Debris Flow and Flooding in Alabama, November 27 to 30, 2011

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
556 AM CST SUN NOV 27 2011

.UPDATE...FOR 12Z AVIATION.

&&

.AVIATION...
WIDESPREAD SHRA AHEAD OF AN APPROACHING CDFNT ARE AFFECTING NORTHERN
AL TERMINALS THIS MORNING. CURRENTLY...A BAND OF LGT/MDT SHRA WITH A
FEW AREAS OF EMBEDDED +SHRA IS ALIGNED ALONG AND JUST EAST OF KHSV.
HOWEVER...FURTHER DEVELOPMENT TO THE SW MAY AFFECT KHSV WITH +SHRA
THRU ABOUT 15Z. AFTERWARDS...THE BAND IS LIKELY TO PROPAGATE EWRD AT
THE SAME TIME THAT LLWS DECREASES. MVFR CIGS ARE LIKELY THRU AT LEAST
21Z AT KHSV. A BRIEF PERIOD OF VFR CONDITIONS IS POSSIBLE BTWN 21-00Z
AS A MOISTURE MINIMUM MOVES ACROSS THE AREA...BUT CONFIDENCE IS STILL
LOW ATTM.

MEANWHILE...MAINLY -SHRA/-RA ARE EXPECTED AT KMSL FOR MOST OF THE
PERIOD WHICH IS LOCATED WITHIN A RELATIVE MOISTURE MINIMUM. A BAND OF
LGT/MDT RAIN CURRENTLY TO THE WEST STRETCHING SW-NE JUST EAST OF THE
MISS RIVER WILL MOVE CLOSER TO NW AL...BUT MAY WEAKEN AS OVERALL
FORCING FOR ASCENT WEAKENS WITH THIS BAND. NEVERTHELESS...LOW-LVL
MOISTURE WILL LIKELY INCREASE SUFFICIENTLY TO BRING LOW-END MVFR CIGS
BACK TO KMSL BY ABOUT 20Z.

&&

.DISCUSSION... /ISSUED 447 AM CST SUN NOV 27 2011/
A HIGHLY AMPLIFIED MID/UPPER-LEVEL FLOW PATTERN IS CURRENTLY IN
PLACE ACROSS NORTH AMERICA...WITH A SHARP LONG WAVE TROUGH EXTENDING
FROM AN ANCHORING UPPER LOW OVER SOUTHERN ONTARIO SOUTHWARD TO THE
LOWER RIO GRANDE VALLEY. THIS LONG WAVE TROUGH IS BEING EFFECTIVELY
BLOCKED BY TWO SEPARATE MID-LEVEL HIGHS...ONE LOCATED OFF THE COAST
THE SOUTHEASTERN U.S. AND THE OTHER POSITIONED OFF THE COAST OF
SOUTHERN CALIFORNIA. THE RESULTANT STRONG SOUTHWESTERLY MID-
TROPOSPHERIC FLOW HAS RESULTED IN ABUNDANT MOISTURE FROM THE
EQUATORIAL REGION OF THE PACIFIC BEING TRANSPORTED NORTHEASTWARD
ACROSS THE TENNESSEE VALLEY...AND SUPPORTING PWAT VALUES IN THE
1.25-1.5 INCH RANGE. STRONG LOW-LEVEL WARM/MOIST ADVECTION AND
INCREASING LARGE SCALE ASCENT HAVE ALLOWED SEVERAL WAVES OF
LIGHT/MODERATE RAIN TO SPREAD NORTHWARD ACROSS THE FORECAST AREA THIS
MORNING...WITH THIS TREND EXPECTED TO CONTINUE THROUGH THE DAY...AS A
STRONG SURFACE COLD FRONT ASSOCIATED WITH THE MID/UPPER- LEVEL TROUGH
PUSHES SLOWLY EASTWARD THROUGH THE DAY. WILL MAINTAIN INHERITED
CATEGORICAL POPS FOR THE ENTIRE REGION...WITH TOTAL RAINFALL AMOUNT
RAGING FROM AROUND 0.5 INCHES IN NORTHWEST ALABAMA TO BETWEEN 1-1.5
INCHES ACROSS THE NORTHEASTERN ZONES. TEMPERATURES WILL BE A BIT MORE
COMPLICATED... WITH THE COLD FRONT EXPECTED TO BE PUSHING INTO
NORTHWEST ALABAMA AROUND 12Z AND EXITING INTO NORTHWEST GEORGIA
AROUND 28/00Z.

