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July 18 NWS National Extended Discussion

EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
200 PM EDT SUN JUL 18 2010

VALID 12Z WED JUL 21 2010 - 12Z SUN JUL 25 2010

FEW MODIFICATIONS WERE MADE TO THE FINAL FRONTS/SEA LEVEL
PRESSURES/500 MB PREFERRED HEIGHT FORECASTS. THE GFS/GFS
PARALLEL/UKMET/CANADIAN GLOBAL/GEFS MEAN ALL STUCK TO EARLIER
SOLUTIONS OR SLIGHT VARIANTS OF THEM...SO MINOR ADJUSTMENTS WERE
MADE TO BLEND WITH THE 12Z GEFS MEAN.

THE MODELS AND ENSEMBLES ARE SHOWING THE BEGINNING OF A PATTERN
TRANSITION IN THE MEDIUM RANGE...AND HAVE THEIR DIFFERENCES TO
SHOW FOR IT.
THE HIGH AMPLITUDE ANTICYCLONE OVER THE NORTHERN PACIFIC FINALLY
IS DISLODGED/RETROGRADES AS HEIGHT FALLS OCCUR WITH THE ALASKAN
CLOSED LOW MOVING SOUTHEAST OVER THE NORTHERN PACIFIC AND
EVENTUALLY INTO BRITISH COLUMBIA AND FURTHER INTO CANADA.

AS THIS OCCURS...THE MODELS/ENSEMBLES HAVE A WIDE SPREAD REGARDING
THE TIMING/PHASING OF INDIVIDUAL WAVES MOVING IN THE ZONAL FLOW
ALONG THE NORTHERN TIER OF THE US AND SOUTHERN CANADA. THE
DIFFERENCES AMONG SOLUTIONS CANCEL EACH OTHER OUT SO THE RESULTANT
MEAN FLOW LOOKS ZONAL...BUT EMBEDDED LOW AMPLITUDE SHORTWAVES WILL
EXIST WITHIN THE FLOW. THE 00Z ECMWF ENSEMBLE MEAN AND 00Z-12Z
GEFS MEAN ARE IN GOOD AGREEMENT AND FORM THE PRIMARY BASIS OF THE
FORECAST...WITH ONLY 10 PERCENT WEIGHTING GIVEN TO THE OPERATIONAL
ECMWF THROUGH DAY SIX. THE PRIMARY FOCUS FOR
SHOWERS/THUNDERSTORMS SHOULD BE THE UPPER MS VALLEY AND ADJACENT
OH VALLEY/UPPER LAKES DAYS 4-5...SPREADING INTO THE REMAINING
LAKES/NORTHEAST THEREAFTER.

THE 00Z-12Z CANADIAN AND UKMET MODELS WERE NOT USED SINCE THEY
TAKE A DISTURBANCE COMING ONSHORE IN BRITISH COLUMBIA AND FORMED A
CLOSED 500 MB LOW OVER SOUTH CENTRAL CANADA BY 00Z SAT 24 JUL. AT
00Z SAT...THE ECMWF/GEFS ENSEMBLE MEMBERS ALL SHOW AN OPEN
WAVE...MAKING THE UKMET/CANADIAN LOOK LIKE LOW PROBABILITY
SOLUTIONS.

TELECONNECTIONS FROM THE LOWER HEIGHTS OVER ALASKA SUPPORT A
NORTHEAST US TROUGH....AND THIS IS SHOWN IN THE GFS/GFS PARALLEL
RUNS AND IN THE 00Z ECMWF/00-012 NCEP ENSEMBLE MEANS. THE
OPERATIONAL ECMWF WAS THE LAST HOLDOUT KEEPING THE HEIGHT FALLS
FURTHER WEST IN THE GREAT LAKES ON SUN 25 JUL. THE 06-12Z
GFS/PARALLEL SOLUTIONS SHOWED A DEEPER CYCLONE OVER SOUTHEAST
CANADA...AND THE MEANS WERE USED TO MITIGATE BOTH THE SPREAD IN
FORECAST INTENSITIES PLUS THE SPREAD IN CYCLONE LOCATION...AS THE
MODEL CYCLONE POSITION CHARTS SHOW A LACK OF CLUSTERING OF
SOLUTIONS AND SPREAD OF LOW LOCATIONS ACROSS SOUTHEAST CANADA INTO
THE MARITIMES.

OVER THE SOUTHEAST..THE CORE OF THE MID LEVEL ANTICYCLONE AND
ATTENDANT HOT CONDITIONS REMAINS IN PLACE THROUGH DAY SIX...WITH
UNCERTAINTY ON DAY SEVEN AS THE MAJORITY OF THE ENVELOPE OF
SOLUTIONS STARTS LOWERING HEIGHTS AND SHIFTING THE POSITIVE
HEIGHT/TEMPERATURE ANOMALY WEST INTO THE DEVELOPING RIDGE OVER THE
CENTRAL HIGH PLAINS IN THE 00-12Z GEFS MEAN SOLUTIONS.

PETERSEN


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