Flash Flood Watch 9, NWS Huntsville, AL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
423 PM CDT SAT SEP 3 2011

.DISCUSSION...
DESPITE AN INCREASE IN CIRROFORM CLOUD COVER FROM TROPICAL STORM LEE
LOCATED ALONG THE COAST OF CENTRAL LOUISIANA...QUIET WEATHER
CONDITIONS PREVAIL ACROSS THE TENNESSEE VALLEY REGION THIS AFTERNOON
LIKELY IN RESPONSE TO A BROAD REGION OF SUBSIDENT AIR ALOFT...TO THE
NORTH OF T.S. LEE AND TO THE SOUTH OF A SMALL MID-LEVEL HIGH CENTERED
OVER THE CENTRAL/SOUTHERN APPALACHIANS. GLOBAL MODELS...SUCH AS THE
UKMET/ECMWF AND GFS...REMAIN IN GOOD AGREEMENT WITH THE EVOLVING
WEATHER EVENT FOR NORTHERN ALABAMA/SOUTHERN MIDDLE TENNESSEE...AS
THEY ALL INDICATE THAT THE PRIMARY MID-LEVEL CIRCULATION ASSOCIATED
WITH T.S. LEE WILL REMAIN NEARLY STATIONARY ACROSS SOUTHERN LOUISIANA
FROM TONIGHT THROUGH TOMORROW -- TRAPPED BETWEEN THE CENTRAL/SOUTHERN
APPALACHIANS MID-LEVEL HIGH AND A SEPARATE MID-LEVEL RIDGE POSITIONED
ACROSS THE SOUTHERN ROCKIES AND NORTHERN MEXICO. HOWEVER...IT APPEARS
THAT THE VORT MAX WILL BEGIN TO EXPAND NORTH-NORTHEASTWARD BY
SUNDAY...AS A STRONG 500 MB TROUGH -- CURRENTLY MOVING EASTWARD
ALONG THE U.S.-CANADIAN BORDER -- SPREADS INTO THE WESTERN GREAT
LAKES/SOUTHERN ONTARIO REGION TONIGHT. AS THIS OCCURS...MID-LEVEL
WINDS ACROSS THE FORECAST AREA WILL VEER TO THE SOUTH AND INCREASE...
ALLOWING A SURGE OF RICH TROPICAL MOISTURE /PWAT VALUES OF 2-2.25
INCHES/ TO MOVE NORTHWARD INTO THE REGION TOMORROW. THE STRONG
WARM/MOIST ADVECTION PROCESSES WILL EFFECTIVELY RESULT IN AN EARLIER
START TO PRECIPITATION TOMORROW...WITH RAINFALL POTENTIALLY BEGINNING
IN THE SOUTHWESTERN HALF OF THE REGION SHORTLY BEFORE 04/12Z AND
SPREADING NORTHWARD THROUGH THE DAY...AS T.S. LEE SHIFTS VERY SLOWLY
NORTH-NORTHEASTWARD ACROSS SOUTHEASTERN LOUISIANA.

MEANWHILE...A SURFACE LOW ASSOCIATED WITH THE GREAT LAKES UPPER
TROUGH WILL DEEPEN ACROSS THE NORTHERN GREAT LAKES AND SOUTHERN
ONTARIO DURING THIS PERIOD...WITH THE SOUTHWESTWARD TRAILING COLD
FRONT LIKELY LOCATED NEAR THE OHIO RIVER ON SUNDAY MORNING.
WIDESPREAD PRECIPITATION IS EXPECTED TO DEVELOP ACROSS THE ENTIRE
AREA BY SUNDAY NIGHT...AS A SMALL VORT MAX DROPPING SOUTH-
SOUTHEASTWARD ACROSS THE UPPER MISSISSIPPI VALLEY RESULTS IN AN
AMPLIFICATION OF THE GREAT LAKES UPPER TROUGH...AND STRENGTHENING
SOUTHWESTERLY WINDS ALOFT OVER THE TENNESSEE VALLEY. WILL INCREASE
POPS INTO THE CATEGORICAL RANGE...AND CONTINUE TO INCREASE RAIN AND
EMBEDDED THUNDERSTORMS AS THE PRIMARY PRECIPITATION TYPE...AS POINT
FORECAST SOUNDINGS INDICATE MOIST ADIABATIC LAPSE RATES AND VERY
LITTLE CAPE. WIDESPREAD RAIN AND EMBEDDED THUNDERSTORMS WILL CONTINUE
FROM MONDAY INTO MONDAY NIGHT...AS ENERGY FROM THE EASTWARD DRIFTING
GREAT LAKES UPPER TROUGH BEGINS TO PHASE WITH THE VORT CENTER
ASSOCIATED WITH T.S. LEE...AND THE ENTIRE SYSTEM BECOMES BLOCKED
BETWEEN THE SOUTHWESTERN U.S. RIDGE AND A SUBTROPICAL HIGH OVER THE
CENTRAL NORTH ATLANTIC. REGARDING TEMPERATURES...HAVE FOLLOWED
CLOSELY THE MAVMOS VALUES THROUGH THE SHORT TERM PERIOD...WITH HIGHS
FALLING FROM THE MID 80S ON SUNDAY TO THE MID 70S ON MONDAY GIVEN THE
INCREASE IN CLOUDS/PRECIPITATION AS WELL AS WEAK LOW-LEVEL COLD
ADVECTION. LOW TEMPERATURES WILL DECLINE THROUGH THE PERIOD AS
WELL...WITH READINGS IN THE MID 50S/AROUND 60 POSSIBLE BY TUESDAY
MORNING AS DRIER LOW-LEVEL AIR FILTERS INTO THE REGION.

