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Area Forecast Discussion

724
FXUS64 KHUN 211149 AAA
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
649 AM CDT SUN SEP 21 2014

.UPDATE...
FOR 12Z TAFS

&&

.DISCUSSION...
/ISSUED 504 AM CDT SUN SEP 21 2014/
RECENT WATER VAPOR SATELLITE DATA SUGGESTS TWO MID-LEVEL VORTICES
CURRENTLY DIGGING INTO THE BASE OF AN AMPLIFYING LONGWAVE TROUGH
ACROSS EASTERN CANADA. THE INITIAL VORT MAX -- CURRENTLY SPREADING
EASTWARD ACROSS LAKE MICHIGAN -- WILL CONTINUE TO PHASE WITH THE
UPSTREAM WAVE DROPPING SOUTHEASTWARD OVER MINNESOTA AS THE COMPOSITE
TROUGH APPROACHES THE EASTERN GREAT LAKES LATER TODAY. ACROSS THE
LOCAL REGION...A WEAK 500 MB SHEAR AXIS SHIFTING ACROSS THE REGION
THIS MORNING HAS RESULTED IN A FEW HIGH-LEVEL CLOUDS BUT LITTLE
IMPACT ON SENSIBLE WEATHER. COOLER TEMPERATURES HAVE ALLOWED PATCHY
FOG TO DEVELOP -- ESPECIALLY IN THE VALLEYS OF NE ALABAMA.

SHORT TERM NWP GUIDANCE CONTINUES TO SUGGEST THAT A STRONG AREA OF
SUBSIDENT AIR ALOFT ORIGINATING SOUTH OF THE AMPLIFYING GREAT LAKES
TROUGH WILL SPREAD INTO THE REGION THIS MORNING/EARLY AFTERNOON WITH
MID-LEVEL HEIGHT RISES QUITE UNFAVORABLE FOR THE DEVELOPMENT OF
PREFRONTAL CONVECTION. HOWEVER...STRONG LOW-LEVEL MASS CONVERGENCE
ALONG THE FRONT WILL LIKELY BE SUFFICIENT TO GENERATE SCATTERED
SHOWERS/THUNDERSTORMS WHICH WILL PRIMARILY IMPACT THE NORTHWEST HALF
OF THE FORECAST AREA BETWEEN 21/21Z-22/00Z. ENVIRONMENTAL CONDITIONS
WILL BECOME MORE FAVORABLE FOR THE DEVELOPMENT OF CONVECTION VERY
LATE THIS AFTERNOON/EARLY EVENING AS HEIGHTS GRADUALLY FALL WITH THE
APPROACH OF THE LONGWAVE TROUGH AXIS FROM THE NORTH. THIS WILL
FAVORABLY OCCUR WITH TIMING OF FRONTAL PASSAGE TO LEAD TO AN INCREASE
IN THE AREAL COVERAGE OF CONVECTION AS THE FRONT PUSHES SOUTHWARD.
THE THREAT FOR PRECIPITATION WILL END FOR ALL ZONES BY 06Z AS THE
COLD FRONT SHOULD COMPLETELY CLEAR THE REGION BY THIS TIME. MODELS
ARE UNANIMOUS IN FORECASTING 20-30 KTS OF DEEP-LAYER SHEAR AND SB
CAPE VALUES OF 1000-1500 J/KG WHICH MAY YIELD A COUPLE OF STRONGER
MULTICELL CLUSTERS CAPABLE OF PRODUCING WIND GUSTS IN THE 35-45 MPH
RANGE.

A VERY PLEASANT EARLY AUTUMN AIRMASS WILL BE FIRMLY IN PLACE ACROSS
THE REGION BY SUNRISE MONDAY...WITH DEWPOINTS IN THE LOWER 50S
FALLING INTO THE MID/UPPER 40S DURING THE PEAK OF EARLY AFTERNOON
BOUNDARY LAYER MIXING. DESPITE ABUNDANT INSOLATION...TEMPERATURES
WILL STRUGGLE TO REACH THE MID/UPPER 70S WITH A GUSTY NORTH-NORTHEAST
WIND OF 10-20 MPH. IT STILL APPEARS AS IF THE SURFACE PRESSURE
GRADIENT ACROSS THE REGION WILL BE TOO STEEP TO ALLOW FOR FULL
DECOUPLING MONDAY NIGHT...BUT TEMPERATURES WILL BE QUITE COOL
NONETHELESS. ALTHOUGH THERE IS SOME DISAGREEMENT AMONG THE
GFS/ECMWF...IT SEEMS REASONABLE TO BELIEVE THAT THE BASE OF THE
PREVIOUS LONGWAVE MAY CUTOFF INTO A MID/UPPER LOW OVER GEORGIA AND
THE CAROLINAS PROVIDING DEEP NORTHERLY FLOW THROUGH TUESDAY.
HOWEVER...LOW-LEVEL FLOW WILL VEER TO THE EAST LIMITING COLD
ADVECTION AND ALLOWING TEMPERATURES TO WARM A FEW DEGREES INTO THE
LOWER 80S.

A COMPLEX MID/UPPER-LEVEL FLOW PATTERN WILL EVOLVE ACROSS THE EASTERN
CONUS DURING THE EXTENDED PERIOD DOWNSTREAM FROM A SHARP TROUGH
EXPECTED TO SPREAD INLAND ALONG THE PACIFIC COAST BY THE END OF THE
WEEK. IT APPEARS THAT THE PREVIOUSLY MENTIONED CUTOFF LOW WILL LIKELY
RETROGRADE ACROSS THE REGION AND WEAKEN DURING THE MIDDLE OF THE
WEEK...WITH AN INCREASING EAST-SOUTHEAST LOW-LEVEL FLOW ALLOWING
MOISTURE TO SLOWLY CREEP BACK INTO THE AREA BY THURSDAY AND FRIDAY.
BOTH HIGHS/LOWS WILL RESPOND BY INCREASING A DEGREE OR SO EACH
PERIOD. THE WARMING TREND MAY BE HALTED EARLY NEXT WEEKEND...AS A
DISTURBANCE ORIGINATING IN THE GULF LIFTS NORTHWARD PROVIDING
ENHANCED CLOUD COVER AND AT LEAST A CHANCE OF SOME MUCH NEEDED
RAINFALL.

70/DD

&&

.AVIATION...
FOR 12Z TAFS...COLD FRONT CURRENTLY OVER SOUTHERN ILLINOIS WILL BE
THE MAIN CONCERN THRU 12Z/22. WEST WINDS WILL BECOME GUSTY BY MID-
MORNING AHEAD OF THE FRONT...AND GUSTS WILL PERSIST THROUGH MUCH OF
THE PERIOD. VFR CEILINGS WILL DEVELOP BY EARLY AFTN. GENERALLY
BELIEVE THIS WILL BE THE MAIN TIMING FOR SHRA/TSRA AS WELL BUT AM NOT
CONFIDENT IN DIRECT IMPACTS TO EITHER TERMINAL. ADDED VCTS AFTER 19Z
AS A COMPROMISE. FRONT SHOULD ARRIVE AFTER 00Z WITH A WIND SHIFT TO
THE NW AND END TO SHRA/TSRA...WITH WINDS BECOMING MORE NORTHERLY
06-07Z AND GRADUAL CLEARING THEREAFTER.

BCC

&&

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

$$

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



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