Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 030357
AFDHUN

AREA FORECAST DISCUSSION...CORRECTION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
952 PM CST MON MAR 2 2015

.UPDATE...
NO CHANGES TO FCST ATTM.

&&

.SYNOPSIS...
/ISSUED 354 PM CST MON MAR 2 2015/

A VERY ACTIVE WEATHER PATTERN IS SHAPING UP AS A POSITIVELY TILTED
UPPER TROUGH OFF THE S CALIFORNIA COASTLINE SLOWLY TRANSLATES
EASTWARD OVER THE NEXT FEW DAYS. IN THE MIDST OF ITS TRANSITIONING
MOVEMENT AND ORIENTATION, SUBTROPICAL JET HAS BEEN INDUCED ON ITS
SOUTHERN FLANK. WITH A JET STREAK ORIENTED N-S FROM BAJA CALIFORNIA
TO THE CENTRAL PLAINS AND THEN W-E ORIENTATION FROM THE CENTRAL
PLAINS TO THE DELMARVA COAST.

THE PROGRESSION OF THIS TROUGH IS BEING CLOSELY WATCHED AS IT
INDUCES STRONG WAA ON TUESDAY INTO WEDNESDAY. THIS COULD MEAN STRONG
AND VERY ISOLATED SEVERE THUNDERSTORMS POSSIBLE ON TUESDAY INTO
WEDNESDAY AHEAD OF THE COLD FRONT. THEN, A VERY COLD ARCTIC AIRMASS
LOOKS TO DROP SE ACROSS THE PLAINS INTO THE TN VALLEY AS THE TROUGH
GAINS MOMENTUM AND MERGES WITH A POLAR JET MID-LEVEL SHORTWAVE
TROUGH MOVING ACROSS THE MIDWEST. THE COMBINATION OF THE SFC AND
UPPER LEVEL FEATURES COULD LEAD TO THE POTENTIAL FOR SIGNIFICANT
WINTER WEATHER IMPACTS ON WEDNESDAY NIGHT INTO THURSDAY.

THERE IS LOW CONFIDENCE ON PLACEMENT OF EXACT WINTRY PRECIP ACCUMULATIONS
RIGHT NOW AND CHANGES TO THE PRECIP TYPE/ACCUMULATION TIMING IS
LIKELY IN THE FOLLOWING FORECASTS.

SL.77

&&

.DISCUSSION...
UNDER CLOUDY SKIES AND A FEW ISOLATED SHRA...TEMPS WERE IN THE
LOW/MID 40S. FOR THE REST OF TONIGHT EXPECT TEMPS TO HOLD STEADY OR
EVEN RISE A FEW DEGREES THANKS TO LIGHT SE WINDS. THOUGHT ABOUT
LOWERING POPS FOR OVERNIGHT BASED ON CURRENT RADAR TRENDS BUT NEW NAM
DATA SHOWS THAT MORE SHRA WILL LIKELY REDEVELOP ARND 09Z AS A WARM FRONT
MOVES NORTH...SO WILL KEEP CURRENT LIKELY POPS. OTHERWISE FCST LOOKS GOOD
ATTM.

07

&&

.AVIATION...
/ISSUED 547 PM CST MON MAR 2 2015/
FOR 00Z TAFS...AN ADVANCING WARM FRONT WILL KEEP THE CHC OF SCT SHRA
OVERNIGHT ACROSS THE TAF SITES. AS THE WARM FRONT MOVES NORTH OF THE
CWA (AFTER 12Z TUESDAY) THERE WILL BE ENOUGH INSTABILITY FOR A FEW
TSRA TUESDAY AFTN. OTHERWISE EXPECT MAINLY MVFR AND IFR CONDITIONS
THRU THE FCST PERIOD FOR BOTH KHSV AND KMSL.

07

&&

.PREV DISCUSSION...
/ISSUED 354 PM CST MON MAR 2 2015/

THOUGH MOST OF THE ATTENTION HAS BEEN PLACED ON THE ABOVE POSSIBLE
IMPACTS THAT ARE LIKELY ACROSS THE TN VALLEY, FOR TONIGHT, SHOWERS
WILL OVERSPREAD THE REGION AS A WARM FRONT LIFTS NORTH TONIGHT.
THERE IS WEAK ELEVATED INSTABILITY (MUCAPES OF 100-200 J/KG) PRESENT
DURING THE EARLY MORNING HOURS ON TUESDAY THAT MAY TRANSLATE TO
UPDRAFTS STRENGTHENING AND RESULTING IN THUNDERSTORMS. THUS, WILL
KEEP THUNDERSTORMS IN THE FORECAST FOR LATE TONIGHT INTO EARLY
TOMORROW MORNING. AN ADDED BONUS FOR TOMORROW IS WARMER DAYTIME
TEMPERATURES AS THE WARM FRONT PASSAGE PLACES THE TN VALLEY INTO THE
WARM SECTOR AND WAA CONTINUES.