IT STILL APPEARS AS IF THE MOST SIGNIFICANT IMPACTS FROM THIS EVENT
WILL COME FROM FLOODING...BOTH IN THE FORM OF FLASH FLOODING EARLY
IN THE EVENT AND RIVER FLOODING THAT MAY PERSIST FOR THE REMAINDER OF
THE WEEK AND INTO NEXT WEEKEND. A FLASH FLOOD WATCH HAS BEEN ISSUED
FOR THE ENTIRE FORECAST AREA FROM SUNDAY AFTERNOON THROUGH TUESDAY
MORNING TO ACCOUNT FOR THIS POTENTIAL. PLEASE REFER TO THE HYDROLOGY
SECTION BELOW...AS WELL AS THE HWOHUN AND FLASH FLOOD WATCH TEXT
PRODUCTS FOR ADDITIONAL INFORMATION. IN ADDITION TO THE HAZARDS FROM
FLOODING...THERE WILL BE A THREAT FROM TORNADOES -- PARTICULARLY FROM
SUNDAY MORNING THROUGH SUNDAY EVENING -- AS THE INITIAL OUTER RAIN
BANDS FROM T.S. LEE SPREAD NORTHWARD INTO THE REGION. STRONG NORTH-
NORTHEASTERLY GRADIENT WINDS WILL EXIST AT THE TIME OF T.S. LEE'S
CLOSEST APPROACH FROM MONDAY AFTERNOON THROUGH TUESDAY AFTERNOON...
AND A WIND ADVISORY HAS BEEN ISSUED TO ADDRESS THIS CONCERN. A HIGH
WIND WARNING MAY BE NECESSARY FOR THE HIGHER TERRAIN AREAS OF
NORTHEAST ALABAMA AND SOUTHERN MIDDLE TENNESSEE DURING THIS PERIOD.

IN THE EXTENDED PORTION OF THE FORECAST...MEDIUM RANGE DATA FROM THE
GFS/ECMWF IS IN CLOSE AGREEMENT THAT THE REMAINDER OF THE WEEK WILL
BE COOL AND CLOUDY...AS THE REGION WILL REMAIN IN A NORTHWEST FLOW
PATTERN ALOFT BETWEEN A MID-LEVEL RIDGE THAT IS FORECAST TO BECOME
ESTABLISHED ACROSS THE NORTHERN ROCKIES/NORTHERN PLAINS...AND A BROAD
REMNANT LONG WAVE TROUGH WHICH IS FORECAST TO LIFT NORTHEASTWARD
ACROSS THE CENTRAL APPALACHIANS/MID-ATLANTIC REGION. BOTH MODELS ALSO
AGREE THAT THE SOUTHERN END OF THE LONG WAVE TROUGH WILL CUT OFF INTO
A LARGE UPPER LOW THAT RETROGRADES WESTWARD ACROSS THE OHIO
VALLEY/UPPER MIDWEST...AND EVENTUALLY ACROSS THE TENNESSEE VALLEY
LATE IN THE FORECAST PERIOD. WILL MAINTAIN MOSTLY CLOUDY SKIES/LIGHT
NORTH TO NORTHWEST WINDS...AND A SLIGHT CHANCE OF RAIN THROUGH THE
PERIOD...WITH HIGHS REMAINING IN THE MID/UPPER 70S AND LOWS IN THE
MID/UPPER 50S.

&&

.HYDROLOGY...
THE LATEST STORM TOTAL QPF PROJECTIONS PAINT A SWATH OF 5-7 INCHES
ACROSS A LARGE PART OF THE HUN CWFA...WITH MOST OF THE
PRECIPITATION FALLING FROM SUNDAY AFTERNOON THROUGH TUESDAY MORNING.
WHILE SOILS ARE RATHER DRY AT THIS TIME...RAINFALL OF THIS MAGNITUDE
WILL EASILY TRANSITION TO RUNOFF AND CREATE SERIOUS FLOODING
CONCERNS. THERE IS STILL SOME UNCERTAINTY ABOUT THE LOCATION OF THE
HEAVIEST RAINFALL...AS IT WILL DEPEND ON THE MOTION AND POSITION OF
THE COLD FRONT.

&&

.PRELIMINARY POINT TEMPS/POPS...
HUNTSVILLE    73  83  69  75  61 /  30  80 100 100 100
SHOALS        71  83  69  74  60 /  40  80 100 100  80
VINEMONT      72  82  69  76  62 /  40  80 100 100 100
FAYETTEVILLE  73  85  68  72  63 /  20  70 100 100 100
ALBERTVILLE   72  81  69  78  67 /  30  80 100 100 100
FORT PAYNE    73  82  70  77  70 /  20  80 100 100 100

&&

.HUN WATCHES/WARNINGS/ADVISORIES...

