Aug 21 Extended Forecast Outlook

EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
329 PM EDT SAT AUG 21 2010

VALID 12Z TUE AUG 24 2010 - 12Z SAT AUG 28 2010

THE MODELS SHOW THE AMPLIFICATION OF THE UPSTREAM FLOW OVER THE
NORTHEAST PACIFIC...WITH A BUILDING E CENTRAL PACIFIC 500 MB RIDGE
LEADING TO HEIGHT FALLS OFF THE WEST COAST BY SAT 28 AUG. THE
ENSEMBLE SPAGHETTI PLOTS SHOW THE ECMWF FAMILY OF SOLUTIONS ON THE
E SIDE OF THE GUIDANCE ENVELOPE WITH THIS FEATURE. THE GEFS FAMILY
OF SOLUTIONS IS FARTHER WEST...AT LEAST A DAY SLOWER IN ITS EWD
TRANSLATION. THE LONGITUDE OF NEXT WEEKENDS W COAST TROF LARGELY
DEPENDS ON THE HARD TO PIN DOWN SHARPNESS OF THE UPSTREAM E
CENTRAL PACIFIC RIDGE.

FINAL DAYS 3-7 FRONTS/SEA LEVEL PRESSURES CONTINUED THE BLEND OF
60% OF THE LATEST 00Z/21 ECMWF ENSEMBLE MEAN AND 40% OF THE NEW
DETERMINISTIC ECMWF. LATEST 12Z/21 DETERMINISTIC MODELS HAVE COME
INTO DECENT AGREEMENT WITH THE OVERALL PATTERN AFFECTING THE LOWER
48 THIS NEXT WEEK...ESPECIALLY CONCERNING THE EVOLUTION OF THE MID
ATLANTIC COASTAL LOW FOR TUE.

HOWEVER ...CONSENSUS OF THE NEW GFS/UKMET/CANADIAN ON THIS FEATURE
IS STILL A TAD E OF THE ASSOCIATED UPPER LOW POSITION OFFERED BY
THE 00Z/12Z/21 ECMWF AND THE ECMWF 00Z/21 ENSEMBLE MEAN. FOR
TUE-WED AUG 24-25...THE AMPLIFIED 850-500 MB TROUGH OVER THE
NORTHEAST TO MID ATLANTIC SUPPORTS A LOW NEAR OR OFFSHORE THE MID
ATLANTIC COAST TO EASTERN NEW ENGLAND. MOVEMENT IS SLOW AS IT IS
FLANKED BY HIGH AMPLITUDE RIDGES BOTH UPSTREAM AND DOWNSTREAM.
THIS SYSTEM LOOKS MORE PROMISING THAN EARLIER FROM NEW 12Z
UKMET/ECMWF/CANADIAN GUIDANCE FOR DROUGHT-RELIEVING RAINS FOR MUCH
OF NY/NEW ENGLAND INCLUDING NYC. SOME PORTIONS OF NY/SRN NEW ENG
COULD EVEN GO FROM DROUGHT TO EXCESSIVE RUNOFF PROBLEMS.

THE DETERMINISTIC ECMWF AND IT ENSEMBLE MEMBERS CONTINUES TO BE ON
THE WEST SIDE OF THE GUIDANCE ENVELOPE WITH THE DEVELOPING
SFC/UPPER LOW. THE 06Z/21 NAM WAS JUST A TAD SE OF THE ECMWF
POSITION OF THE MID ATLANTIC UPPER LOW FOR 12Z 24 AUG. ENSEMBLE
SPAGHETTI PLOTS SHOWED A BIT MORE SPREAD TODAY ON THIS E COAST
UPPER LOW. THE ECMWF AND ITS ENSEMBLE MEMBERS CLUSTERED SHARPLY ON
THE W SIDE OF THE GUIDANCE ENVELOPE....WITH THE CORRESPONDING GEFS
FAMILY WELL ON THE ERN SIDE. SOME UNCERTAINTY HERE...BUT WE ARE
STICKING WITH THE ECMWF/ECMWF ENSEMBLE MEAN SOLUTION DUE TO ITS
BETTER LONG TERM VERIFICATION.

OUT WEST...NEW DETERMINISTIC GUIDANCE IS IN GOOD AGREEMENT WITH
THE HEAVILY WEIGHTED AND PREFERRED 00Z/21 ECMWF ENSEMBLE
MEAN...EXCEPT THAT THE CANADIAN IS A SLOW OUTLIER.
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 3-5...SUBSIDING A LITTLE ON DAY SIX.
THIS SHOULD CONTINUE HOT CONDITIONS ACROSS THAT REGION. ON DAY
SEVEN...HEIGHTS FALL IN THE SOUTHWEST AS THE DEEPENING PACIFIC
TROUGH APPROACHES THE COAST. THIS SHOULD NUDGE THE ANTICYCLONE
EAST DOWNSTREAM TOWARDS THE SOUTHERN PLAINS WITH A NEW BURST OF
HEAT FOR THAT AREA.

THE 00Z/21 ECMWF DEVELOPED A NEW SFC LOW WITHIN THE PERSISTENT
SHEAR ZONE OVER THE NORTHERN GULF OF MEXICO.. NEW 12Z MODEL RUNS
ARE LESS DEFINED WITH THIS FEATURE....INCLUDING THE NEW ECMWF. FOR
NOW EXPECT A LITTLE ABOVE CLIMO CHANCES OF SHOWERS ALONG THE WRN
GULF COAST AREA THU-SAT DAYS 5-7.

FLOOD


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