Flat Rock, Alabama Weather Forecast Discussion

441
FXUS64 KHUN 162336
AFDHUN
Area Forecast Discussion
National Weather Service Huntsville AL
636 PM CDT Tue Sep 16 2025
...New AVIATION...
.NEAR TERM...
(Rest of Today and Tonight)
Issued at 146 PM CDT Tue Sep 16 2025
As anticipated, a cluster of showers has developed across southern
Kentucky into northern middle Tennessee and begun producing
lightning as the previously mentioned upper level disturbance
rotates around the backside of the upper level low to our east.
Low level lapse rates in excess of 8.5C/km may support a strong
wind threat with any established updrafts, though intensity should
be limited by a temperature inversion at 700mb. Additionally,
PWAT values in the 1.2in-1.3in range could support locally heavier
rainfall. In the mid - upper levels, N to NNE flow at 15-25kts
will allow these showers/t-storms to advance southward with time,
generally reaching the southern middle Tennessee region by the
late afternoon. The threat of t-storms should wane as the sun sets
and instability begins to diminish, though rain will likely
persist overnight. However, there is uncertainty in current model
guidance regarding the coverage of rain, and for this reason we
have kept PoPs at 30%. Temperatures are on track to reach
forecasted highs, with most of the region currently at u80`s to
l90`s. Areas that receive rainfall in addition to river valleys
and lakes will be prone to patchy fog development through the
overnight into early morning timeframe as partly cloudy skies
persist with temperatures cooling into the m60`s to u60`s
overnight.
Previous Discussion:
In the mid/upper-levels, NNE flow of 15-25 knots will continue
across the TN Valley today, before diminishing into the 5-15 knot
range overnight as an upper-level cyclone shifts north-
northeastward across eastern portions of NC/VA. At the surface,
our forecast area will remain within a region of light SSE-
light/variable flow between a weakening area of high pressure
across western VA/NC and an ill-defined surface trough arcing from
the Lower MS Valley northeastward into northern portions of the
TN Valley. Latest model guidance suggests that at least 1-2
distinct but fairly small vorticity maxima (embedded within the
flow around the low to our east) will provide sufficient lift to
generate scattered showers and thunderstorms (beginning early this
afternoon in the general vicinity of the surface confluence axis
to our north), which should spread generally southward with time
and impact the local forecast area later today. For this reason,
we have increased POPs into the low chance range for the entire
CWFA this afternoon. Although the combination of afternoon
temperatures in the m80s-l90s and dewpoints in the u50s-l60s will
yield CAPE of 500-1000 J/kg (perhaps a bit higher in the
northwest), a shallow inversion in the 600-700 mb layer will
likely limit updraft intensity, yielding lightning, locally heavy
rainfall and perhaps a brief wind gust up to 30-40 MPH as the main
storm impacts.
Present indications are that there will be essentially no change
in the general synoptic pattern (described above) overnight, and
we will continue to advertise a low chance POP for the entire
forecast area. Although the risk for lightning and thunder will
gradually diminish due to nocturnal stabilization of the boundary
layer, POPs may actually be a bit on the low side and may need to
be increased in future forecast updates. In anticipation of mostly
cloudy skies and little change in the thermodynamic properties of
the local airmass, overnight lows will be a bit warmer (compared
to previous nights) and in the l-m 60s.
&&
.SHORT TERM...
(Wednesday through Friday)
Issued at 1115 AM CDT Tue Sep 16 2025
Short range model data is in fairly good agreement in depicting a
lighter (5-15 knot) northerly flow aloft to continue across the
TN Valley on Wednesday and Wednesday night, in the wake of an
upper low that will begin to accelerate northeastward across the
Mid-Atlantic states before phasing with the northern branch of the
jet. Although a couple of additional (but weaker) disturbances
aloft may initiate additional showers and thunderstorms on
Wednesday afternoon/evening, latest solutions from both the CAMs
and global models suggest a lower spatial coverage, which may
ultimately be focused south of the TN River (as a slightly drier
airmass will begin to spread into the region from the northeast).
For this reason, we have indicated a gradual decreasing trend in
rain chances beginning tomorrow afternoon/evening. Highs tomorrow
will once again be in the m80s-l90s, and with similar values of
CAPE anticipated, impacts from any thunderstorms should be similar
to what we experience today. Lows on Thursday morning will remain
in the l-m 60s.
During the period from Thursday-Friday, a 500-mb shortwave ridge
will translate southeastward across the region to the east of a
complex upper low (featuring several embedded circulations) over
the northern Plains. Profiles will dry considerably as this
occurs, and with PWAT values predicted to fall from 1.2-1.3"
(currently) to near or slightly less than 1" we will maintain a
dry forecast for this timeframe. A subsequent increase in
afternoon temperatures is also expected, with highs for most
valley locations reaching the l-m 90s by Friday.
&&
.LONG TERM...
(Friday night through Tuesday)
Issued at 856 PM CDT Mon Sep 15 2025
Our pattern looks to change slightly through the long term with
the blocking pattern finally being disrupted. By the start of the
weekend troughing will have significantly elongated over the
Atlantic allowing ridging to cover most of the eastern CONUS.
Without any resistance, high pressure ridging looks to gradually
be pushed into the Atlantic as a deepening trough builds in the
plains and pushes east through the start of the new work week.
Locally this will result in a slow increase in Pops as the
attached surface cold front looks to brush the TN Valley. Models
currently struggle to take the front all the way through thus Pops
remain at or below 30% from Saturday through Tuesday. Coverage of
showers and storms will have an impact on our temperatures as
well leading to a little bit more uncertainty in seeing any
significant break from the summer like conditions. We will
continue to monitor this trend as it draws closer.
&&
.AVIATION...
(00Z TAFS)
Issued at 636 PM CDT Tue Sep 16 2025
Isolated to scattered convection moving southward around 10 mph
over middle Tennessee, continued on a dissipating trend. There
are slim chances that some of this activity could reach parts of
the area this evening, so have a VCSH for the mid/latter evening.
A 30 percent rain chance cannot be ruled out in the late evening
and part of the overnight. Odds for rain appeared to be lower with
later model runs; may remove the PROB30 group should later runs
continue a lower PoP trend. VFR weather is otherwise expected for
the TAF.
&&
.HUN WATCHES/WARNINGS/ADVISORIES...
AL...None.
TN...None.
&&
$$
NEAR TERM...AM
SHORT TERM....70
LONG TERM....RAD
AVIATION...RSB

NWS HUN Office Area Forecast Discussion