AL...FLASH FLOOD WATCH FROM SUNDAY AFTERNOON THROUGH TUESDAY MORNING FOR
     THE FOLLOWING ZONES: COLBERT...CULLMAN...DEKALB...FRANKLIN
     AL...JACKSON...LAUDERDALE...LAWRENCE...LIMESTONE...MADISON...
     MARSHALL...MORGAN.

     WIND ADVISORY FROM 10 AM MONDAY TO 7 PM CDT TUESDAY FOR THE
     FOLLOWING ZONES: COLBERT...CULLMAN...DEKALB...FRANKLIN AL...
     JACKSON...LAUDERDALE...LAWRENCE...LIMESTONE...MADISON...
     MARSHALL...MORGAN.

TN...FLASH FLOOD WATCH FROM SUNDAY AFTERNOON THROUGH TUESDAY MORNING FOR
     THE FOLLOWING ZONES: FRANKLIN TN...LINCOLN...MOORE.

     WIND ADVISORY FROM 10 AM MONDAY TO 7 PM CDT TUESDAY FOR THE
     FOLLOWING ZONES: FRANKLIN TN...LINCOLN...MOORE.

&&

70/DD

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
309 PM CDT SAT SEP 3 2011

ALZ001>010-016-TNZ076-096-097-041000-
LAUDERDALE-COLBERT-FRANKLIN AL-LAWRENCE-LIMESTONE-MADISON-MORGAN-
MARSHALL-JACKSON-DEKALB-CULLMAN-MOORE-LINCOLN-FRANKLIN TN-
309 PM CDT SAT SEP 3 2011

THIS HAZARDOUS WEATHER OUTLOOK IS FOR NORTH ALABAMA AND PORTIONS
OF SOUTHERN MIDDLE TENNESSEE.

.DAY ONE...TONIGHT...

THE PROBABILITY FOR WIDESPREAD HAZARDOUS WEATHER IS LOW.

ISOLATED THUNDERSTORMS WILL DEVELOP TONIGHT ACROSS THE TENNESSEE
VALLEY.

.DAYS TWO THROUGH SEVEN...SUNDAY THROUGH FRIDAY...

RAIN AND THUNDERSTORM CHANCES WILL INCREASE ACROSS THE ENTIRE
TENNESSEE VALLEY FROM SUNDAY THROUGH THE MIDDLE OF NEXT WEEK AS
ABUNDANT TROPICAL MOISTURE SPREADS NORTHWARD IN ADVANCE OF TROPICAL
STORM LEE. CONDITIONS COULD DETERIORATE ACROSS THE TENNESSEE VALLEY
AS EARLY AS SUNDAY AND HAZARDOUS WEATHER COULD PERSIST THROUGH
WEDNESDAY.

HEAVY RAINFALL WILL PROVIDE THE POTENTIAL FOR SIGNIFICANT FLOODING
AND FLASH FLOODING ACROSS THE TENNESSEE VALLEY. WIDESPREAD RAINFALL
TOTALS THROUGH TUESDAY ARE EXPECTED TO BE FIVE TO SEVEN INCHES AND
LOCALLY HIGHER AMOUNTS WILL BE POSSIBLE. A FLASH FLOOD WATCH IS IN
EFFECT FOR THE ENTIRE TENNESSEE VALLEY FROM SUNDAY AFTERNOON UNTIL
EARLY TUESDAY MORNING. PLEASE CONSULT THE FLASH FLOOD WATCH FOR
ADDITIONAL DETAILS.

AS HEAVY RAINFALL RUNS OFF INTO AREA STREAMS AND RIVERS...RIVER
FLOODING WILL BECOME A POSSIBILITY AS EARLY AS MONDAY...WITH IMPACTS
POTENTIALLY LINGERING THROUGH THE END OF THE WEEK.

AS THE OUTER BANDS OF TROPICAL STORM LEE MOVE TOWARD THE TENNESSEE
VALLEY...ISOLATED TORNADOES WILL BECOME A POSSIBILITY LATE SUNDAY
MORNING THROUGH SUNDAY EVENING.

IN ADDITION...WINDS WILL INCREASE ON MONDAY...WITH SUSTAINED WINDS
OF 20 TO 30 MPH GUSTING TO 40 MPH AT TIMES. A WIND ADVISORY IS IN
EFFECT FROM MONDAY MORNING THROUGH TUESDAY AFTERNOON. PLEASE CONSULT
THE WIND ADVISORY FOR ADDITIONAL DETAILS.

ALL RESIDENTS AND VISITORS ARE URGED TO CLOSELY MONITOR FORECASTS
AND ADVISORIES ISSUED BY THIS OFFICE AND THE NATIONAL HURRICANE
CENTER.

.SPOTTER CALL TO ACTION STATEMENT...

ACTIVATION OF STORM SPOTTERS AND EMERGENCY MANAGEMENT PERSONNEL MAY
BE POSSIBLE SUNDAY THROUGH TUESDAY.

DD/CCC/JLL