Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 281145
AFDHUN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
545 AM CST SUN DEC 28 2014

.DISCUSSION...
WATER VAPOR AND EVENING RAOB DATA CONFIRM THE PRESENCE OF A STRONG
BELT OF SOUTHWESTERLY FLOW IN PLACE ACROSS MUCH OF THE CONUS THIS
MORNING -- BETWEEN A SUBTROPICAL RIDGE CENTERED NEAR THE BAHAMAS AND
A COLD UPPER LOW WHICH IS FORECAST TO SPREAD EASTWARD ACROSS NORTHERN
ONTARIO/JAMES BAY AND INTO QUEBEC BY MONDAY MORNING. AT THE
SURFACE... A RATHER STRONG COLD FRONT CONTINUES TO SLOWLY APPROACH
THE FORECAST AREA FROM THE NORTHWEST -- WITH DEEP ATMOSPHERIC LIFT TO
THE NORTHWEST OF A SURFACE WAVE LEADING TO A FAIRLY WIDESPREAD
RAINFALL EVENT LAST NIGHT. SHORT TERM NWP GUIDANCE INDICATES THAT
THE SURFACE WAVE WILL TRACK EASTWARD INTO CENTRAL ALABAMA TODAY AND
WEAKEN...WITH THE AREA OF WIDESPREAD RAINFALL EXPECTED TO DISSIPATE
AND LIFT NORTHEASTWARD BY LATE MORNING. HOWEVER...A SHORTER
WAVELENGTH MID-LEVEL TROUGH EMBEDDED WITHIN THE BROADER SOUTHWEST
FLOW ALOFT IS CURRENTLY MOVING ACROSS THE RIO GRANDE VALLEY. THIS
FEATURE IS PROGGED TO WEAKEN AS IT ENTERS A REGION OF STRONG MID-
LEVEL CONFLUENCE...BUT ITS REMNANTS WILL LIKELY SHIFT DIRECTLY ACROSS
THE REGION THIS EVENING. INSPECTION OF VORTICITY FIELDS AND
ISENTROPIC PROGS SUGGESTS THAT UPWARD VERTICAL MOTIONS WILL BE QUITE
WEAK WITH THIS SYSTEM...BUT INCREASINGLY FAVORABLE FLOW IN THE
850-700 MB LAYER WILL RESULT IN ELEVATED WARM ADVECTION ATOP THE COOL
SURFACE AIRMASS YIELDING ANOTHER ROUND OF LIGHT RAIN -- BEGINNING IN
NW ALABAMA LATE THIS AFTERNOON AND SPREADING EASTWARD THROUGH THE
EVENING. CLOUDS AND AT LEAST A SLIGHT CHANCE OF LIGHT RAIN WILL
PERSIST THROUGH THE DAY ON MONDAY...AS ANOTHER VERY WEAK MID-LEVEL
SHEAR AXIS TRANSLATES ACROSS THE REGION IN SOUTHWEST FLOW ALOFT.
REGARDING THE FLASH FLOOD WATCH...WE HAVE DROPPED THE NW
ALABAMA/SOUTHERN TENNESSEE COUNTIES AS THIS REGION SHOULD EXPERIENCE
MUCH LIGHTER PRECIP AMOUNTS THAN OTHER LOCATIONS.

MEANWHILE...ALL GLOBAL MODELS REMAIN CONSISTENT IN FORECASTING A
SHORTWAVE TROUGH TO AMPLIFY OVER THE PACIFIC NORTHWEST LATER
TONIGHT AS A HIGH AMPLITUDE RIDGE BUILDS NORTHWARD ALONG THE COAST OF
BRITISH COLUMBIA. THE RESULTANT LONGWAVE TROUGH WILL ASSUME A NEARLY
ZONAL ORIENTATION...WHICH IS A HIGHLY FAVORABLE PATTERN FOR THE
SOUTHWARD MOVEMENT OF ARCTIC AIR INTO THE CENTRAL/EASTERN CONUS.
INDEED...MOST MODELS AGREE THAT A 1055+ MB SURFACE HIGH BUILDING INTO
THE NORTHERN PLAINS LATER TODAY/TONIGHT WILL CONTINUE TO SURGE
SOUTHWARD/EASTWARD ON MONDAY -- REACHING THE LOCAL AREA SOMETIME
EARLY TUESDAY MORNING. ANTECEDENT NORTHERLY LOW-LEVEL FLOW IN THE
WAKE OF TODAY`S COLD FRONT WILL MASK THE ARRIVAL OF THE ARCTIC
AIRMASS TO SOME EXTENT...BUT NOTICEABLE CHANGES WILL OCCUR IN OTHER
SENSIBLE WEATHER ELEMENTS AS DEWPOINTS DROP WELL INTO THE 20S BY
TUESDAY NIGHT. LOWS IN THE LOWER/MID 20S ON WEDNESDAY/THURSDAY
MORNING WILL MARK THE COLDEST TEMPS SINCE MID-NOVEMBER...WITH MORNING
WIND CHILL VALUES IN THE 15-20 RANGE FOR MOST LOCATIONS AND IN THE
10-15 RANGE IN ELEVATED TERRAIN.

