Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 170129

Area Forecast Discussion
National Weather Service Huntsville AL
829 PM CDT Fri Mar 16 2018

.NEAR TERM...(Tonight)
Issued at 828 PM CDT Fri Mar 16 2018

Little to no precip has fallen across the area this evening thanks
to a very dry atmosphere earlier today that has been slow to saturate.
LAPS soundings show the low levels have mostly saturated with temps
in the upper 50s/lower 60s and dew points in the lower 50s as of 8PM.
However, mid/upper levels have dried just a bit. So we will have to
resaturate to get any moderate rainfall in the next few hours.
Synoptically, the upper low is sitting over SE Nebraska with a weak
surface low just to its south. As these two pushed northeast this
afternoon, the lack of lift, combined with the dry air, kept the area
mostly dry. Speaking of that, hires guidance continues to push back
the onset of rainfall this evening and have therefore pushed the POPs
later as well. This new forecast introduces slight chance pops
slowly west to east by 6z. These showers will be aided by a mid level
wave moving along the westerly flow at the base of the upper trough
and a 40kt LLJ pushing in from the west. The jet will cap us in the
mid levels, per soundings, but some instability aloft thanks to
moderately steep lapse rates and enough moisture could still give us
a rumble or two of thunder after 9z. There could be some small hail
and gusty winds with these storms if they do develop.

Given the increasingly cloudy skies and mild southwesterly flow,
temps may only drop a couple more degrees through the rest of the
overnight period.

.SHORT TERM...(Saturday through Sunday)
Issued at 248 PM CDT Fri Mar 16 2018

The initial wave will push to the east of the forecast area early
Saturday morning, with the bulk of the rainfall exiting the area by
daybreak. The upper low will continue to weaken as it moves into the
Ohio Valley during the day on Saturday. Meanwhile, a weak frontal
boundary will trail west-southwestward from the parent low. Given
that this boundary will remain well to the north of the area for most
of the day, southerly flow will continue to advect warmer and moist
Gulf air across the region. A lack of strong forcing and limited
moisture will limit much, if any, diurnal convection during the day,
despite efficient destabilization during the afternoon. The
aforementioned frontal boundary will swing through the area late
Saturday afternoon and evening, as the upper/surface lows continue
eastward. This may help generate some isolated thunderstorms as it
moves through.

Any precip that does develop along the boundary will quickly move out
of the area late Saturday night, with northerly winds bringing some
slightly drier air. Temperatures will still be on the mild side,
given some cloud cover, with overnight lows in the mid to upper 40s.
Sunday looks to be pleasant across the TN Valley, with light
northerly flow shifting to the east by the afternoon. Skies will
remain cloudy through the day and keep temperatures in the upper 60s,
but the area will remain dry as we watch our next system take shape
to the west.

.LONG TERM...(Sunday night through Thursday)
Issued at 248 PM CDT Fri Mar 16 2018

Active pattern continues early next week as an area of low pressure
deepens across the Central/Southern Plains Sunday night and moves
into the Mid South and Tennessee Valley region Monday and Monday
night. This system will bring the potential for numerous showers and
thunderstorms and severe weather, especially Monday afternoon through
Monday night. Ahead of this system a warm front will begin to lift
northward Sunday night, serving as a focus for potentially some deep
convection across portions of Central and South Alabama. It is
uncertain how far north this boundary will lift overnight and where
the axis of heaviest rainfall will set up. However, models continue
to trend further to the south over Central Alabama where WPC has
introduced a slight risk area for excessive rainfall. This will
bear watching as model QPF show the potential for 1-2 inches
developing along and near this boundary during the Sunday night

There are still model discrepancies on how long this convection will
linger and the timing of this warm front as it lifts north. Some
guidance, such as the ECMWF and Canadian solutions maintain this
activity well into the morning on Monday across Central and South
ALabama. This would have a significant impact on the establishment
of a warm sector later on in the afternoon on Monday as the Tennessee
Valley may be cutoff from the best moisture rich environment when
the cold front moves through late Monday afternoon and evening into
Monday night. Additionally, there are other uncertainties,
specifically with respect to the track of the surface low. The GFS
shows the feature further to the northwest closer to the Missouri
Bootheel and eventually the lower Ohio Valley. The NAM is much
further to the south, passing it over Southern Middle Tennessee
Monday evening. As one would expect, this NAM solution (if the
boundary layer destabilizes sufficiently) would place the Tennessee
Valley under a greater threat for severe weather due to the
combination of the best instability and wind shear coinciding
simultaneously around 21z Monday to 03z Tuesday.

Some caution when using the 12z NAM given that this more southerly
track is a bit of an outlier, and some of the higher instability
parameters noted (CAPE, Helicity, etc) may be overdone due to some
convective feedback issues within the model. Still, this southerly
track would support a strong LLJ in a favorable position, enhancing
low-level and deep layer shear during the afternoon/evening period.
Localized backed winds in northeast Alabama ahead of developing pre-
frontal trough/dryline would create looped hodographs and a shear
profile more favorable for a tornado threat (in addition to large
hail and damaging winds). Should something close to this scenario
play out, a line of supercells developing west of the area around
mid afternoon and moving across Northern Alabama and Southern Middle
Tennessee as the dryline sweeps east would certainly be plausible.
However, (and I can`t stress this enough), the exact details
regarding the thermodynamics and dynamics with this event are very
uncertain. If the track of the low changes slightly or the boundary
layer doesn`t recover as much by the afternoon, these will create big
changes with respect to the threats and magnitude of the event.

For this forecast package, have trended much more toward a model
blend, favoring thunderstorms Sunday night and early Monday morning
along and south of the area. Then, more scattered activity Monday
morning, followed by some more organized strong to potentially
severe storms along the dryline/trough in the afternoon and evening.
Am most confident (given all of the model data we`ve looked at), this
environment will be capable of producing large hail (perhaps up to
golfball size) as well as damaging winds. And, if the right dynamics
and thermodynamics come together, a couple tornadoes are possible.

After the front sweeps east of the region on Tuesday, cooler and
eventually drier air will filter into the Tennessee Valley for the
remainder of the week. GFS/ECMWF both should low clouds and wrap
around moisture on Tuesday so have increased PoPs slightly to
low-end scattered. Drier air moves in Wednesday, promoting sunny, but
cooler temperatures with highs in the 50s and lows in the 30s through


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 613 PM CDT Fri Mar 16 2018

Terminals are dry right now outside of a shower or two that may
linger another hour at KHSV. Another batch of rain ahead of an upper
disturbance will move in from the west after 05z and move east. This
should be scattered showers. An isolated thunderstorm is possible
after 8-9z but the chances are too low to put in the TAFs. Winds will
gradually be shifting from the south to the southwest overnight and
be around 10kts. CIGS will likely drop to IFR with the approach of
the rainfall and remain around 900-1500ft through late Saturday
morning. Confidence is not high on the exact category of the CIGS but
visibility values should stay above 3SM. A mostly dry cold front
will move through Saturday afternoon with winds shifting to the
northwest with gusts around 20kt, possibly higher for an hour or two.





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