Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 032354
AFDHUN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
554 PM CST TUE MAR 3 2015

.UPDATE...
FOR 00Z TAFS

&&

.DISCUSSION...
/ISSUED 521 PM CST TUE MAR 3 2015/
HAD A REPORT OF SOME LOCALIZED DENSE FOG OVER CULLMAN COUNTY...SO
WENT AHEAD AND ADDED AREAS OF FOG IN THE ZONES/FCST. ALSO WILL GO
WITH ISOLATED THUNDER WORDING...DUE TO FAIRLY STABLE CONDITIONS
ACROSS THE CWA TONIGHT.

07

.PREV DISCUSSION...
/ISSUED 350 PM CST TUE MAR 3 2015/

A VERY WARM AND HUMID AIRMASS IS IN PLACE ACROSS THE TN VALLEY AS A
WARM FRONT MOVES NORTH/NE FROM CENTRAL AL/GA. WIDESPREAD SHOWERS AND
POSSIBLY THUNDERSTORMS ARE EXPECTED TONIGHT AS ELEVATED INSTABILITY
AND WIND SHEAR INCREASES. THIS WILL OCCUR AHEAD OF THE WIDELY
ANTICIPATED MERGING OF THE SUBTROPICAL AND POLAR JETS TAKING PLACE
AS TWO MID-LEVEL SHORTWAVE TROUGHS (ONE IN SUBTROPICAL JET AND ONE IN
POLAR JET) ACROSS THE PLAINS AND UPPER MIDWEST TONIGHT INTO TOMORROW.
AS THIS TRANSITION OCCURS, A HIGH SPEED UPPER JET STREAK IS FORECAST
TO MOVE JUST TO THE NORTH OF THE TN VALLEY INDUCING A STRONG LLJ
TONIGHT. THIS WILL ACT TO STEEPEN LOW-LEVEL LAPSE RATES LIKELY JUST
ENOUGH TO CAUSE SOME STRONGER UPDRAFTS WITHIN ISOLATED TO SCATTERED
THUNDERSTORMS FROM SOUTH TO NORTH ALONG THE WARM FRONT. A LOOK AT
INSTABILITY PARAMETERS INDICATES 0-3 KM MUCAPES BTWN 300-500 J/KG AS
THE WARM FRONT PASSES. MEANWHILE, WIND SHEAR PARAMETERS SHOW MODERATE
TO HIGH WIND SHEAR AT 0-3 KM 100-300 M2/S2, AND A 40-50 KTS LLJ. ALL
OF THIS MEANS THAT WE COULD HAVE SOME ROTATING UPDRAFTS AND POSSIBLE
SUDDEN GUSTS OF WIND UP TO 50 MPH WITH THE STRONGEST STORMS WITHIN
THE DOWNDRAFT OF THESE STORMS. THOUGH ROTATING UPDRAFTS ARE POSSIBLE,
THE THREAT FOR TORNADOES WILL LIKELY BE RATHER LOW AND SHORT-LIVED.
THE PRIMARY TIME PERIOD FOR THIS CONVECTIVE ACTIVITY WILL BE 6 PM
THROUGH AROUND 3 AM WEDNESDAY MORNING.

THE THREAT FOR SHOWERS AND THUNDERSTORMS WILL REMAIN LATE ON
WEDNESDAY MORNING. HOWEVER, A WARMING OF THE LAPSE RATES ALOFT LOOKS
TO OCCUR--POSSIBLY LIMITING THUNDERSTORM ACTIVITY INITIALLY BEFORE
THE ARCTIC COLD FRONT APPROACHES. HOWEVER, GIVEN THE COMPLEX TIMING
OF THE ARCTIC FRONT AS THE POSITIVELY TILTED TROUGH APPROACHES, HAVE
LEFT THUNDERSTORMS IN THE MORNING FOR NOW. MED RANGE GUIDANCE
CONTINUES TO SLOW THE PROGRESSION OF THE COLD FRONT AND THAT HAS
AFFECTED THE ONSET OF FROZEN PRECIPITATION ACROSS THE TN VALLEY AND
SUBSEQUENT ACCUMULATIONS. SO BASED ON ANALYSIS OF CURRENT MODEL
TRENDS, EXPECT THE FRONT TO PASS OVER NW AL/S MIDDLE TN REGION ON
WEDNESDAY AFTERNOON AROUND 18Z AND GRADUALLY PASS OVER THE REST OF
THE TN VALLEY BY 00Z. KEEP IN MIND THOUGH THAT DAYTIME TEMPERATURES
AHEAD OF THIS FRONT ON WEDNESDAY COULD BE AS HIGH AS 70 DEGREES!

