Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 181103 AAA
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
603 AM CDT THU SEP 18 2014

.UPDATE...
FOR 12Z TAFS

&&

.DISCUSSION...
/ISSUED 504 AM CDT THU SEP 18 2014/
THE FORECAST AREA REMAINS BENEATH A NORTHWEST FLOW ALOFT THIS
MORNING...BETWEEN A MID-LEVEL RIDGE CENTERED ACROSS NORTHEASTERN
MEXICO AND A LONGWAVE TROUGH ENCOMPASSING MUCH OF EASTERN NORTH
AMERICA. A MID-LEVEL VORTICITY MAXIMA EMBEDDED IN THIS FLOW WILL
CONTINUE TO SHIFT EAST-SOUTHEASTWARD ACROSS KENTUCKY/TENNESSEE THIS
MORNING...AND STRENGTHENING FLOW ASSOCIATED WITH THIS FEATURE HAS
ENHANCED CONVERGENCE ALONG THE 850MB FRONT WHICH IS CURRENTLY
SITUATED ACROSS SOUTHWEST TENNESSEE. AS WAS INDICATED BY SEVERAL
MODELS YESTERDAY...CLUSTERS OF ELEVATED CONVECTION DEVELOPED IN THE
VICINITY OF THIS 850MB FRONT SHORTLY AFTER MIDNIGHT -- WITH THIS
ACTIVITY CONTINUING TO TRACK SOUTHEASTWARD INTO THE REGION BEING
STEERED BY NORTHWEST FLOW ALOFT. MID-LEVEL LAPSE RATES ON THE 00Z OHX
SOUNDING SUGGEST SUFFICIENT ELEVATED INSTABILITY AND MOISTURE TO
MAINTAIN CONVECTION...AND WILL INCREASE MORNING POPS FOR THE SOUTHERN
MIDDLE TENNESSEE/NE ALABAMA CONTINUES BASED ON CURRENT TRENDS IN
RADAR DATA. THE COVERAGE/INTENSITY OF CONVECTION WILL TEND TO
DIMINISH AS THE 500MB WAVE CONTINUES EAST-SOUTHEASTWARD ACROSS THE
SOUTHERN APPALACHIANS...WITH ONLY A FEW SHOWERS/STORMS POSSIBLE
ACROSS NW ALABAMA THIS AFTERNOON. HOWEVER...A SIMILAR CONVECTIVE
PATTERN MAY UNFOLD ACROSS THE REGION TONIGHT AS ANOTHER/STRONGER
500MB WAVE -- CURRENTLY ACROSS THE MISSOURI VALLEY -- DROPS
SOUTHEASTWARD OVER THE REGION. THERE IS SOME UNCERTAINTY AMONG THE
MODELS REGARDING POSITION OF THE 850MB FRONT BY TONIGHT AND AMOUNT OF
DRY ADVECTION THAT WILL OCCUR TO THE NORTHEAST OF THIS BOUNDARY...
AND FOR THIS REASON HAVE ONLY INCLUDED SLIGHT CHANCE POPS FOR THE
SOUTHWEST HALF OF THE FORECAST AREA.

BY FRIDAY/FRIDAY NIGHT...THE CORE OF THE EASTERN CONUS LONGWAVE
TROUGH WILL SHIFT EASTWARD INTO THE CANADIAN MARITIMES WHILE THE
SOUTHERN PORTION OF THE LONGWAVE CUTS OFF INTO A WEAK CIRCULATION
ACROSS THE SOUTHEASTERN ATLANTIC COAST/FLORIDA PENINSULA. THIS WILL
OCCUR AS A MID-LEVEL RIDGE BUILDS NORTHWARD ACROSS THE ARKLATEX
REGION...WHICH WILL EFFECTIVELY ALLOW MID-LEVEL FLOW TO VEER TO THE
NORTH ON FRIDAY AND EVENTUALLY NORTHEAST BY FRIDAY NIGHT/SATURDAY.
MID-LEVEL HEIGHTS WILL RISE DURING THIS PERIOD...PROVIDING AN
ENVIRONMENT MORE UNFAVORABLE FOR CONVECTION...ESPECIALLY WITH THE
LITTLE TO NO DEEP-LAYER FORCING FOR UVM EVIDENT AT THIS TIME. AT THE
SURFACE...A WEAK INVERTED TROUGH WILL RETROGRADE ACROSS THE REGION
FRIDAY/FRIDAY NIGHT...WITH LOW-LEVEL FLOW INCREASING FROM THE EAST-
SOUTHEAST IN ITS WAKE. STRENGTHENING WARM ADVECTION WILL YIELD AN
INCREASE IN THICK STRATUS CLOUDS BY LATE FRIDAY NIGHT/SATURDAY
MORNING...WITH A FEW SPRINKLES OF RAIN POSSIBLE IN THIS REGIME ON
SATURDAY MORNING. HOWEVER...DUE TO LIMITED LOW-LEVEL MOISTURE RETURN
ON WESTERN FLANK OF A STRONG SURFACE HIGH SHIFTING INTO THE WESTERN
NORTH ATLANTIC...WILL KEEP POPS FOR SATURDAY AT 10 PERCENT.

