Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 182348

Area Forecast Discussion
National Weather Service Huntsville AL
648 PM CDT Sun Mar 18 2018

For 00Z TAFS.


.NEAR TERM...(Tonight)
Issued at 317 PM CDT Sun Mar 18 2018

High clouds continue to hang on ahead of a warm front that has begun
to slowly inch its way north from Central and South Alabama. Current
radar and satellite data (along with surface obs) depict this
boundary roughly across the I-20 corridor of Central Mississippi and
into the US-80/I-85 corridor of Central Alabama. The upper storm
system is taking shape to the southwest of the region over Texas as a
plume of dry air surges into the Texas Hill Country and deeper
convection develops across the ArkLaTex and Northern Gulf Coast of
Mississippi and Louisiana.

This warm front will moves northward Sunday evening into Sunday
night across the region, as this low pressure system over the
Central/Southern Plains deepens overnight and moves toward the lower-
Mississippi Valley. Weak isentropic lift will generate light showers
late this afternoon, which will increase coverage by late
evening/midnight as the boundary moves north into the Tennessee
Valley. This will quickly spread numerous showers and scattered
thunderstorms into Northern Alabama and eventually Southern Middle
Tennessee. Soundings indicate a fairly moist profile, with PWATs
around 1.2 inches, so would expect some locally heavy downpours with
this activity. Timing of the warm front passage still looks to mainly
occur between 03-09z, and given its forward movement do not think a
heavy rainfall/flooding threat will exist. This front, however, will
serve to veer winds to the south and potentially set the stage for a
moist, unstable air mass later on Monday afternoon.

.SHORT TERM...(Monday and Monday Night)
Issued at 317 PM CDT Sun Mar 18 2018

The warm front will be exiting the region by 12z Monday morning,
leaving a moist, but capped environment in its wake. Thus, am
expecting a shield of low stratus, with perhaps some light rain
showers and storms through mid morning hours. There is still some
uncertainty on how long this activity will persist which would have
an impact on the thermodynamic environment (and thus severe
potential) later in the afternoon. However, latest mesoscale models
and lower-resolution synoptic scale models hint at this activity exiting
by around 14-15z at the latest. Additionally a "dry slot" feature
aloft should help to erode any of the dense mid/high clouds by late
morning/midday, allowing for breaks in the sky cover and further
boundary layer destabilization by the afternoon. The aforementioned
capping inversion will weaken by early afternoon as heating permits
and should dissipate entirely by mid/late afternoon as strengthening
LLJ over Central Mississippi moves northeast into the Plateau region
of Middle/East Tennessee. These winds will also help to advect some
richer moisture into the region, with low to mid 60s dewpoints
expected to arrive at or just after 18z.

Drier air to the west along the Mississippi Valley and the deeper,
Gulf moisture to the east will create a pseudo dryline/cold front
feature across West Tennessee and Northern Mississippi which will
sweep east with the surface low, which is progged to move from
Memphis, TN, (18z) to around Crossville, TN, by 00-03z. Timing of
this dryline/front has slowed just a bit in recent model runs. This
would delay the arrival of potential storms for our area by an hour
or two and also allow the boundary layer further destabilize (give
the cap more opportunity to weaken/break). Once the cap does break,
models have come into agreement that a very favorable environment for
long-lived severe thunderstorms will develop, especially along and
east of the Interstate 65 corridor (where locally backed surface
winds will enhance the tornado threat).

As mentioned, models have come into agreement regarding the
environmental parameters, showing (at the very least) a moderately
unstable environment. As has been the case the past several days, the
NAM has lead the charge in showing the greater instability
parameters, and honestly a pretty volatile environment. The NAM
(while slightly overdone) does hint at up to 2000-2500 J/kg of
MLCAPE and 0-3 km SRH values of 300-500 m2/s2. The GFS/ECMWF are less
aggressive with this destabilization, but the bottom line is even if
some of the lower-end of the forecast parameters are realized, it
will be a very active (and potentially dangerous) severe weather set-
up for Monday afternoon and evening. The main area of storm
initiation will likely be the between 18-20z across Northwest Alabama
into Middle Tennessee as the LLJ surges north. This feature will
create some very strong 0-6 km bulk shear of 60-70 kts and up to
30-40 kts of 0-1/0-3 km shear. the result will be an environment very
conducive for rotating supercells (possibility in a line) that will
sweep across the region along the dryline/front through mid/late