GIVEN ALL OF THAT, AS THE FRONT PASSES, TEMPERATURES WILL RAPIDLY
FALL AFTER 18Z THROUGH EARLY THURSDAY MORNING. HOWEVER, TEMPERATURES
SHOULD NOT FALL TO FREEZING UNTIL CLOSE TO MIDNIGHT ACROSS NW AL AND
S MIDDLE TN WHICH SHOULD TRANSLATE TO A PRECIP TYPE CHANGE TO
FREEZING RAIN AND SLEET. THIS FREEZING RAIN/SLEET AND LIKELY A MIX
WITH RAIN (OVER SE PORTIONS OF AREA) AS WELL SHOULD CONTINUE TO MOVE
ACROSS THE REST OF THE AREA. AS TEMPERATURES BEGIN TO FALL ACROSS THE
REST OF THE AREA, THE FREEZING RAIN AND SLEET MIX WILL GRADUALLY
SHIFT TO THE SOUTHEAST WITH FREEZING RAIN LIKELY EXTENDING THROUGH
THURSDAY MORNING AT LEAST OVER OUR SOUTHEAST COUNTIES. A CHANGEOVER
TO ALL SNOW SHOULD THEN OCCUR LATE ON THURSDAY MORNING INTO THE
AFTERNOON. WITH THE HIGHEST QPF AMOUNTS EXPECTED TO BE ACROSS NW AL/S
MIDDLE TN, ICE ACCUMULATIONS OF 0.2-0.3 INCHES LOOK PROBABLE AND HAVE
ISSUED A WINTER STORM WATCH FOR THOSE AREAS. THERE DOES APPEAR TO BE
THE POSSIBILITY OF SNOW ACCUMULATIONS AS WELL MAINLY OVER THE AREAS
THAT HAVE BEEN BELOW FREEZING LONGER. HOWEVER, THE QPF IS LOWER
AFTER SUNRISE AND HAVE ONLY INCLUDED UP TO ONE INCH (HIGHEST AMOUNTS
OVER SOUTHERN MIDDLE TN) OF SNOW ACCUMULATION.

TEMPERATURES ARE NOT EXPECTED TO RISE ABOVE FREEZING ON THURSDAY
WHICH WILL MEAN THAT WHATEVER RAIN/ICE/SNOW ACCUMULATIONS OCCUR WILL
LIKELY NOT EVAPORATE MUCH. ONE GOOD NEWS/BAD NEWS ITEM IS THAT STRONG
CAA ON THE BACK SIDE OF THE FRONT WILL TRANSLATE TO 10-15 MPH
NORTHERLY WINDS WITH HIGHER GUSTS. THIS COULD HELP EVAPORATION SOME
BUT COULD ALSO CAUSE ICED TREES TO FALL. THE OTHER POSSIBLE
MITIGATING FACTOR IS THAT GROUND AND ROAD SURFACES WILL BE QUITE WARM
PRIOR TO COLD FRONT PASSAGE. LASTLY, THERE COULD BE SOME CLOUD BREAKS
LATE ON THURSDAY AFTERNOON BUT NOT SURE IT WILL BE ENOUGH TO MELT OR
EVAPORATE MOISTURE BEFORE TEMPERATURES NOSEDIVE RAPIDLY AFTER SUNSET
INTO THE TEENS BY EARLY FRIDAY MORNING.

SL.77

LONG TERM (FRIDAY THROUGH TUESDAY)...

A PERIOD OF QUIETER WEATHER IS FORESEEN FOR MOST OF THE EXTENDED
PERIOD AS A MID-UPPER CONFLUENT PATTERN DEVELOPS ACROSS THE MID
SOUTH AND TN VALLEY OVER THE WEEKEND.  FAST MOVING SHORTWAVES IN THE
NORTHERN STREAM WILL RAPIDLY SKIRT BY TO OUR NORTH ON SATURDAY INTO
SUNDAY.  THE SWRN U.S. LOW EVOLUTION IS STILL UP FOR DEBATE, BUT
SOME OF THIS ENERGY MAY STREAK ACROSS THE GULF SOUTH AS WELL DURING
THIS PERIOD, BUT NEITHER BRINGING PRECIP OR A FRONTAL SYSTEM INTO
OUR REGION.

A SURFACE RIDGE WILL BROADEN ITSELF W-E ACROSS THE OZARKS TO THE MID
ATLANTIC STATES BY EARLY NEXT WEEK.  TEMPS WILL MODERATE RAPIDLY,
BUT NOT DUE TO WARM ADVECTION, BUT MORE TO COLUMN WARMING FROM
SUBSIDENCE AND INCREASING SOLAR HEATING EACH DAY.  THE SWRN U.S.
UPPER LOW IS PROGGED TO EVENTUALLY DISLODGE ITSELF BY EARLY TO MIDDLE
OF NEXT WEEK INTO TX OR PERHAPS INTO THE LWR MS VALLEY. TOO EARLY TO
INTRODUCE HIGH POPS, BUT WILL GO WITH SCHC POPS DAYS 6-7 AND VERY
NEAR SUGGESTED MODEL BLENDS GIVEN THE TIMING AND LOCATION DISPARITY
BETWEEN THE GFS AND ECWMF.

&&

.AVIATION...
FOR 00Z TAFS...ARCTIC CDFNT ACROSS CENTRAL MO WHILE A WARM FRONT
EXTENDED ACROSS CENTRAL AL. THIS COMBINATION OF FRONTS WILL CONTINUE
THE CHC OF RAIN ACROSS THE TAF SITES THRU MUCH OF THE FCST PERIOD.
THERE COULD ALSO BE AN ISOLATED TSRA ACROSS THE TN VALLEY MAINLY
THIS EVENING. THE CDFNT WILL BEGIN TO PASS OVER KMSL ARND 20Z AND
KHSV BY 21Z WEDNESDAY. OTHERWISE EXPECT IFR CONDITIONS FOR THE NEXT
24 HOURS AT BOTH TAF SITES.

07

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...WINTER STORM WATCH FROM LATE WEDNESDAY NIGHT THROUGH THURSDAY
     AFTERNOON FOR ALZ001>005.

TN...WINTER STORM WATCH FROM LATE WEDNESDAY NIGHT THROUGH THURSDAY
     AFTERNOON FOR TNZ076-096-097.

&&

$$

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


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