AS THE UPPER TROUGH MOVES CLOSER, A VERY STRONG UPPER JET STREAK
WILL SETUP TO THE NORTH OVER THE MID-UPPER MS RIVER VALLEY INDUCING
A LLJ OF 50-60 KTS ON TUESDAY MORNING. TEMPERATURES WILL LIKELY TOP
OUT IN THE MID TO UPPER 60S ON TUESDAY AFTERNOON. WITH THE STRONG
WAA AND SATURATED PROFILE ALOFT, WAA SHOWERS AND THUNDERSTORMS ARE
POSSIBLE WITHIN THE WARM SECTOR. HOWEVER, THE HIGHEST THREAT WILL
ACTUALLY BE DURING TUESDAY NIGHT INTO WEDNESDAY AS THE UPPER JET
MOVES CLOSER AND WIND SHEAR INCREASES SUBSTANTIALLY. SRH VALUES ARE
OFF THE CHARTS WITH SOME MODEL RUNS (VALUES BTWN 300-600 M2/S2 AT
0-3 KM). INSTABILITY VALUES LOOK TO BE RATHER PALTRY (BTWN 50-200
J/KG). WITH THESE VALUES, IT APPEARS WE WILL HAVE A THREAT FOR A QLCS
DEVELOPING AHEAD OF THE SLOW MOVING COLD FRONT WITH DAMAGING WINDS
AND SPIN-UP TORNADOES THE PRIMARY THREATS.

THE MED RANGE MODELS HAVE BEEN SHOWING A GENERALLY SLOWER TREND IN
THE MOVEMENT OF THE TROUGH AND ACCOMPANYING SFC COLD FRONT FOR
WEDNESDAY THROUGH THURSDAY. THIS IS TO BE EXPECTED GIVEN THE COMPLEX
MOVEMENT AND CHANGE IN ORIENTATION OF THE TROUGH. THIS HAS
INTRODUCED ANOTHER LAYER OF COMPLEXITY AS THE PRECIP TYPE FOLLOWS
THE FRONT AND EVENTUALLY CAA BEGINS ALOFT. THUS, HAVE MADE
ADJUSTMENTS TO KEEP SOUTHERN PORTIONS OF THE CWA WARMER ON WEDNESDAY
WITH THE POSSIBILITY OF HIGHS ACTUALLY OCCURRING DURING THE DAYTIME
(AS OPPOSED TO ORIGINALLY THOUGHT IN THE EARLY MORNING TIME). THIS
HAS ALSO PUSHED BACK THE TRANSITION FROM SHOWERS/THUNDERSTORMS EARLY
WEDNESDAY MORNING TO ALL RAIN ALONG WITH THE CHANGE IN WINTRY
PRECIPITATION. A SIGNIFICANT WARM NOSE IS SEEN IN ALL OF THE MODEL
RUNS WITH A PRONOUNCED SHALLOW INVERSION INDICATIVE OF THE ARCTIC
FRONT. EVEN WITH THIS, THE SFC TEMPS LOOK TO BE SLOWER TO DROP BELOW
FREEZING DURING THE DAY ON WEDNESDAY EVENING. FORECAST SOUNDINGS
SHOW THAT SIGNIFICANT COLD AIR ADVECTION EXISTS BELOW AN OVERRUNNING
AREA OF STRONG MID-LEVEL WARM AIR ADVECTION (SW FLOW ALOFT). IN
ADDITION, OVERRUNNING MID-LEVEL SHORTWAVE TROUGHS MOVING ALONG THE
SW FLOW ALOFT ATOP A SATURATED COLUMN COULD YIELD SIGNIFICANT ICE
ACCUMULATIONS. WITH THE TIMING AND OVERALL SETUP OF FORECAST QPF,
HAVE MADE ADJUSTMENTS TO THE FORECAST TO REFLECT A SLOWER TRANSITION
TO FREEZING RAIN ON WEDNESDAY EVENING OVER FAR NW AL. SO, GIVEN ALL
OF THAT, THE WINTRY PRECIPITATION STARTS AFTER 00Z OVER FAR NW AL/S
MIDDLE TN AS FREEZING RAIN FOLLOWED BY A FREEZING RAIN/SLEET MIX
AFTER 06Z WITH A SLOW TRANSITION TO SNOW/SLEET AFTER 09-12Z. THIS
PRECIP TYPE TRANSITION SHOULD SLOWLY PROGRESS ACROSS THE ENTIRE TN
VALLEY OVERNIGHT.

ACCUMULATIONS ARE DIFFICULT TO DETERMINE WITH THE VERY WARM SFC
TEMPS IN PLACE JUST PRIOR TO THE FRONTAL PASSAGE. IN FACT, ICE
ACCUMULATIONS MAY ACTUALLY START ON ELEVATED SURFACES AND VEGETATION
FIRST BEFORE GRADUALLY IMPACTING THE ROADS OVERNIGHT WEDNESDAY NIGHT
INTO THURSDAY MORNING. SO, FOR NOW, HAVE INCLUDED A SHARP GRADIENT
IN SNOWFALL ACCUMULATIONS ACROSS THE ENTIRE CWA WITH A MAXIMUM OVER
NW AL/S MIDDLE TN GIVEN THE ORIENTATION OF THE MAX QPF FORECAST AND
PROFILE COOLING TIMING. ICE AMOUNTS UP TO 0.3 INCHES AND SNOWFALL
FROM 0.5-2.0 INCHES ARE LIKELY.

THEN, BY THURSDAY MORNING, TEMPERATURES ARE EXPECTED TO BE STARTING
IN THE LOW TO MID 20S WITH HIGHS ONLY EXPECTED TO RISE CLOSE TO 30-
32 DEGREES. COMBINE THESE TEMPERATURES WITH NORTHERLY WINDS OF 10-15
MPH AND ICING ON VEGETATION AND ROADS, AND THERE VERY WELL COULD BE
SIGNIFICANT HAZARDOUS TRAVEL AND TREE DAMAGE.

TEMPERATURES FINALLY RISE BACK ABOVE FREEZING ON FRIDAY AS ZONAL
FLOW RETURNS. SOUTHERLY FLOW RETURNS ON SATURDAY WITH TEMPS RISING
INTO THE 50S BY THEN AND AGAIN ON SUNDAY.

SL.77

&&

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

$$

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


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