Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 221944
AFDHUN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
144 PM CST MON DEC 22 2014

.DISCUSSION...
A RATHER INTERESTING AND CHALLENGING FORECAST FOR THE TENNESSEE
VALLEY OVER THE NEXT SEVERAL DAYS. LATEST SURFACE ANALYSIS SHOWS AN
ELONGATED LOW ACROSS THE NORTHERN/CENTRAL PLAINS WITH A TRAILING
COLD FRONT EXTENDING THROUGH THE KC METRO AREA THROUGH THE ARKLATEX
INTO THE TEXAS HILL COUNTRY. CLOSER TO HOME, LOW CLOUDS CONTINUE TO
PREVAIL ACROSS THE TENNESSEE VALLEY WITH TEMPERATURES RANGING FOR THE
UPPER 40S TO LOWER 50S. THE LOW LEVEL FLOW REMAINS EAST/SOUTHEAST AND
IS BEING SUBSTANTIALLY INFLUENCED BY A LARGE AREA OF HIGH PRESSURE
SITUATED FROM THE CANADIAN MARITIMES THROUGH THE MID ATLANTIC REGION.
THIS "WEDGE" OF COLD AIR WILL PLAY A SIGNIFICANT FACTOR IN OUR
SENSIBLE WEATHER OVER THE NEXT SEVERAL DAYS.

EARLY THIS AFTERNOON, REGIONAL RADARS SHOWED A BROAD AREA OF SHOWERS
EXTENDING FROM LOWER ALABAMA NORTHWARD THROUGH THE TENNESSEE VALLEY.
THIS SHOWER ACTIVITY HAS EXPANDED QUICKLY IN SPATIAL EXTENT OVER THE
LAST SEVERAL HOURS. THE LATEST HIGH RESOLUTION MODELS (HRRR/HIGH RES
WINDOWS) CONTINUE TO SHOW THE AFOREMENTIONED LOW LEVEL INVERSION
(MODIFIED WEDGE) WILL BE HARD TO ERODE DESPITE A STRENGTHENING LOW
LEVEL JET AND ASSOCIATED WARM AIR ADVECTION. IN FACT...THE MODELS
TEND TO ERODE THE APPALACHIAN WEDGE TOO RAPIDLY IN THIS TYPE OF SETUP
SO WILL TREND TOWARD THE MORE PESSIMISTIC TIMING. THIS WILL RESULT IN
PRIMARILY ELEVATED INSTABILITY ACROSS NORTH ALABAMA THIS EVENING AND
OVERNIGHT WITH SOMEWHAT FAVORABLE LAPSE RATES IN THE 900-700 MB LAYER
TO SUPPORT CONVECTIVE UPDRAFTS. DESPITE AN INCREASING WIND FIELD AND
SOMEWHAT FAVORABLE KINEMATICS THE PRIMARY THREAT WITH ANY STORMS
TONIGHT WOULD BE SMALL HAIL. HAVE ALSO INCREASED QPF AMOUNTS SLIGHTLY
FROM PREVIOUS PACKAGE PER WPC GUIDANCE AND MODEL TRENDS.

MOISTURE DEEPENS SIGNIFICANTLY AFTER 12Z TUESDAY WITH PROFILES
BECOMING MOIST ADIABATIC TO NEAR 500MB. THE 12Z NAM TRIES TO SLOWLY
ERODE THE WEDGE WITH A NARROW RIBBON OF BOUNDARY LAYER INSTABILTY
NOSING MAINLY INTO OUR SOUTHERN COUNTIES. THIS MAY BE A BIT GENEROUS,
BUT CANNOT RULE OUT SOME SURFACE (OR NEAR SURFACE BASED STORMS)
MAKING THEIR WAY NORTH TOWARD OUR REGION. LOW LEVEL SHEAR/BULK SHEAR
PARAMETERS ARE QUITE ROBUST PER THE THE STRONGLY DEEPENING
SURFACE-H7 LOW SO ANY STORMS THAT DEVELOP WILL CERTAINLY HAVE TO BE
MONITORED FOR THE POTENTIAL TO AT LEAST ISOLATED WIND GUST POTENTIAL.
HOWEVER, THE PRIMARY CONCERN GIVEN THE SLOW MOVING AND ROBUST NATURE
OF THIS SYSTEM WILL BE THE RAINFALL. THE LATEST TRENDS FROM THE SHORT
RANGE MODELS POINT TO STORM TOTAL PRECIP AMOUNTS FROM 1.5 TO LOCALLY
2.5 INCHES ACROSS THE TENNESSEE VALLEY REGION.