OTHERWISE...GLOBAL MODELS CONTINUE TO SHOW EXCELLENT AGREEMENT IN
FORECASTING THE NORTHERN END OF THE LONG WAVE TROUGH TO GRADUALLY
DEEPEN AND SHIFT NORTHEASTWARD ACROSS THE SOUTHERN PORTION OF HUDSON
BAY LATER TODAY...AS THE SOUTHERN END OF THE TROUGH BEGINS TO DEEPEN
INTO A STRONG CUTOFF LOW SOMEWHERE ACROSS MISSOURI OR ARKANSAS. THIS
WILL LEAD TO A CONTINUATION OF WIDESPREAD LIGHT/MODERATE RAINFALL
EVEN IN THE POST FRONTAL AIR MASS...AS LARGE SCALE FORCING FOR UPWARD
VERTICAL MOTION AND ELEVATED WARM ADVECTION CONTINUE TO INCREASE. IT
STILL APPEARS AS IF LIFT WILL BE MAXIMIZED DURING THE PERIOD FROM
LATE MONDAY AFTERNOON THROUGH TUESDAY MORNING...AS ALL GUIDANCE
SUGGESTS THAT THE CUTOFF UPPER LOW WILL CONTINUE TO GRADUALLY DEEPEN
AND LIFT NORTHEASTWARD FROM THE LOWER MISSISSIPPI VALLEY REGION...
ACROSS NORTHERN ALABAMA AND INTO THE CENTRAL APPALACHIANS BY TUESDAY
EVENING...AS ANOTHER MID-LEVEL WAVE DROPS SOUTHEASTWARD ACROSS THE
NORTHERN ROCKIES AND PLAINS. OVERALL...EXPECT PRECIPITATION CHANCES
TO REMAIN VERY HIGH DURING THE DAY ON MONDAY...BUT THE P-TYPE SHOULD
BECOME MORE CONVECTIVE IN NATURE AS THE STRONG FORCING FROM THE UPPER
LOW BEGINS TO IMPINGE ON THE REGION. THUS...HAVE INDICATED SHOWERS ON
MONDAY...AS OPPOSED TO THE STRATIFORM RAINFALL EXPECTED TODAY AND
TONIGHT. MOST LOCATIONS WILL LIKELY EXPERIENCE BETWEEN 2-3 INCHES OF
RAINFALL THROUGH THE SHORT TERM PERIOD...OVER A PROLONGED PERIOD OF
TIME WE DO NOT ANTICIPATE THIS TO GENERATE MAJOR FLOODING CONCERNS.

STRONG VERTICAL MOTION...COUPLED WITH THE PREEXISTING COLD POCKET OF
AIR ALOFT AND STRONG LOW-LEVEL COLD ADVECTION ON THE SOUTHWESTERN
SIDE OF A DEEPENING SURFACE LOW SHIFTING NORTHWARD ACROSS EASTERN
TENNESSEE/KENTUCKY WILL SET THE STAGE FOR A TRANSITION TO RAIN/SNOW
MIX BY MONDAY EVENING...WITH FORECAST SOUNDINGS INDICATING FAVORABLE
CONDITIONS FOR PURE SNOW ON MONDAY NIGHT/TUESDAY MORNING.
PRECIPITATION SHOULD THEN TRANSITION BACK TO A RAIN/SNOW MIXTURE LATE
ON TUESDAY MORNING...BEFORE PROFILES WARM TO THE POINT OF SUPPORTING
A COLD RAIN ON TUESDAY AFTERNOON WITH ANY LINGERING PRECIPITATION AS
THE LOW LIFTS NORTHEASTWARD AND AWAY FROM THE LOCAL AREA. DUE TO
RECENT WARM TEMPERATURES...WE ARE STILL ONLY ANTICIPATING A LIGHT
DUSTING OF SNOW...WITH THE MOST SUBSTANTIAL ACCUMULATIONS LIKELY TO
OCCUR ON GRASSY SURFACE IN HIGHER TERRAIN. HOWEVER...THE POTENTIAL
DOES EXIST FOR ELEVATED INSTABILITY TO GENERATE SOME CONVECTIVE
SNOWFALL...AND THIS MAY LEAD TO LOCALLY HIGHER SNOWFALL RATES...
ENHANCING THE CHANCES FOR LOW-END ACCUMULATIONS. THIS PORTION OF THE
FORECAST WILL HAVE TO BE REFINED AS WE DRAW CLOSER TO THE ACTUAL
EVENT.

