Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 240537
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1137 PM CST Fri Feb 23 2018

.UPDATE...
For 06Z TAFS.

&&

.NEAR TERM...(Rest of tonight)
Issued at 946 PM CST Fri Feb 23 2018

A mid-level subtropical high centered just east of the FL peninsula
is forecast to weaken overnight, as an amplified longwave trough over
the western CONUS begins to deepen and assume a more neutral tilt
while it spreads eastward over the Rockies. Near term model guidance
suggests that generally weak deep-layer forcing for ascent will
persist across the TN Valley overnight, with 12-hr 500-mb height
falls on the order of 30-40 m likely due mainly to strengthening
lower-tropospheric WAA. However, within the deep southwest flow
regime downstream from the western CONUS trough, a subtle disturbance
has provided sufficient ascent to generate a vast region of light-
moderate rain, currently extending from northeast MS across the
western half of our CWFA and into middle TN. This area of
precipitation is expected to shift northeastward and away from the
region after Midnight, along with the instigating mid-level wave.

Additional showers and perhaps a few thunderstorms will be possible
through the early morning hours tomorrow as low-level cyclogenesis
across the southern Plains sends a secondary warm front northward
through the region. However, we expect this activity to remain
isolated-widely scattered at best, and POP/weather grids have been
adjusted to reflect this reasoning. Although an overcast mid-level
cloud deck currently encompasses the region, ceilings are expected to
lower as the secondary warm front lifts northward, and lows have
been raised by a degree or so to account for both current obs and
expectations of lowering cloud bases after Midnight.

.SHORT TERM...(Saturday and Saturday Night Sunday)
Issued at 315 PM CST Fri Feb 23 2018

Attention quickly turns to the potential for strong to severe
thunderstorms late Saturday afternoon through Saturday night as a
upper-level trough swings a cold front into the region. Still a
somewhat uncertain forecast, but there is moderate confidence that
some scattered severe thunderstorms will occur, mainly Saturday
evening and overnight. The best thermodynamic environment will likely
be present during the afternoon and early evening, while the better
dynamics will occur late evening and overnight.

Ahead of the cold front, dense low-level cloud cover will linger
from earlier predawn hour rainfall. However, drier air aloft will
help to create some breaks in these clouds and allow for some heating
and destabilization of the boundary layer, especially during the
afternoon. The one mitigating factor regarding this dry air may be
that it acts to wash out (somewhat) some of the richer moisture in
place at the surface. However, model progs continue to show deep SW
flow aloft, which would help to reinforce this air mass and thus
maintain our low to mid 60s dewpoints through the day and into the
evening and overnight hours.

Model soundings indicate a decent thermodynamic environment to
support strong to severe thunderstorms across most of the region by
21z. The NAM in particular favors this destabilization in the low-
levels Around 1000 J/kg of SBCAPE, with 500-700 J/kg of ML/MU CAPE by
21-00z Saturday afternoon and evening. Some concern does lie that a
few strong storms may develop along/near any weak mesoscale
boundaries that could be put out by tonight`s activity. However,
without a true dynamic forcing mechanism, am expecting a bulk of the
severe weather threat to be confined to the evening and overnight
period (when the better forcing) will occur.

After 00z, models show a fairly stout 60-80 kt LLJ develop across
Mississippi and streak NE into Middle Tennessee, clipping far
Northwest Alabama. The feature will help to enhance a line of strong
to severe storms just west of the Mississippi River around 00z that
will race eastward with time. There is some model disagreement with
the orientation of the jet and when this line of storms will arrive.
Current thinking has this activity making into to the MS/AL an hour
or so before midnight and moving across the area from west to east
during a 06-12z timeframe. Due to the enhancement of the LLJ, shear
profiles will help maintain storm structure and organization (even as
instability drops off after sunset). The best environment will be
west of the region in the enhanced risk area. However, am thinking an
organized severe threat will exist, especially in areas west of I-65
where the dynamics will be better. Damaging winds will be the
primary risk with this -- especially in bowing segments and line
surges. With even less instability to work with east of I-65, do
think the threat will be more marginal -- especially given the timing
which will likely be after 09z. While not the most ideal for
tornadoes, there will be enough low-level turning and speed shear
present in the 0-1 km range to warrant a mention for a brief, weak
tornado -- mainly across far Northwest Alabama.

Total QPF looks to be between 1 to 1.5 inches tonight through Sunday
morning. We`ll certainly need to watch for localized heavy rainfall
and flooding late Saturday night -- especially in areas where
training occurs. However, this threat does not appear widespread
enough to consider at watch at this time.

.LONG TERM...(Sunday night through Thursday)
Issued at 315 PM CST Fri Feb 23 2018

It looks as though the front will push as far south as a line from
near Chattanooga southwest to near Birmingham by 6 AM. As the
morning goes on, this front should push further southeast, but slowly
stall a bit further south near central Alabama by the afternoon. At
this point, believe this precipitation should be in the form of rain
behind some sort of line of showers and storms to our south.This
could produce another quarter to half an inch or so of rainfall
Sunday morning. This would only aggravate any minor flooding or
river flooding issues that could be ongoing.

By Sunday afternoon, rain chances will drop off quite a bit as drier
air and subsidence moves into northern Alabama. Couldn`t rule out a
few light showers lingering near and south of the TN river into the
early afternoon hours, but amounts should be less than one tenth of
an inch.

Models are in quite a bit of disagreement how far west additional
heavier rainfall behind the front may develop Sunday night into
Monday as a strong (but dry) longwave trough axis quickly moves east
from Missouri and into the Tennessee Valley. GFS/NAM keep the
boundary much further south as this upper level feature approaches
and they only develop one to two tenths of an inch of precipitation
over our southern counties. ECMWF produces quite a bit more, another
half an inch to an inch of rainfall. At this point, going with an
average of all of them, but leaning toward drier solutions of the
GFS/NAM.

A fairly strong 1025 mb high builds into the area on Monday in most
models, again the ECMWF lags a bit of precipitation behind the front
over the area. However, again think this is a bit of an outlier
solution. This will keep temperatures a bit cooler, but still warm.
Dry weather looks to continue through Tuesday of next week at least.
By the middle of the week, another warm front pushes north with an
approaching amplified longwave trough axis over the desert southwest.
This system could bring us some stronger to maybe severe storms
either Tuesday night or Wednesday (more likely on Wednesday), as it
approaches the Tennessee Valley. Much warmer temperatures will return
as well.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1135 PM CST Fri Feb 23 2018

Light/mod ra/shra continue to spread ewd across parts of NW/N cntrl
AL this late Fri evening. Even with the precip, VFR conds prevail at
the two main terminals with predom just some mid level cloud cover
streaming into the area with the shra activity. Cigs are xpcted to
lower closer to 3-4K ft overnight, as S winds increase near 10KT.
Cigs may briefly lower into the MVFR cat/2-3K ft early Sat morning,
as additional ra/shra move across the area. There may actually be a
break in the precip during the day Sat, thereby allowing the cigs to
climb above 3K ft during the early afternoon hrs, before additional
precip develops/moves into the area late in the TAF period. Sfc winds
out of the SSW are also xpcted to increase near 12-14KT with higher
gusts Sat morning, and persist for much of the day.

&&

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

$$

NEAR TERM...70/DD
SHORT TERM...AMP.24
LONG TERM...KTW
AVIATION...09


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