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Aug 20 Extended Forecast Technical Discussion

EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
318 PM EDT FRI AUG 20 2010

VALID 12Z MON AUG 23 2010 - 12Z FRI AUG 27 2010

THE LONGWAVE MEAN PATTERN STARTS OFF THE MEDIUM RANGE WITH MEAN
TROFS ALONG EACH COAST AND ENDS UP PRETTY MUCH THE SAME. IN THE
INTERIM...A MAJOR SYS WILL CROSS THRU THE POLAR WESTERLIES NEAR
THE US CANADIAN BORDER....WHILE A PIECE OF ENERGY DROPS S OVER THE
NEW ENG AND MID ATLANTIC STATES ....CLOSING OFF AN UPPER LOW IN
THE SUN/MON DAY 3-4 TIME FRAME. THIS FEATURE COULD BRING NEEDED
RAIN TO PARCHED AREAS OF NY/NEW ENG AS IT WILL BE SLOW MOVING AND
HAVE PLENTY OF MOISTURE TO WORK WITH. TEMPS AS LOW AS -12C NEAR
THE CENTER OF THE 500MB UPPER LOW MAY PROVIDE SOME INSTABILITY AS
WELL ON THE BACK EDGE OF THE DEEPER MOISTURE.

FINAL PROGS STAYED WITH THE UPDATED PRELIM PATTERN BASED ON 70% OF
THE NEW ECMWF 00Z/20 ENSEMBLE MEAN AND 30% 00Z/20 DETERMINISTIC
ECMWF. NO CHANGES WERE MADE EXCEPT OVER NORTHWESTERN SATES DAYS
6-7 WHERE CONSENSUS OF THE LATEST 12Z GFS/UKMET/UKMET GUIDANCE WAS
NOTICEABLY SLOWER BRINGING A MAJOR SHORTWAVE ONSHORE BY THU DAY 6.
THE 12Z/20 CANADIAN... HOWEVER...LOOKED LIKE A SLOW OUTLIER HERE.

CONSENSUS OF THE NEW 12Z DETERMINISTIC MODELS GENERALLY SUPPORTS
OUR EARLIER CONTINUITY EXCEPT THAT SPREAD REMAINS ALONG THE MID
ATLANTIC/NEW ENG AREAS DAYS 3-4...AND OVER THE PACIFIC NW DAYS
6-7.THE 00Z ECMWF ENSEMBLE MEAN STILL LOOKS QUITE REASONABLE IN
LIGHT OF NEW GUIDANCE EXCEPT FOR BEING A BIT FAST THE END OF NEXT
WEEK BRINGING HEIGHT FALLS INTO THE NWRN CONUS. THE
DETERMINISTIC 12Z/20 ECMWF CONTINUES TO BE ON THE WEST SIDE OF THE
GUIDANCE ENVELOPE WITH THE DEVELOPING MID ATLANTIC UPPER LOW...AND
IS JOINED IN THIS REGARD BY THE 12Z/20 UKMET AND NAM.

FOR MON-WED...THE AMPLIFIED 850-500 MB TROUGH OVER THE NORTHEAST
TO MID ATLANTIC SUPPORTS A LOW NEAR OR OFFSHORE THE MID ATLANTIC
COAST. MOVEMENT IS SLOW AS IT IS FLANKED BY HIGH AMPLITUDE RIDGES
BOTH UPSTREAM AND DOWNSTREAM. THIS SYSTEM WILL CONTINUE TO BE
WATCHED CLOSELY FOR HEAVY RAIN POTENTIAL FOR PORTIONS OF THE MID
ATLANTIC/NY/NEW ENGLAND.

THE MEAN 500 MB ANTICYCLONE STAYS OVER THE SOUTHWEST US...WHERE
THE ECMWF AND GEFS ANOMALIES CONTINUE TO SHOWS 1-2 STANDARD
DEVIATIONS ABOVE NORMAL DAYS 3-6...SUBSIDING A LITTLE ON DAY
SEVEN. THIS SHOULD CONTINUE HOT CONDITIONS ACROSS THAT REGION.

PETERSEN/FLOOD


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