From Storm Prediction Center

MESOSCALE DISCUSSION 0546
NWS STORM PREDICTION CENTER NORMAN OK
0203 PM CDT THU APR 23 2009

AREAS AFFECTED…MIDDLE/EASTERN TN…EXTREME NORTHEAST AL AND
NORTHWESTERN GA

CONCERNING…SEVERE POTENTIAL…WATCH UNLIKELY

VALID 231903Z – 232100Z

BANDS OF THUNDERSTORMS WILL BE ON THE INCREASE ACROSS THE REGION
THIS AFTERNOON WITH A RISK OF ISOLATED STRONGER STORMS PRODUCING
HAIL UP TO ONE INCH IN DIAMETER.

THE BAND OF MID-LEVEL CONVECTION THAT MOVED EAST ACROSS THE REGION
EARLIER TODAY HAS LIKELY MOISTENED PROFILES ALONG THE BASE OF THE
ELEVATED MIXED LAYER. AS A RESULT…DESPITE SLIGHT WARMING ALOFT
ASSOCIATED WITH THE BUILDING UPPER RIDGE…THERMAL BUOYANCY HAS
LIKELY INCREASED/INHIBITION DECREASED IN VICINITY OF THE WARM FRONT
DRAPED NORTHWEST-SOUTHEAST ACROSS MIDDLE TN AND NORTHWEST GA.

ANOTHER BAND OF MID-LEVEL CONVECTION…BEING DRIVEN BY PERSISTENT
WEAK LOW/MID-LEVEL WARM/MOIST ADVECTION…HAS DEEPENED ACROSS MIDDLE
TN AND LIGHTNING HAS INCREASED SINCE 18Z. LATEST 4KM NSSL-WRF…NAM
AND RUC SUGGEST THE ACTIVITY WILL DEVELOP SOUTHEAST ALONG THE WARM
FRONT INTO NORTHWEST GA…INCLUDING THE ATLANTA METRO AREA THROUGH
00Z.

12Z SOUNDINGS SHOWED THAT THE REGION RESIDES ALONG EASTERN PERIPHERY
OF VERY STEEP 700-500 MILLIBAR LAPSE RATES /GREATER THAN 7.5 DEGREES
C PER KM/. EFFECTIVE VERTICAL SHEAR OF 35-40 KNOTS COUPLED WITH
MAGNITUDE OF LAPSE RATES WILL RESULT IN ISOLATED STRONG STORMS…BUT
LONGEVITY OF ANY GIVEN UPDRAFT WILL BE TEMPORALLY SHORT OWING TO
MODEST INSTABILITY. STRONGER STORMS MAY YIELD HAIL…GENERALLY UP
TO ONE INCH IN DIAMETER AND PERHAPS GUSTY WINDS.

PROSPECTS FOR ONLY ISOLATED SEVERE WEATHER WILL PRECLUDE A WEATHER
WATCH.

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