NO MAJOR CHANGES WILL BE MADE TO THE EXTENDED PORTION OF THE
FORECAST...WITH DRY CONDITIONS EXPECTED TO PREVAIL AS THE FLOW
ALOFT BECOMES MORE ZONAL IN THE WAKE OF A DEPARTING TROUGH AXIS
ACROSS THE SOUTHEASTERN U.S. AND ANOTHER STRONG UPPER LOW FORECAST TO
CUTOFF ACROSS THE SOUTHWESTERN UNITED STATES. COLD TEMPERATURES WILL
REMAIN ACROSS THE REGION THROUGH THE END OF THE WEEK...WITH RETURN
FLOW NOT DEVELOPING UNTIL NEXT WEEKEND AT THE EARLIEST.

&&

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

AVIATION...KDW
DISCUSSION...70/DD

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE HUNTSVILLE AL
444 AM CST SUN NOV 27 2011

ALZ001>010-016-TNZ076-096-097-271900-
LAUDERDALE-COLBERT-FRANKLIN AL-LAWRENCE-LIMESTONE-MADISON-MORGAN-
MARSHALL-JACKSON-DEKALB-CULLMAN-MOORE-LINCOLN-FRANKLIN TN-
444 AM CST SUN NOV 27 2011

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

.DAY ONE...TODAY AND TONIGHT...

LIGHT TO MODERATE RAIN WITH AN OCCASIONAL EMBEDDED HEAVY SHOWER WILL
CONTINUE ACROSS THE AREA TODAY. TOTAL RAINFALL AMOUNTS AROUND ONE TO
THREE INCHES ARE STILL ANTICIPATED...WITH THE HIGHEST AMOUNTS LIKELY
OCCURRING IN NORTHEASTERN ALABAMA AND SOUTHERN MIDDLE TENNESSEE.
WHILE WIDESPREAD FLOODING IS NOT EXPECTED...THE RISK OF FLOODING WILL
INCREASE IF HEAVY SHOWERS TRACK REPEATEDLY OVER THE SAME LOCATIONS.
URBAN AREAS WOULD BE THE MOST SUSCEPTIBLE TO MINOR FLOODING DUE TO
RUNOFF FROM PAVED SURFACES...AND NARROW VALLEYS IN THE EAST SUCH AS
THE PAINT ROCK AND DEKALB VALLEYS WHERE ANY RUNOFF WOULD BE MORE
CONFINED.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY...

PERIODS OF LIGHT RAIN WILL CONTINUE THROUGH MONDAY...BUT COLDER AIR
WILL BEGIN TO FILTER INTO THE REGION ON MONDAY NIGHT. LIGHT RAIN
SHOWERS ARE EXPECTED TO MIX WITH AND THEN CHANGE OVER TO LIGHT SNOW
SHOWERS GRADUALLY ON MONDAY NIGHT. RELATIVELY WET AND WARM GROUND
CONDITIONS WILL HELP TO MELT FALLING SNOW...BUT SNOW SHOWERS MAY BE
HEAVY ENOUGH TO AMOUNT IN BRIEF MINOR ACCUMULATIONS MAINLY ON GRASSY
SURFACES. ANY MINOR ACCUMULATIONS WILL BE MOST LIKELY IN THE HIGHER
ELEVATIONS. DUE TO THE LIKELIHOOD THAT SNOW WILL NOT BE ABLE TO
ACCUMULATE ON ROAD SURFACES...TRAVEL PROBLEMS ARE NOT EXPECTED AT
THIS TIME. THE PRECIPITATION WILL LIKELY CHANGE OVER TO LIGHT RAIN
BEFORE ENDING ON TUESDAY.

.SPOTTER CALL TO ACTION STATEMENT...

ACTIVATION OF STORM SPOTTERS AND EMERGENCY MANAGEMENT PERSONNEL
IS NOT ANTICIPATED AT THIS TIME.

KDW