Area Forecast Discussion
Issued by NWS Huntsville, AL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS64 KHUN 180519

Area Forecast Discussion
National Weather Service Huntsville AL
1219 AM CDT Sun Mar 18 2018

For 06Z TAFS.


.NEAR TERM...(Tonight and Sunday)
Issued at 836 PM CDT Sat Mar 17 2018

Strong/svr tstms earlier over portions of mid TN have since moved
well E into E TN/extreme NE GA, leaving quiet wx conditions in place
over the cntrl TN Valley area. Weak cold front is still just to the
NW over mid TN into extreme NW MS. Current temps ahead of the front
remain quite warm this early Sat evening, with values predom around
the 65-70F range. These temps though should begin to quickly fall
later tonight more into the mid/upper 40s, as the front drops into
the area nearing midnight. The quiet wx pattern should continue well
into early Sun morning, with mainly just some lingering high clouds
in place across the area. As such, latest forecast has been updated
to remove the slight chc prob for evening showers/tstms across NE AL
and to refresh some of the current/hourly grids based on the latest
trends. No other major changes were needed attm.

Issued at 330 PM CDT Sat Mar 17 2018
This boundary will quickly drop into Central Alabama by 03-06z.
However, moisture trapped in the inversion of the boundary layer
overnight will generate low stratus late tonight into Sunday morning.
This stable air mass will persist into Sunday, before some mixing
helps to erode cloud cover somewhat. However, isentropic lift will
begin to increase Sunday afternoon as a deepening low pressure over
the Plains begins to move east toward the Mississippi Valley. The
best rainfall chances will exist along a warm front across
South/Central Alabama. After 21z, this boundary will begin lift
north, gradually increasing shower coverage (especially south of the
Tennessee River) late in the day. thus, an isolated thunderstorm or
two will also be possible. Cloud cover will keep highs in check
somewhat on Sunday, but limited sunshine will help to warm us into
the mid to upper 60s.

.SHORT TERM...(Sunday Night through Monday Night)
Issued at 330 PM CDT Sat Mar 17 2018

More complicated part of the forecast begins Sunday Night and
especially on Monday , which has the potential to be a very active
severe weather day. A warm front will lift northward Sunday evening
into Sunday night across the region, spreading numerous showers and
scattered thunderstorms into the Tennessee Valley. Soundings
indicate a fairly moist profile, with PWATs around 1.2 inches, so
would expect some locally heavy downpours with this activity. Some
limited instability aloft may allow for a localized strong
thunderstorm, but believe the severe threat will be low. It appears
that the movement of the main warm front may take place roughly
between 03-09z (perhaps even earlier) and given its movement do not
think a heavy rainfall/flooding threat will exist. However, this
front will serve to veer winds to the south and potentially set the
stage for a moist, unstable air mass later on in the afternoon.

Models are coming into better agreement regarding the track of a
surface low on Monday and the timing of it`s associated dryline/cold
front feature that will provide the source of lift for thunderstorms
in the afternoon. The primary uncertainty deals with the morning and
early afternoon boundary layer, how fast the capping inversion will
break, and how the thermodynamic environment will evolve in the
afternoon. Wind shear will not be in question, with a 60 kt LLJ
punching NE from Central Mississippi into Northern Alabama and the
Cumberland Plateau region of East Tennessee by 21z Monday. This
feature would create 60-70 kts of bulk shear, with 30-40 kts of LL
0-1 km shear. The result could be a line/wave of organized rotating
thunderstorms, supercellar in nature and/or organized strong, bowing
segments (if the thermodynamics are sufficient).

The surface low moving quickly from near Memphis, TN, at 18z to near
Crossville, TN, by 00-03z, would suggest a fast moving cold/dryline
moving across Northern Alabama during the afternoon. Given the
earlier end to precipitation expected by Monday and the low-level
jet progged to develop ahead of the front, think we`ll see some
breaks in the clouds which will allow for destabilization by
early/mid afternoon. The LLJ may help enhance a low-level moisture
surge just ahead of this boundary 18-21z on Monday, with dewpoints
potentially reaching the low to mid 60s. Additionally, this
destablization can be seen in the NAM-12, NAM-3km, GFS, and even
ECMWF soundings which show at least a moderately unstable atmosphere
environment with MLCAPE values up to 1500 J/kg per the GFS and even
higher values on the NAM (though think that may be overdone).
Coinciding with this moisture plume and LLJ, impressive SRH/EHI
values in the 21-00z timeframe bullseye over Northern and Eastern
Alabama. Should these values come close to reality, we could
see numerous severe storms and certainly a greater tornado risk.
Adding to this would be the possibility of a localized backed wind
component east of Interstate 65, which would support the looped
hodographs noted on model soundings during the 21-00z timeframe at KHSV
and FTP.

However, there is still uncertainty on how much of this thermodynamic
environment (particularly the one shown by the NAM) will be realized
for the reasons stated above. As such, have favored a blend of the
GFS/ECMWF for the forecast once again, through am more confident that
an outbreak of severe weather (of some kind) is more likely Monday
afternoon and evening. Large hail may end up being the biggest
threat, with the progged shear and hail CAPE values supporting
golfball to possibility tennis ball hail with the strongest storms.
Given the strong wind shear, damaging winds will also be a threat
with any of this activity. And as mentioned above, there are several
reasons for concern that at least a few tornadoes are possible.
Should confidence increase that this more volatile environment is
possible, may have to include "strong tornado" wording in graphics,
the HWO, and other products in later forecasts. However, enough
uncertainty currently exists to exclude a mention of that at this

Models show the front rapidly exiting the region by 03z, bringing a
swift end to the thunderstorm and severe weather threat. Northwest
winds will filter in cooler, dry air quickly, clearing sky cover and
dropping temperatures back into the upper 40s by Tuesday morning.

.LONG TERM...(Monday night through Friday)
Issued at 330 PM CDT Sat Mar 17 2018

By Tuesday morning, the initial wave of cooler air will have moved
into the TN Valley, but another upper wave and attendant cold front
with surface cold advection will be moving across the area during the
day. Currently, temperatures are not expected to climb out of the 50s
for nearly all locations, and NW winds around 10-15 mph will make
temperatures feel a little cooler. This next upper wave, of Pacific
origin, will have some low/mid-lvl moisture in attendance and
lift may be sufficient in the cloud-bearing layer to produce light
rain across the area. Precipitation looks rather light though, with
amounts likely to remain below one tenth of an inch on Tuesday. As
the system departs the area on Tuesday night, some of the lingering
precipitation could mix with or change to some light flurries before
ending in the higher terrain. Low temperatures on Wednesday will
likely be in the 30s, but most if not all locations may remain above
freezing at the surface.

Conditions will be seasonably chilly on Wednesday with continuing
cold advection and high pressure to our west. However, clouds are
expected to begin clearing out during the day, adding some better
insolation to the region than on Tuesday. Still, high temperatures
are not expected to climb out of the 50s for Wednesday.

For Thursday and beyond, a gradual warm up will commence as deepening
southwesterly flow in the central CONUS leads to a building longwave
ridge which will move into the region. Global models then indicate a
more zonal and progressive upper flow pattern evolving across the
CONUS by early next weekend. A weak front may move into the area and
instigate some shower activity by late Friday or Saturday.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1219 AM CDT Sun Mar 18 2018

VFR conditons will prevail through the TAF period however, MVFR CIGS
around 1,500ft are possible between 09-12Z. Light rain will start to
spread into the TAF sites 00-02Z.





For more information please visit our website
at is the U.S. government's official web portal to all federal, state and local government web resources and services.