGREEMENT/CONTINUITY ARE NOT PARTICULARLY GREAT EITHER FOR SPECIFICS OF SHRTWVS/FRONTAL SYSTEMS CARRIED ALONG BY THE MEAN FLOW ALONG THE WRN 2/3 OF THE US-CANADIAN BORDER. ...GUIDANCE EVALUATION AND PREFERENCES... FOR THE ERN CONUS/WRN ATLC SYSTEM... THROUGH THE 18Z CYCLE AN AVG OF THE 18Z GEFS/12Z ECMWF MEANS SEEMED TO OFFER THE BEST INTERMEDIATE TRACK FOR THE SFC LOW WHILE OPERATIONAL GUIDANCE AS A WHOLE SUPPORTED A DEEPER ADJUSTMENT TO SOME DEGREE. THUS INCORPORATED A BLEND OF THE ENSEMBLE MEANS AND 18Z GFS/12Z ECMWF BUT WITH LOW ENOUGH WEIGHTING TO DOWNPLAY THE ECMWF WHICH STRAYED TOWARD THE ERN PORTION OF THE ENSEMBLE SPREAD BY DAY 4 FRI. SOLNS ARE STILL VARYING WITH HOW AMPLIFYING NRN STREAM ENERGY REACHING THE GRTLKS AND PSBLY INTO THE OH VLY BY THE START OF THE PERIOD THU WILL EVOLVE... AND THE INTERACTION WITH EJECTING SHORT RANGE PLAINS ENERGY. ALBEIT WITH GROWING SPREAD WITH TIME... THERE HAD BEEN A RELATIVE CLUSTER AMONG THE SOLNS MENTIONED ABOVE EARLY ON TO RECOMMEND THE COMPROMISE OF THEIR IDEAS. OVERALL THE 00Z GUIDANCE GENERALLY SHOWS A STRONGER OH VLY/MID ATLC UPR LOW THUS LEADING TO A FARTHER WWD SFC TRACK AT SOME FCST HRS. THE 00Z GFS IS LOOKING LIKE A DEEP EXTREME WITH ITS UPR LOW THOUGH... WITH THE GEFS MEAN AND NEW ECMWF SUPPORTING MORE MODERATE DEPTH. AT LEAST COMPARED TO EARLIER CYCLES THIS CYCLE REPRESENTS IMPROVEMENT FOR THE TRACK OF MID LVL ENERGY EVEN IF OTHER ATTRIBUTES REMAIN IN QUESTION. WITHIN THE FAST FLOW ACROSS THE NRN TIER... THERE IS SOME DEGREE OF CONSISTENCY WITH THE LEADING SHRTWV/SFC FRONT THU-FRI BUT OTHERWISE GUIDANCE REFLECTS A NOTICEABLE TREND TOWARD FLATTER AND MORE PROGRESSIVE FLOW COMPARED TO EARLIER DAYS. THIS RESULTS IN A FASTER SERIES OF FRONTS THAT WILL HAVE A TENDENCY TO WEAKEN AS THEY REACH BEYOND THE NRN PLAINS. AT THE MOMENT THERE IS GENERAL AGREEMENT TOWARD A BETTER DEFINED SHRTWV ALOFT REACHING CNTRL CANADA AND THE NRN PLAINS BY NEXT MON... SUPPORTING A LEADING COLD FRONT THAT MAY REACH FARTHER EWD THAN PRIOR FRONTS IN THE SERIES. FARTHER WWD MDLS/ENSMEANS AGREE FAIRLY WELL WITH THE IDEA OF NERN PAC ENERGY BRINGING ANOTHER TROUGH TO THE PAC NW BY DAY 7 MON. GIVEN THE PROGRESSIVE NATURE OF THIS STREAM AND CHANGES THAT GUIDANCE HAS MADE OVER RECENT DAYS IT WOULD NOT BE SURPRISING TO SEE SOME ASPECTS OF THIS FCST CHANGE IN THE NEAR FUTURE. BASED ON PREFS FOR THE WRN ATLC SYSTEM AND TO REFLECT AVBL CLUSTERING UPSTREAM... DAYS 3-5 THU-SAT START WITH A BLEND OF THE 18Z GFS/ECMWF AND THEIR RESPECTIVE MEANS. CONFIDENCE IN GFS DETAILS OVER E-CNTRL CANADA DECREASES AFTER SAT SO DAYS 6-7 SUN-MON EXCLUDE THAT SOLN FROM THE BLEND. ...SENSIBLE WEATHER HIGHLIGHTS... DURING THE FIRST HALF OF THE PERIOD DIFFS IN TRACK FOR WRN ATLC LOW PRES STILL LEAD TO GREATER THAN AVG UNCERTAINTY IN COVERAGE/INTENSITY OF RNFL NEAR THE EAST COAST AND LOCATIONS THAT MAY BE AFFECTED BY ENHANCED WINDS. IMPROVED CLUSTERING IN GUIDANCE FOR PATH OF BEST DEFINED UPR ENERGY EARLY IN THE PERIOD AT LEAST RAISES CONFIDENCE IN THE POTENTIAL FOR SOME RNFL OVER THE APLCHNS/MID ATLC THOUGH LIKELY NOT TO THE EXTENT OF THE 00Z GFS GIVEN ITS EXTREME DEPTH ALOFT. CONFIDENCE REMAINS LOWER IN HOW MUCH MSTR REACHES THE NORTHEAST DUE TO SENSITIVITY IN THE SFC LOW TRACK. AS NRN PAC FLOW CONTINUES ALONG THE US-CANADIAN BORDER... ASSOC FRONTS SHOULD HELP TO EXPAND AREAL COVERAGE OF RNFL WITH TIME FROM THE NRN-CNTRL ROCKIES INTO THE GRTLKS. IN RELATIVE TERMS HEAVIEST ACTIVITY SHOULD OCCUR LATE IN THE PERIOD. COOL TEMPS EARLY IN THE PERIOD OVER THE EAST SHOULD STEADILY TREND WARMER TO NEAR/ABOVE NORMAL. ASIDE FROM NEAR NORMAL READINGS OVER THE SOUTH... THE REMAINDER OF THE CONUS SHOULD SEE ABOVE NORMAL READINGS FOR MOST OF THE PERIOD. RAUSCH $$

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