Greenland ULL Block moving east --> active weather forecast for CONUS end-of-week
Posted by JAC on 6/29/2009, 8:34 am



DAY 4-8 CONVECTIVE OUTLOOK 
  NWS STORM PREDICTION CENTER NORMAN OK
  0334 AM CDT MON JUN 29 2009
 
  VALID 021200Z - 071200Z
 
  ...DISCUSSION...
 
  MODEL CONSENSUS IS THAT BLOCKING PATTERN WILL GRADUALLY BREAK DOWN
  BY DAY 5 WITH FLOW TRANSITIONING TO MORE OR LESS ZONAL PROGRESSIVE.
  AS THE UPPER PATTERN DEAMPLIFIES...POSITIVE THETA-E ADVECTION WILL
  INCREASE OVER THE PLAINS/MID MS VALLEY REGION AND IN CONJUNCTION
  WITH EVAPOTRANSPIRATION WILL RESULT IN A GRADUAL INCREASE IN LOW
  LEVEL MOISTURE IN VICINITY OF WNW-ESE ORIENTED BAROCLINIC ZONE.
  PATTERN WILL BE CONDUCIVE FOR STORMS TO DEVELOP OVER THE PLAINS AND
  THE MS VALLEY AND SPREAD SEWD WITHIN ZONE OF ISENTROPIC LIFT NORTH
  OF THE BOUNDARY WITH SEVERAL MCS CLUSTERS POSSIBLE. THE BOUNDARY
  WILL REMAIN CLOSE ENOUGH TO THE STRONGER MID LEVEL FLOW FOR
  ORGANIZED SEVERE STORMS TO DEVELOP. SEVERE THREAT WILL BE MODULATED
  BY SERIES OF IMPULSES MOVING ESEWD THROUGH THE BELT OF STRONGER
  WLYS. WHILE SOME THREAT FOR SEVERE STORMS IS POSSIBLE DAY 4 IN THIS
  REGION...THREAT IS EXPECTED TO INCREASE IN THE DAY 5-6 TIME FRAME.
 
  ..DIAL.. 06/29/2009



PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
319 AM EDT MON JUN 29 2009

VALID 12Z FRI JUL 03 2009 - 12Z MON JUL 06 2009

THE MODELS AND THEIR ENSEMBLES SHOW GOOD AGREEMENT THAT THE
BLOCKING UPPER LOW SOUTHEAST OF GREENLAND WILL BEGIN TO EJECT
TOWARD EUROPE ON DAY 3...WHICH THEN ALLOWS FOR MORE PROGRESSIVE
FLOW AND INCREASING UNCERTAINTY TO ACCOMPANY THE DAY-TO-DAY
DETAILS OVER THE CONUS THROUGH THE MEDIUM RANGE PERIOD. THE
LARGEST UNCERTAINTY APPEARS TO ACCOMPANY THE STRONGER FLOW
DEVELOPING OVER WESTERN CANADA AND ITS POTENTIAL INTERACTION WITH
THE WEAKENING LOW OVER THE GREAT LAKES...WITH THE FASTER GFS
RECEIVING THE LEAST SUPPORT FROM OTHER DETERMINISTIC/ENSEMBLE
GUIDANCE. THUS...WILL
CONTINUE TO EMPHASIS THE SLOW SIDE OF THE GUIDANCE...INCLUDING THE
NEW ECMWF/CANADIAN FOR THIS INTERACTION...WHICH MAINTAINS
CONTINUITY/PERSISTENCE WITH PRIOR FORECASTS. THE NEXT LARGEST
UNCERTAINTY INVOLVES THE DEEPENING UPPER LOW OVER THE BERING
SEA...WITH THE MAJORITY OF GUIDANCE FAVORING A DEEPER
SCENARIO...AND ITS SUBSEQUENT INFLUENCE ON A DOWNSTREAM TROUGH
NEARING THE WESTERN CONUS AROUND DAY 5. GIVEN THE MIX OF PREFERRED
GUIDANCE...WITH AN ECMWF/CANADIAN/GEFS MEAN BLEND PREFERRED OVER
THE EASTERN HALF OF THE CONUS...AND A GFS/CANADIAN/GEFS MEAN BLEND
PREFERRED OVER THE PACIFIC/WEST COAST...THIS FORECAST CONTAINS
BELOW AVERAGE CONFIDENCE...WITH THE GEFS MEAN GENERALLY USED TO
CREATE THE PRESSURES/FRONTS FOR DAYS 5-7.

JAMES



EAST...
THE BLOCKING UPPER LOW SOUTHEAST OF GREENLAND BEGINS TO
LIFT...WHICH ALLOWS THE UPSTREAM ELEMENTS TO GRADUALLY MOVE. MODEL
AGREEMENT IS EXCELLENT UNTIL THE SHORTWAVE PATTERN BEGINS TO
DEAMPLIFY (ABOUT DAY 5)...WITH THE 00Z GFS FASTEST TO TRANSITION
OUT OF THE BLOCK AND THE ECMWF/MEANS ABOUT 1/2 DAY SLOWER.
PERSISTENCE AND MODEL CONSENSUS SUPPORT THE SLOWER SCENARIO.  THE
MODELS/ENSEMBLE MEAN SHOW A DEEP LAYER TROUGH/EMBEDDED CLOSED LOW
MOVING EAST FROM CENTRAL TO EASTERN CANADA THIS
PERIOD...EVENTUALLY REINFORCING THE EASTERN CANADA/GREAT LAKES
LONGWAVE TROUGH. THE 06Z GFS TRENDED MUCH SLOWER THAN THE 00Z RUN
IN MOVING THE CLOSED 500 MB EAST FRI THROUGH SAT...AND IT BUILDS A
HIGHER AMPLITUDE 500 MB RIDGE ACROSS THE GREAT LAKES THAN THE
MODEL/ENSEMBLE CONSENSUS.  CONFIDENCE IS LOW IN THIS SOLUTION...AS
THE DEEP LAYER RIDGE OVER THE HIGH PLAINS/FRONT RANGE SHOULD LEAD
TO LOWER HEIGHTS DOWNSTREAM IN THE EAST. THE 12Z GFS WAS SLOWER
LIFTING OUT THE GREAT LAKES CLOSED LOW SFC/ALOFT...AND TRENDED
TOWARD THE PREFERRED SOLUTION...ALTHOUGH ITS 700-500 MB HEIGHTS
ARE A FEW DM DEEPER THAN CONSENSUS/00-12Z NCEP ENSEMBLE MEANS. 

THE 00Z ECMWF MADE A LARGE CHANGE FROM THE 12Z RUN AND IS STRONGER
SFC/ALOFT WITH THE SYSTEM CROSSING NEAR LK SUPERIOR SAT 04 JUL
INTO QUEBEC SUN 05 JUL.  THE RUN TO RUN CHANGES IN BOTH THE ECMWF
AND GFS LEAD TO THE PREFERENCE FOR THE 00-12Z ENSEMBLE MEAN
SOLUTIONS. CONFIDENCE IS NOT HIGH IN PICKING ANY ONE
SOLUTION...AND THE 12Z GEFS MEAN HAS SHOWN GOOD RUN TO RUN
CONTINUITY. 

PETERSEN

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Greenland ULL Block moving east --> active weather forecast for CONUS end-of-week - JAC, 6/29/2009, 8:34 am
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