FORTUNATELY...MEDIUM RANGE GUIDANCE FOR THE BEGINNING OF THE
EXTENDED PERIOD INDICATES A NEARLY ZONAL FLOW ALOFT AFTER THE ARCTIC
AIRMASS ARRIVES WITH LITTLE INITIAL THREAT FOR PRECIPITATION.
HOWEVER...A LARGE UPPER LOW IS PROGGED TO CUTOFF FROM THE NORTH
AMERICAN LONGWAVE TROUGH AND DRIFT SOUTHWARD ALONG THE PACIFIC COAST
EARLY THIS WEEK BEFORE GRADUALLY OPENING INTO A WAVE AS IT EJECTS
NORTHEASTWARD ACROSS THE CENTRAL PLAINS BY MID/LATE WEEK. THIS WILL
INDUCE A SOUTHWEST FLOW ALOFT ONCE AGAIN BY THURSDAY NIGHT...WITH THE
POTENTIAL FOR SEVERAL SMALL MID-LEVEL WAVES TO ENHANCE CHANCES FOR
PRECIPITATION. FORECAST SOUNDINGS FROM THE GFS/ECMWF ARE IN EXCELLENT
AGREEMENT THAT IF PRECIPITATION DOES INDEED DEVELOP BETWEEN 6-12Z
FRIDAY IT WILL LIKELY BEGIN AS SLEET...BEFORE TRANSITIONING TO A COLD
RAIN LATER FRIDAY MORNING WHEN THE ENTIRE THERMAL PROFILE WARMS
ABOVE FREEZING. WE HAVE ADDED SLEET TO THE OFFICIAL FORECAST/HWO...
AND THIS IS CERTAINLY AN EVENT THAT WILL NEED TO BE MONITORED CLOSELY
OVER THE COMING DAYS. WIDESPREAD RAIN WILL LIKELY PERSIST
FRIDAY...BUT AS THE PARENT MID- LEVEL WAVE BECOMES NEGATIVELY TILTED
FRIDAY NIGHT SUFFICIENT INSTABILITY WILL EXIST FOR SHOWERS.
PROBABILITIES FOR THUNDERSTORMS WILL INCREASE ON SATURDAY...AS AN
UNSTABLE AIRMASS /DEWPOINTS IN THE LOWER 60S/ IS ADVECTED NORTHWARD
IN ADVANCE OF THIS DYNAMIC WEATHER SYSTEM. THIS EVENT WILL ALSO NEED
TO BE MONITORED CLOSELY FOR THE POTENTIAL FOR SEVERE THUNDERSTORMS...
AS A SSWLY LOW-LEVEL JET OF 50-60 KNOTS WILL LIKELY EXIST IN THE
WARM SECTOR.

70/DD

&&

.AVIATION...
FOR 12Z TAFS...FLIGHT RESTRICTIONS ARE GENERALLY XPCTED FOR MUCH OF
THE FORECAST PERIOD...AS -RA/SHRA...AHEAD OF A COLD FRONT TO THE
W...CONTINUE TO MOVE NEWD ACROSS THE AREA. CONDS MAY ALTERNATE
BETWEEN IFR/MVFR THIS MORNING...IN/NEAR AREAS OF MODERATE RA/SHRA.
CIG HEIGHTS MAY THEN BECOME MORE STAGNANT AROUND 1K FT LATER THIS
MORNING...WITH -RA/SHRA CONTINUING. PRECIP MAY BEGIN TO TAPER OFF
FROM THE W NEAR 06Z SUN...AS THE COLD FRONT MOVES EWD ACROSS THE
AREA...WITH PREDOM MVFR VIS AND CIGS JUST UNDER A 1K FT.

09

&&

.PRELIMINARY POINT TEMPS/POPS...
HUNTSVILLE    51  41  52  37 / 100  90  20  20
SHOALS        50  38  49  36 / 100  90  20  20
VINEMONT      54  40  52  37 / 100  90  20  20
FAYETTEVILLE  50  39  48  34 / 100  90  20  20
ALBERTVILLE   53  44  52  40 / 100  90  20  20
FORT PAYNE    56  44  55  39 / 100  90  20  20

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...FLASH FLOOD WATCH THROUGH THIS EVENING FOR ALZ004>010-016.

TN...NONE.
&&

$$

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


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