Area Forecast Discussion
Issued by NWS Huntsville, AL

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477
FXUS64 KHUN 272332
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
632 PM CDT Sat May 27 2017

.UPDATE...
For 00Z TAFS.

&&

.NEAR TERM...(This afternoon and Tonight)
Issued at 250 PM CDT Sat May 27 2017

Convection continues to rapidly develop over southern MO near the
intersection of a remnant outflow boundary and a warm front. Further
E/SE towards the TN Valley satellite imagery is showing cloud
features more representative of a stable/capped environment
suggesting the CAP evident in 12z soundings was still in place to
some extent. The concern for the rest of the evening and into tonight
is, where does the eventual MCS track and will we see convection
develop closer to the area. There is a boundary evident in satellite
across central AR that extends east through central TN. Need to watch
this area closely as it could spark storms this afternoon and would
line up with what some of the hires guidance was showing with two
potential rounds of storms over the next 12-24 hours. Highly
uncertain though.

The storms to our NW have a better shot at being the main severe
event tonight. Mesoanalysis and guidance is showing a Theta-E axis
extending E/SE out of central MO down into central TN. This also
lines up with Corfidi vectors this evening, which show the potential
movement of the MCS into the area tonight. Timing of this remains a
question as there has been very little consistency in guidance over
the past few runs and even the hourly HRRR runs are very different.
With that in mind have opted to go with a slightly larger time range
for when we could see the MCS drop into the area.

A few things we do have confidence in is the amount of CAPE and shear
we have this afternoon through Sunday morning. Instability of between
2,500 J/kg up to 4,000 J/kg is possible over the TN Valley. Surface
to 6km shear will be between 40-50kts with low level shear increasing
to around 20-25kts later tonight. These values are highly conducive
to severe storms with damaging winds and large hail the main threats.
Depending on the exact movement of the MCS if it moves in a more
S/SE fashion it will be able to ingest some additional low level
helicity and a few circulations are possible along the leading edge
of the MCS (very low confidence though).

For the forecast tonight tended to lean more towards the ensemble CAM
guidance and some time lagged ensemble models. This would keep PoPs
in the 30-50 percent range through close to midnight, then begin to
increase PoPs out of the NW to SE with the possible MCS moving into
the area. It will be a warm and humid night with lows remaining in
the 70s and dewpoints in the upper 60s to around 70 degrees. Will be
paying close attention to the trends and movement of storms tonight.

.SHORT TERM...(Sunday through Monday)
Issued at 250 PM CDT Sat May 27 2017

If the MCS does move through tonight much of the hires guidance has
it through the area shortly after sunrise Sunday. However, the cold
front will still be lingering off the NW and should spark another
round of convection that will drop S/SE through the area Sunday
afternoon/night. Timing again is very uncertain with the MCS
potentially limiting activity as we may be too worked over. Plus, the
main synoptic forcing with the front will be off to the NE. CAPE and
shear parameters are a bit lower on Sunday and the mid-level dry air
we have over the area today will be gone. So, the potential for
the stronger damaging winds may not be as great. But hail and some
stronger wind gusts are still possible with storms along the front.
The forecast uses more of a blend because of the lingering
uncertainty but if we don`t have the MCS come through we could end up
with some severe storms on Sunday with the front.

With the front losing it`s main forcing it begins to slow down as it
moves through the area Sunday into Monday. This could shift the
threat from severe to more of a heavy rain and flooding threat on
Monday. Did stick to a blend of guidance for PoPs on Monday and kept
50 to 60 PoPs for much of the area during the day. Thunderstorm
chances begin to decrease as we lose heating Monday evening. With the
increased rain chances and cloud cover temps will be a little
"cooler" on Monday compared to today and Sunday. Hoping to see some
more consistency in the guidance in the next few runs.

.LONG TERM...(Monday night through Friday)
Issued at 250 PM CDT Sat May 27 2017

A broad upper trough remains over N. MN at 00z Monday with a cold
front that will essentially be stalled across N. AL. The upper low
remains wedged between strong high pressure over the N. Atlantic and
the N. Pacific with all the energy well to our north. Locally, a mid
level ridge anchored off the SE coast will allow short waves to ride
along it SW to NE across N. AL. Continued SW flow at the surface
will keep ushering moisture into the area as well. The upper low
finally opens up and meanders eastward on Friday. At the same time,
a weak upper low develops over the midwest and will track eastward.

What this all means is that isolated to scattered thunderstorms will
essentially stay in the forecast for the entire long term period,
Monday night through Saturday night. The highest coverages will be
in the afternoon but isolated showers/storms will probably linger
each evening. Shear is non existent most of the week so would
expected convectively induced showers and maybe a strong storm
possible each afternoon. The dynamics for the weekend look stronger
with greater instability so we`ll watch that more in coming
forecasts.

Temperatures for the week will be near normal with highs in the
lower 80s and lows in the lower to mid 60s.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 632 PM CDT Sat May 27 2017

Unsettled weather this TAF, as a series of convective systems move
NW to SE across the region. Even in the short term, there is low
confidence regarding timing of these systems. One of the models is
forecasting thunderstorm complexes now across the KY/TN border and
SE MO should move across this area in the late evening. Other output
has those systems impacting the region during the overnight. Stayed
with the quicker solution this issuance. Given an unstable atmosphere
across the region, strong erratic wind gusts are expected as storm
complex traverses the region. Have lowered VIS/CIG values into the
lower MVFR range, with IFR or lower values possible. MVFR CIGs
should continue to after sunrise Sunday, with scattered convection
into the early morning.

&&

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

$$

NEAR TERM...Stumpf
SHORT TERM...Stumpf
LONG TERM...LN
AVIATION...RSB


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