Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 270912

Area Forecast Discussion
National Weather Service Huntsville AL
412 AM CDT Sat May 27 2017

.NEAR TERM...(Today)
Issued at 412 AM CDT Sat May 27 2017

The uncertainties remain quite high regarding the forecast over the
next 24 to 48 hours. A weakening ridge remains over the forecast
area, while a shortwave was moving eastward across the Mid West. This
impulse was sparking convection generally along the
Missouri/Arkansas border. Closer to home, dry conditions persisted
across the TN Valley as of 4 am, with low clouds beginning to stream
eastward over the area. Temperatures remained in the upper 60s to
lower 70s, with dewpoints already in the lower 60s. Expect the quiet
conditions to continue through the remainder of the predawn hours,
with temperatures remaining near steady.

The mid/upper ridge is expected to weaken further during the day,
with some weak height falls forecast this afternoon as the shortwave
to our west moves eastward. Many questions remain as to the
convective forecast, as several scenarios could play out. For
starters, we will be watching the cluster of storms developing over
Missouri/Arkansas to see how that progresses. Although short term
guidance is somewhat differing on exactly where the MCS ends up,
their general timing seems to be fairly consistent. The varying
solutions have the MCS remaining to our north altogether or decaying
as it moves eastward this morning. With either solution, this would
leave the potential for the TN Valley to see convective initiation
during the afternoon along any outflows that are generated from the
MCS. There is still some potential for the MCS to make a more
southeastward turn and impact the forecast area, however at this
point, the steering flow is a bit too eastward for this to occur.
Therefore, am leaning towards the MCS decaying but clipping extreme
northern AL and our TN counties, with the potential for redevelopment
along an outflow this afternoon. Most of this activity will push
east of the area just before sunset, though some lingering storms
will be possible. With that said, confidence is not high at all,
especially given that the MCS is still well to our west. So the
forecast will likely change once we have a better idea of the
mesoscale features later this morning.

Looking at the severe potential, parameters still suggest that if
storms do impact this forecast area later in the day they will have
the potential to reach severe criteria. As stated in previous
discussion, dewpoints will continue to rise into the upper 60s to
lower 70s. We will likely have enough time to warm up before we see
any activity, and with temperatures currently in the upper 60s, it
won`t take much of a warm up. Guidance has come down how strong the
capping inversion is today, due to a weaker trend in the ridge. So
storms will be able to tap into the high amount of instability. Wind
shear still looks to be in the 40 to 50 kt range, so thunderstorms
will be able maintain their updrafts and remain organized. Soundings
show most of the instability within the hail growth zone, so the
development of hail is likely, though may be limited due to the warm
atmosphere and wet bulb zero heights above 12000 feet. Given drier
air aloft and DCAPE values in the nearing 1000 j/kg, a greater
potential will be damaging straight line winds. Stronger storms will
also carry the threat for very heavy rainfall. Although it does not
look like a set up for training storms, if we do see slower movement
to storms, a localized flash flood threat will exist.

.SHORT TERM...(Tonight through Monday)
Issued at 412 AM CDT Sat May 27 2017

An upper level trough will move into the Central Plains region late
tonight, with a cold front trailing southward from the parent low
over the midwest. Another shortwave is expected to develop on the
southeast periphery of the trough and move eastward overnight.
Models tend to agree on the development of another MCS late tonight,
however their agreement on the overall timing/solution of this
system is very different. It looks like the greatest potential for
the TN Valley will be this MCS moving in during the early morning
hours and perhaps lingering through midday, before exiting the area
or decaying completely. Depending on when this system reaches the
area will determine the severity. Guidance develops another cap at
850 mb overnight, though not sure how much I buy into that. But with
this system potentially moving in during the predawn hours, there
may not be as much instability to work with. Even still, if the low
level cap does not develop, there will be plenty of surface based
CAPE remaining across the area as our dewpoints remain near 70
degrees and we don`t cool off too much. Additionally, bulk shear
values remain sufficient for organized activity. So, these storms
will still carry the potential for a damaging straight line wind
threat and severe hail. The highest potential will generally be
north of the TN River.

This activity will push out of the area by midday, leaving a
potential lull in the activity. There may be an isolated thunderstorm
Sunday afternoon, but most of the area is expected to remain dry.
The aforementioned cold front will continue to progress eastward
with showers and thunderstorms reaching NW AL during the early
evening hours on Sunday. Guidance differs somewhat on the coverage
of thunderstorms, given the relatively weak forcing associated with
this front. However, there is some agreement on the timing. The
storms will continue to move slowly eastward through the evening and
overnight hours, but may not make too much of a southern progression
as the cold front looks to stall across our southern continues.
Although most of the upper level dynamics will remain well north of
the area, there will still be a chance for an isolated strong to
marginally severe thunderstorm along the front. The same parameters
will be in place as mentioned above, though shear may drop off just a
bit overnight Sunday and into Monday.

The heavy rain threat will also be possible with storms that move in
during the early morning hours on Sunday and again along the front.
However, the overall flash flooding threat will be contingent on how
much rainfall we see today. Again, not anticipating any training
storms, but with PW values reaching the 90 percentile, this potential
will need to be monitored.

Most of the activity will be across the southeast portion of the
forecast area at the start of Memorial Day, though isolated to
scattered thunderstorms will persist through the day. Although we
may see a higher coverage of storms than currently forecast,
confidence was not high enough to have anything higher than 50
percent. Fortunately, it looks like the severe potential will
finally come to an end across the area on Monday, as the wind shear
continues to diminish.

.LONG TERM...(Monday night through Friday)
Issued at 412 AM CDT Sat May 27 2017

The long term period is somewhat uncertain as models show the front
remaining stalled just to our south. This will keep the chance for
isolated to scattered thunderstorms in the forecast for Tuesday and
Wednesday, generally south of the TN River. The front will finally
move south of the area late Wednesday, with near zonal flow
persisting aloft. Diurnally driven thunderstorms will be possible on
Thursday and Friday, given the high amount of moisture and the lack
of subsidence. Temperatures will be near seasonal norms, with highs
in the mid 80s and lows in the 60s.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1234 AM CDT Sat May 27 2017

Only some scattered cirro-cumulus out there right now, but models
do show a broken deck of VFR cigs developing over the next couple of
hours at both terminals. After 27/12Z, models lower cigs even more
to around 2000 feet and keep mvfr cigs through 18Z on Saturday. Some
models do show precipitation pushing into the terminals, but other
show none developing until Saturday night. So will continue to leave
precipitation out of both terminals on Saturday.





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