As one would expect, all modes of severe weather are possible in
this environment. With the melting level progged to be around 8 kft
or so and abundant CAPE in the -10C to -30C, feel confident we will
see some big hail. Given the strong updrafts, this hail could
potentially reach golfball to tennis ball size. Additionally, with
such a robust low-level jet, damaging winds will be possible with any
of the storms that develop. The growing concern, however, is the
potential tornado threat, especially for areas east of Interstate 65.
There, models are all hinting at a localized backed wind profile.
Combined with the strong speed shear in the low-levels and
directional shear aloft, hodographs exhibit a classic "sickle shape",
which has been observed in many strong tornado events. Additionally,
the combination of this shear/instability may be maximized per the
NAM and GFS just ahead of this boundary around 00z across the HSV
metro and especially Northeast Alabama. Thus, a few tornadoes are
possible, and a strong tornado or two cannot be ruled out --
especially with any supercell that can become dominant.

As this line of supercells pushes east toward the AL/GA border
between 00-03z, instability will drop off slightly and the very
strong shear will begin to become more dominant. Thus, line of
supercells may morph into an intense QLCS as it exits the region into
Northwest Georgia and Central Alabama, enhancing the damaging wind
threat there. Behind this line of storms, precipitation will end
quickly along with the severe threat. In fact, clouds may dissipate
enough to result in a partly/mostly clear night over the region. Cold
air advection will knock temperatures back into the 40s by early
Tuesday morning.

.LONG TERM...(Tuesday through Saturday)
Issued at 317 PM CDT Sun Mar 18 2018

By Tuesday morning, the broad upr trough will be swinging eastward
across the TN/OH Valley Region, while surface lows take shape along
the Atlantic Coast. Cold air advection will be ongoing across the
area, with a likely reinforcing shot of cooler air during the
afternoon. Meanwhile, attendant moisture from the N. Pac airmass will
increase during the day, brining chances for rain back to the area.
Rainfall will likely be light however, and only about a tenth of an
inch or less is expected for most locations on Tuesday through
Tuesday night. Amidst the cold air advection, increasing cloud cover
and chances for rain, temperatures may only climb about 5-10 degrees
from morning lows on Tuesday.

As usual, the colder air will be moving in and temperatures dropping
into the 30s as precipitation moves out of the area. Currently, it
looks as though all precipitation will likely end as rain, late
Tuesday night. However, a brief ice pellet or snow flurry cannot be
ruled out mainly for the higher terrain of NE Alabama and the
Cumberland Plateau in TN. Nevertheless, chances appear too low still
and this was not included at this time in the official forecast.

Despite clouds clearing from west to east on Wednesday, and
increasing sunshine, temperatures may be cooler on Wednesday
afternoon than on Tuesday as the high pressure axis moves just to our
west during the afternoon. With a clear or mostly clear sky possible
on Thursday morning and with dew point temperatures potentially
falling into the upper 20s, some locations could experience a freeze
for a few hours on Thursday morning.

A gradual warm up will commence on Thursday as a trough builds
initially into the western CONUS and a ridge with deepening S-SW
flow develops in the Plains States and moves eastward. Some
discrepancies exist as expected between the next upr wave and cold
front expected to impact the region. But, the global models indicate
a rather quick transition from a developing ridge to a near zonal
flow pattern by the early weekend, with low pressure moving rapidly
across the OH Valley region on Saturday. What is appearing likely is
that at least a part of the west CONUS trough will be ejected into
the Plains and Midwest, but what is not easy to determine yet is the
southern extent of its influence, including on the TN Valley.
Nevertheless, this has been a persistent large scale feature and thus
some POPs are retain for the weekend period. Weak instability during
the period may produce a few thunderstorms, but only slight chances
were mentioned owing to the generally weak instability and forcing
over our region.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 648 PM CDT Sun Mar 18 2018

While mid level clouds/prevailing VFR conds persist across the area
this late Sun afternoon, -ra/shra are beginning to develop more
across portions of NW AL and spread ewd. This activity could result
in some vis reductions/MVFR cigs later this evening as ra/shra
become a bit more heavy in some locations. Precip is xpcted to
gradually taper off to the E heading into the morning period Mon,
although lower clouds/cigs in the 1-2K ft range may linger well into
the late morning hrs. A developing storm system and attendant cold
front out of the srn Plains states will then track ewd Mon afternoon
and into the wrn half of the area late in the TAF period. Shra/tsra
are xpcted to develop along/ahead of this front as they spread into
NW/N cntrl AL after 21Z. Sfc winds ahead of the front will increase
out of the S/SE in the 10-12KT range with higher gusts late Mon
morning and continue thru the afternoon hrs.





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