SHOWERS AND EMBEDDED STORMS WILL PERSIST ACROSS THE REGION TUESDAY
NIGHT AS THE SURFACE FRONT APPROACHES SLOWLY FROM THE WEST. THERE ARE
SOME DISCREPANCIES BETWEEN THE MODELS IN TERMS OF THE FRONTAL
PASSAGE TIMING. IF THE SLOWER OPERATIONAL NAM VERIFIES, WE COULD SEE
ANOTHER WINDOW DURING THE DAY WEDNESDAY FOR CONVECTIVE DEVELOPMENT IN
OUR EASTERN COUNTIES. FOR NOW, TRENDED SLIGHTLY TOWARD THAT
DIRECTION AND SHOWED A TEMPORAL PROGRESSION (POPS AND TEMPS) FROM
WEST TO EAST DURING THE DAY. THE COMBINATION OF THE DRY SLOT AND
STRONG COLD AIR ADVECTION WEDNESDAY AFTERNOON SHOULD LEAD TO FAIRLY
GUSTY WESTERLY WINDS BEHIND THE FRONT. HAVE ADJUSTED WINDS UP
SLIGHTLY FOR THAT TIME PERIOD. THE UPPER TROUGH AXIS AND ASSOCIATED
VORT MAX WILL MOVE ACROSS WEDNESDAY NIGHT. WOULD NOT BE SHOCKED TO
SEE A FEW FLURRIES ACROSS THE FAR NORTHERN AREAS AS THE COLD AIR
FINALLY WORKS ITS WAY IN, BUT OVERALL TEMP/MOISTURE PROFILES ARE
MARGINAL AND WILL LEAVE OUT FOR NOW.

IN THE EXTENDED RANGES...THE HIGHLY AMPLIFIED FLOW PATTERN WILL
CONTINUE ACROSS THE CONUS THROUGH THE HOLIDAY WEEK. IN THE WAKE OF
THE DEEPENING SURFACE LOW AND ASSOCIATED COLD FRONT, A DRY AND
RATHER PLEASANT (ALBEIT COOL) CHRISTMAS DAY IS IN STORE FOR THE
TENNESSEE VALLEY AS TRANSIENT U/L RIDGING IMPACTS THE REGION. THE
LATTER PORTION OF THE WEEK WILL SEE A MODEST WARMING TREND AS
SOUTHERLY LOW LEVEL FLOW BECOMES ESTABLISHED AROUND THE BACKSIDE OF
THE DEPARTING SURFACE HIGH.

THIS CLEARING TREND WILL UNFORTUNATELY BE SHORT LIVED HOWEVER AS THE
NEXT UPPER LEVEL SYSTEM TO IMPACT OUR AREA WILL ALREADY BE TAKING
SHAPE ACROSS THE INTERMTN REGION ON FRIDAY. THIS WILL INDUCE
CYCLOGENESIS ACROSS THE CENTRAL PLAINS WITH A COLD FRONT APPROACHING
THE TENNESSEE VALLEY LATE SATURDAY/EARLY SUNDAY. THERE ARE SOME
FAIRLY SIGNIFICANT DISCREPANCIES WITH THE STRENGTH/TIMING (AND
POTENTIAL PHASING) OF THIS PARTICULAR SYSTEM AMONGST THE MID RANGE
SUITE OF GUIDANCE. FOR THE TIME BEING, HAVE TRENDED TOWARD A BLEND OF
THE ECMWF/GFS AND INCLUDED LOW CHANCE POPS FOR THE SATURDAY THROUGH
MONDAY TIME FRAME WHILE AWAITING LATER MODEL TRENDS/CONFIRMATION.

15

&&

.AVIATION...
/ISSUED 1052 AM CST MON DEC 22 2014/
FOR 18Z TAFS... IFR CONDITIONS ARE EXPECTED FOR THE ENTIRE PERIOD.
LOW CLOUDS WILL PRODUCE BR AND SPORADIC DZ. SCT SHRA WILL ALSO OCCUR
THIS AFTERNOON INTO TONIGHT AS AN UPPER LEVEL SYSTEM APPROACHES.
ATTM, BRIEF MODERATE RAIN IS EXPECTED WHICH COULD LOWER VSBYS
TEMPORARILY, BUT PROBABLY NOT FOR A LONG ENOUGH PERIOD TO INCLUDE IN
THE TAFS.

AK

&&

.PRELIMINARY POINT TEMPS/POPS...
HUNTSVILLE    52  64  51  57 /  70  90  90  50
SHOALS        52  63  47  51 /  70  90  90  50
VINEMONT      52  64  51  56 /  70  90  90  50
FAYETTEVILLE  50  62  51  58 /  70  90  90  60
ALBERTVILLE   51  63  52  58 /  70  90  90  60
FORT PAYNE    49  62  53  59 /  60  90  90  60

&&

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

$$

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


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