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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

                            
000
FXUS64 KHUN 212006
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
206 PM CST Thu Feb 21 2019

.NEAR TERM...(Tonight)
Issued at 206 PM CST Thu Feb 21 2019

A potentially significant flash flood threat is evolving for tonight
(and through Friday) for southern middle TN and portions of north
Alabama. The next round of moderate to heavy rain is not lifting
north-northeast as an 850 mb (elevated) warm front lifts back
northward. The nocturnal LLJ tonight will increase intensity and
coverage of this band of showers and thunderstorms as elevated
instability increases. The mesoscale models and NAM all indicate a
continually redeveloping solid band of training showers and 
thunderstorms through tonight into Friday from north central MS
through the Shoals/NW Alabama into southern middle TN. Will elaborate
more in the next section. With the threat of flash flooding
increasing, and being at nighttime when the threat for motorists is
higher, will transition to a Flash Flood Watch for southern middle TN
and most of northwest/north central AL. Will keep the areal flood
watch in effect in our southeast counties where existing flooding and
lessor QPF is expected. Thanks for neighboring WFO coordination on
this.

.SHORT TERM...(Friday through Saturday)
Issued at 206 PM CST Thu Feb 21 2019

Although WPC basin QPF is showing 1.5-2 inches through Friday night,
convective rain rates may reach 0.5 to 1.0 inch per hour or better at
times. This could cause rapidly rising water on already saturated and
in some cases flooded soils. Thus, flash flooding will become a 
distinct hazard.

By Friday evening, the 850 mb front and heavy rain area will shift 
northward, although the surface warm front will also be moving 
through. Low clouds, light rain and perhaps some drizzle may be 
possible Friday night until Saturday morning when deeper mixing 
should occur. On Saturday, a tight pressure gradient takes shape with
a wide area of 50-60kt flow at 850 mb and 925 mb flow of 40-50kt. We
may need to hoist a wind advisory, particularly with a higher risk of
tree falls occurring due to the saturated soils. Depending on if the
clouds can break at times Saturday, highs should still reach the 70s
for most, although the CAD/wedge will limit clearing and keep
temperatures/dew points much cooler in far northeast AL.

Not much change regarding the threat of severe weather Saturday
night. It appears that any pre-frontal convection may be possible
after 22-23Z, with the QLCS pushing east with the front from approx
01-02Z thru 07-09Z. Forecast hodographs indicate a rather straight
line and strongly sheared profile. 1km SRH values will still be
sufficient for mesovortices and tornadoes along this line. If
instability were to be higher, a threat of pre-frontal supercells
would also grow, but confidence in this remains somewhat low.


.LONG TERM...(Saturday night through Wednesday)
Issued at 206 PM CST Thu Feb 21 2019

Gusty northwesterly winds between 15 and 20 mph with gusts up to 30 
mph will hold on in the morning hours on Sunday, behind the departed 
front as it moves into eastern Georgia. Much cooler and drier air 
will move into the Tennessee Valley behind it. Despite abundant 
sunshine, highs should only climb into the 50s, with maybe a few 
lower 60s in southern Cullman county. Thankfully, a prolonged period 
of dry weather is expected, as a surface high remains in place over 
the southeast through the day on Tuesday at least. This should 
continue to keep the area dry and cooler through Tuesday with highs 
continuing to be in the 50s and lows in the 30s.

Monday morning looks like the coldest period, before upper level 
flow becomes more zonal and the surface high weakens. From this 
point on in the extended, models diverge significantly. Models show 
upper level forcing associated with a weak longwave trough axis 
pushing east through zonal flow into the southeast and stalling 
midweek into the later portions of next week somewhere in the 
southeastern CONUS. GFS is much wetter and brings deeper moisture and
more precipitation northward as far north as Tennessee and northern 
Alabama by Wednesday. However, ECMWF keeps this moisture and any 
precipitation in the Gulf of Mexico. So for now went with a blended 
approach to the forecast, introducing isolated chances of rain 
Tuesday night and keeping them in the forecast through Wednesday 
night. Daytime maximum temperatures will warm slightly during this 
period, but generally remain in the upper 50s to lower 60s. 

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1118 AM CST Thu Feb 21 2019

Moderate rain will spread into north AL over the next few hours. The
rain will lower visibility to around 3-4SM (MVFR) to along with the
already MVFR ceilings of 015-025agl. Tonight, showers and a few
thunderstorms are expected to continue, mainly over northwest/north
central AL. Ceilings will likely drop back below 010agl (IFR) with
moderate to occasionally heavy rain expected. The greatest chance of
thunderstorms with heavy rain will be at KMSL after 12Z (mainly
14-18Z with a TEMPO group), dropping visibility to 2sm or less. This
is also possible at KHSV, so have included VCTS.


&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Flood Watch through late Friday night for ALZ007>010-016.

     Flash Flood Watch through late Friday night for ALZ001>006.

TN...Flash Flood Watch through late Friday night for TNZ076-096-097.

&&

$$

NEAR TERM...17
SHORT TERM...17
LONG TERM...KTW
AVIATION...17


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at weather.gov/huntsville.