THE ARKLATEX MID-LEVEL RIDGE WILL CONTINUE TO BUILD ACROSS THE
REGION ON SATURDAY NIGHT IN ADVANCE OF ANOTHER STRONG SHORTWAVE
TROUGH DIGGING SOUTHEASTWARD ACROSS MANITOBA/SASKATCHEWAN -- WHICH
WILL SERVE TO DEEPEN THE EASTERN NOAM LONGWAVE TROUGH ONCE AGAIN. A
WEAK MID-LEVEL SHEAR AXIS AND ASSOCIATED ZONE OF SURFACE CONVERGENCE
WILL SHIFT SOUTHEASTWARD ACROSS THE REGION ON SUNDAY RESULTING IN THE
DEVELOPMENT OF ISOLATED/WIDELY SCATTERED SHOWERS AND STORMS BY
AFTERNOON. THE COVERAGE OF THIS ACTIVITY WILL BE LIMITED BY MEAGER
LOW-LEVEL MOISTURE PRIOR TO FRONTAL PASSAGE. A FEW SHOWERS AND STORMS
WILL PERSIST INTO SUNDAY EVENING...BEFORE A STRONGER COLD FRONT
ASSOCIATED WITH THE AMPLIFYING LONGWAVE TROUGH SPREADS SOUTHWARD
BETWEEN 9-15Z MONDAY. A BRIEF SHOT OF EVEN COOLER/DRIER AIR IS IN
STORE ON MONDAY/MONDAY NIGHT...WITH DEWPOINTS FALLING INTO THE UPPER
40S/LOWER 50S IF NOT EVEN SLIGHTLY LOWER. A WEAK DISTURBANCE IN
NORTHWEST FLOW ALOFT WILL LEAD TO AN INCREASE IN CLOUD COVER BY THE
END OF THE EXTENDED PERIOD. TEMPERATURES AND DEWPOINTS WILL INCREASE
GRADUALLY AS LOW-LEVEL FLOW SHIFTS TO THE EAST-SOUTHEAST.

70/DD

&&

.AVIATION...
FOR 12Z TAFS...
SHOWERS AND ISOLATED THUNDERSTORMS WILL CONTINUE TO IMPACT KHSV
THROUGH ABOUT 15Z THIS MORNING, WHEN CONVECTION WILL BECOME MORE
ISOLATED IN NATURE. CONDS MAY DIP INTO LOWER-MVFR, BUT VFR CONDS WILL
PREVAIL OVERALL. AT KMSL, COVERAGE OF CONVECTION WILL BE A BIT MORE
SPOTTY, SO ONLY VCSH WAS MENTIONED THROUGH 15Z THIS MORNING. THROUGH
THE TAF PERIOD, CLOUDS WILL GRADUALLY DISSIPATE AND LIFT, LEAVING A
FEW LOWER CLOUDS AND SCATTERED HIGH CLOUDS BY 00Z TODAY. SOME PATCHY
FOG IS POSSIBLE AT BOTH SITES TOMORROW MORNING DUE TO THE RAINFALL
THIS MORNING, BUT WILL ONLY MENTION MVFR CONDS FOR NOW.

12

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

FOR MORE INFORMATION PLEASE VISIT OUR WEBSITE
AT WEATHER.GOV/HUNTSVILLE.



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