Posted by JAC on 4/17/2009, 7:58 am, in reply to "Next Severe Weather: Tuesday 4/21, SE Coast"

EXTENDED FORECAST DISCUSSION NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD 155 PM EDT THU APR 16 2009 VALID 12Z SUN APR 19 2009 - 12Z THU APR 23 2009 MEDIUM RANGE MODEL CONTINUITY CONFIDENCE HAS DECREASED CONSIDERABLY THIS CYCLE GROUP WITH EARLIER SHORTWAVE DISCONINUITY AND A PHASING PROBLEM STILL IN THE WORKS. NOW THE LONGWAVE PATTERN CHANGE POTENTIAL IS SHOWING UP IN OPERATIONAL RUNS. SOUTHWESTERN CONUS CLOSED LOW IN SPLIT FLOW ALOFT WILL CONT TO MOVE INTO THE CENTRAL PLAINS DAY 3 SUN. GFS RUNS CONT TO DROP MORE NRN STREAM ENERGY INTO THIS SYSTEM RESULTING IN A MORE PHASED AND CLOSED OFF MID LEVEL LOW COMING THRU THE OH VALLEY LATE MON AND TUES BEFORE EXITING OFF THE NEW ENG COAST DAY 6 WED. ECMWF/UKMET/CMC DO NOT SHOW THIS PHASING OR AT LEAST DROP IN NRN STREAM ENERGY LATER AS THE MID LEVEL TROF MOVES THRU THE UPPER OH VALLEY AND NORTHEAST LATE TUES AND WED. UKMET IS THE MOST AGRESSIVE AND LEAST PROGRESSIVE FORMING AN INTENSE DEEP CUTOFF LOW CENTERED OVER THE GREAT LAKES REGION LATE DAY 5 TUES. ENS MEANS OF 00Z RUNS GFS AND ECMWF ARE QUITE SIMILAR THRU DAY 5 TUES WITH A MEAN DEEP ERN CONUS TROF AND WRN CONUS RIDGING. HPC PREFERENCE IS TO KEEP A NON PHASED SOLUTION TO THE EVOLVING CENTRAL AND ERN CONUS TROF THRU DAY 5 WHILE KEEPING UP WRN CONUS RIDGING. DAYS 6 AND 7... THE EXTENT OF ERN PAC RIDGING AND AN APPROACHING SHORTWAVE TROF DROPPING DOWN THE B.C. COAST INTO THE PAC NORTHWEST BECOMES A PROBLEM. OP MODELS OF GFS/CMC AND ECMWF ARE QUITE STRONG WITH THE N PAC RIDGE AND DOWNSTREAM TROF AND RAISE HTS CONSIDERABLY OVER THE ERN HALF OF CONUS DAYS 6 AND ESP DAYS 7 AND 8. ENS MEANS OF ECMWF AND GFS ARE MUCH MORE SUBDUED WITH THIS SOLUTION WITH LESSER ERN PAC/PAC NORTHWEST AMPLIFICATION AND MAINTAIN A WEAKER RESIDUAL DOWN STREAM ERN CONUS TROF. THE LAGGED AVERAGE FORECAST TRENDS OF BOTH GFS AND ECMWF OVER THE PAST 4 RUNS SHOW THE STRONG RECENT RUN CHANGES WITH BOTH THE NW PAC TROF AND ERN CONUS RIDGE AND A COMBINED BLENDED SOLUTION OF THE PAST 4 RUNS OF BOTH GFS AND ECMWF YIELDS A MEAN DAY 7 THURS SOLUTION VERY SIMILAR TO THE 00Z ECMWF ENS MEAN I.E. A MEAN TROF ALONG THE WEST COAST/PLAINS RIDGING AND A TROF PERSISTING OVER THE ERN QUARTER OF CONUS. THIS IS A MUCH MORE SUBDUED SOLUTION THAN THE DAY 7 OP RUNS OF GFS AND ECMWF. PERSISTENCE AND ENS SOLUTION WOULD SUGGEST BEING CONSERVATIVE WITH THIS PATTERN CHANGE. HOWEVER SOME FORECAST MJO SOLUTIONS INDICATE THAT A PROGRESSION FROM A PHASE 4 TO PHASE 5 MJO INDICATES A COMPOSITE SOLUTION THAT FULLY SUPPORTS THE OP MODEL SOLUTIONS. OTHER MJO GUIDANCE INDICATES A MUCH WEAKER AND AN INDETERMINANT PHASE. TELECONNECTIONS BASED ON BOTH AGREED UPON POSITIVE AND NEG HT ANOMALIES FROM BOTH OP MODELS AND ENS MEANS OFFER SOME SUPPORT BUT BASICALLY TEND TOWARDS A FLATTER ERN CONUS FLOW AND A WEKER PAC NW TROF. THESE MIXED SIGNALS SUGGEST THAT WE BE CONSERVATIVE WITH THE PATTERN CHANGE AT LEAST FOR TODAY AND HPC DAY 6 AND 7 SOLUTIONS LEAN HEAVILY ON THE ECMWF ENS MEAN/BLENDED LAGGED AVERAGE FORECAST TREND AND OP ECMWF. THIS RESULTS IN PATTERN SOMEWHAT CLOSE TO BUT WITH LESSER THERMAL EXTREMES OF THE 00Z ECMWF AND 06Z GFS. SEE HPC DERIVED H500 FIELDS. 12Z GFS GUIDANCE HAS GONE AWAY FROM ITS 00Z CENTRAL MS/LOWER OH VALLEY PHASING SOLUTION DAYS 3 AND 4 TO YIELDING A VERY BROAD DEEP TROF AND CLOSED EXITING THRU NRN NEW ENG DAY 6. UPSTREAM KICKER TROF APPROACHING THE PAC NORTHWEST IS SLOWER THAN ITS 00Z RUN DAYS 4-6 BUT CATCHS UP BY DAY 7 WITH A STRONG DEEP CLOSED LOW IN THE PAC NORTHWEST. DOWNSTREAM CHANGES ARE DRAMATIC AS NEW 12Z GFS IS ABOUT 300 METERS LOWER WITH ERN CONUS HTS DAYS 6 INTO DAY 7. THIS POSSIBILITY WAS ACCOUNTED FOR EARLIER BY ECMWF AND GFS ENS MEANS WITH HPC USING THE ECMWF MEAN FOR PROGS AND MID LEVELS. CMC MAINTAINS GOOD CONTINUITY AND CONTS TO BE SIMILAR TO THE GFS/ECMWF ENS MEANS AT DAY 6. UKMET ALSO MAINTAINS CONINUITY WITH A LARGE DEEP CYCLONE OVER THE GREAT LAKES REGION AT DAY 6 EARLIER DERIVED HPC PROGS AND H500S REMAIN VALID EMPHASING A LATE PERIOD SIG NWRN CONUS TROF/LEE OF ROCKIES-HIGH PLAINS RIDGING AND A CONTD ERN CONUS TROF WITH A WEAKENING TENDENCY. NO CHANGES TO AFTN FINALS FROM EARLIER PRELIMS. ROSENSTEIN