Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 240007

Area Forecast Discussion
National Weather Service Huntsville AL
707 PM CDT Sun Sep 23 2018

For 00Z TAFS.


.NEAR TERM...(This Evening and Tonight)
Issued at 235 PM CDT Sun Sep 23 2018

A cold front remains draped just to the northwest of the area and is
serving as a focus for a broad area of thunderstorms stretching from
the ArkLaTex into the Central Appalachians. Any of the deeper forcing
from this feature is displaced to our northwest, and most of our
scattered activity has been a result of the deep moisture in place
and diurnal heating. The latest CAMs continue to favor this more
scattered convection through this evening, with perhaps some more
numerous clusters pushing into far Northwest Alabama and Southern
Middle Tennessee between 21-03z.

A 19z SPC mesoanalysis indicates an axis of instability from Central
Mississippi into Western Tennessee, with some values of 2000 J/kg of
SBCAPE and 1000 J/Kg of MLCAPE just clipping Northwest Alabama.
These values are generally less across North Central and Northeast
Alabama. As a result, would think that any threat for a stronger
storm would be more focused across areas just to the north and west
of the region. Any storms that do get going will be very short-
lived, with lightning being the main threat to those outdoors. PWATs,
however, have climbed to around 1.7 to 1.9 inches, making heavy
downpours a much bigger threat with this activity. As was the case
yesterday, due to the somewhat limited coverage, am not expecting a
widespread heavy rainfall/flood threat. However, we`ll need to watch
rainfall amounts and stream flow guidance in areas where training may
occur -- most likely across Northwest Alabama.

The aforementioned front will not move much overnight, though it may
start to creep a little further to the SE, closer to far NW Alabama.
For this reason, will leave likely PoPs across the NW Tennessee
Valley through 03z this evening. However, most of the diurnal
convection will dissipate after sunset. Still, a few lingering
outflow boundaries and the front will keep isolated to widely
scattered showers a possibility overnight. Given the lack of fog last
night and the lower clouds expected to develop overnight, will keep
a mention of patchy fog out of the forecast for now -- though the
boundary layer would support it if some clearing were to occur.

.SHORT TERM...(Monday through Tuesday)
Issued at 235 PM CDT Sun Sep 23 2018

Models are still a little uncertain as to the progress of the
aforementioned front early this week. Most solutions either keep it
just out of the area (over N. Mississippi and Middle Tennessee) or
bring it into portions of Northern Alabama. For this reason,
uncertainties remain on storm coverage and QPF early this week.
Regardless, guidance is suggesting the potential for several
shortwaves/vort-max`s to rotate through, thanks to a gradually
amplifying upper-trough over the Great Plains. All this to say, there
appears to be a slightly better opportunity for scattered to
numerous showers and thunderstorms both Monday and Tuesday, thanks to
this better forcing.

As discussed above, moisture will not be an issue as southwest flow
will help to reinforce some of this deep Gulf moisture in place over
the region. With PWATs near 2", it will not take much for heavy
downpours to occur with even moderate shower activity. Thus, we`ll
have to watch areas where slow-moving, heavy rainfall activity sets
up in the coming days (even with as dry as it`s been). The good news
is that at least earlier in the week, most areas should be able to
absorb some of this rain, provided it doesn`t all occur at once. The
thermodynamic and shear profile would not favor any organized and/or
strong thunderstorm activity at this time -- just a heavy rainfall
potential in areas where training occurs.

.LONG TERM...(Tuesday night through Saturday)
Issued at 235 PM CDT Sun Sep 23 2018

The main window for locally heavy rainfall and the highest confidence
for shower/thunderstorm coverage appears to be from Tuesday night
through the day on Thursday. A stronger mid/upper-level trough will
push through the Ohio Valley which should shove whatever is left of
this boundary into the area. In addition to the frontal lift, a weak
850 mb LLJ ramps up across North Alabama into East Tennessee
Tuesday night, placing the area in the right entrance region. Thus,
confidence is increasing for the potential for numerous to
widespread showers and storms during this period.

Though the rainfall will not come all at once, the latest storm
total QPF for our area is running between 2" to 3", with higher
amounts across Northwest Alabama through 12z Friday morning. In
situations like these there will most certainly be locally higher
amounts. Thus, there will be at least a localized flash flood threat
during the middle of the week and forecast QPF trends will need to
be monitored. Given that there is still some uncertainty on how
precipitation and rainfall earlier in the week will evolve, will
just highlight this possibility in the HWO for now. Given the cloud
cover and rain chances, we may be looking at upper 70s highs (but
upper 60s lows) during this timeframe should sky conditions remain

Beyond Thursday night into Friday, there continues to be evidence
that a broad subtropical ridge may form along the Northern Gulf Coast
late this week into the weekend. This would force the remnant
boundary back north and result in some drier weather. Thus, the
arrival of true "autumn" weather will likely have to be postponed, at
least for a few more weeks. In this situation, we`d be looking at
mid to upper 80s during the day and 60s at night -- with southerly
flow maintaining the Gulf moisture over the region.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 707 PM CDT Sun Sep 23 2018

A subtle low-level convergence boundary will remain the focus for
widespread rain/embedded tsra this evening, before beginning to lift
nwd and away from the region after 06Z. Due to the proximity of this
boundary to nw AL, we will include prevailing -RA and VCTS in TAFMSL
thru 06Z. However, current radar imagery also warrants including VCSH
for TAFHSV, but only thru 03Z. Otherwise, we anticipate VFR
altostratus cigs btwn FL080-180 to persist overnight along with ESE
flow of 3-5 knots. There is some concern that convection may
redevelop along a lingering outflow bndry across central AL and move
nwd into the region prior to 12Z, but at this time we will only
include a tempo group for development of a lower stratus deck btwn
10-14Z. Although the sfc boundary will be even further displaced from
the TN Valley tomorrow, a passing upper-level wave will initiate
fairly widespread convection in the moist/deep southerly flow regime
locally. We will reintroduce VCTS at 15Z, with PROB30 groups included
for both terminals btwn 18-22Z to indicate the increasing likelihood
of MVFR cigs/vsby in tsra.





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