Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 262350 AAB
AFDHUN

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
650 PM CDT Sun Mar 26 2017

.NEAR TERM...(Tonight)
Issued at 650 PM CDT Sun Mar 26 2017

Updated the gridded forecast to remove showers and thunderstorm
chance this evening. With satellite trends indicating a clear or
mostly clear sky, have also adjusted sky grids, with increasing
clouds and possible patchy fog development holding off until after
midnight.

.SHORT TERM...(Monday through Tuesday)
Issued at 238 PM CDT Sun Mar 26 2017

The shortwave trough moving across the Ozarks will move across a
conditionally unstable airmass and provide the necessary wind
shear/lift to support severe weather over the ARK/TX/OK regions
tonight into the early morning hours on Monday. The surface low will
move to the northeast just ahead of the trough. As this occurs the
warm conveyor belt will increase in strength across the northwest
gulf Coast inland to the MS River Valley. This along with cooling
lapse rates aloft will provide a conditionally unstable environment
(1000-2500 J/kg) across the MS and TN River Valleys late Monday
morning into the afternoon/evening. Also noted in the profile is a
low-level capping inversion that slowly erodes during the afternoon.
Bulk shear values (deep layer) are around 35-45 kts during peak
daytime heating. The main limitation for the TN Valley will be the
strength/extent of a lifting mechanism. The latest 12Z GFS run shows
that there could be a weak shortwave trough/vorticity axis moving
across between 16Z-23Z which could provide forcing for convection.
Given the uncertainty with the spatial coverage am not confident if
this afternoon episode will materialize, and have structured the
POPs/Wx to reflect a lower POP in the afternoon. However, if any
storms develop, with the high amount of instability in the hail
growth zone, wind shear, and eroding cap aloft could see very large
hail and damaging winds with discrete cells that initiate in the
afternoon. Could even see supercells with this environment (curved
hodographs, low-level veering wind profile) with rotation especially
further west closer to the surface low. However, not confident enough
in the the threat continuing into the evening as the trailing cold
front moves east over N MS--contributing to additional discrete
cells developing. However, the diurnally driven sb instability
decreases which will keep rising parcels elevated in nature as steep
lapse rates in the hail growth zone remain.

The instability does decrease further east and anticipating that
storms will likely weaken as they progress east especially during the
evening hours. Then, the southerly flank of the cold front becomes
oriented from SW to NE--slowly moving east as the shortwave trough
moves east over the OH River valley, and the sfc low moves off to the
northeast along the front. As the front approaches within the
convectively overturned environment, could see some additional
showers late on Monday night/Tuesday morning before the front stalls
somewhere over N Al. This front may provide additional storm
development on Tuesday afternoon (mainly over southern portion of
CWA). On temperatures, though this is a front, little in the way of
temperature change is anticipated with daytime highs in the mid 70s
expected on Tuesday as much of the `cool` air stays to the northwest.
Then, with light winds and clearing clouds, could see additional fog
development on Tuesday night within the `warm` sector over portions
of the CWA.

.LONG TERM...(Tuesday night through Saturday)
Issued at 238 PM CDT Sun Mar 26 2017

An active pattern looks to continue into the long term with a deep
cut off low over the TX panhandle Wed afternoon. A warm front will
be stretched out east of that, most likely across central MS/AL, to
our south. Guidance is in good agreement that this warm front will
push north Wed evening/Thu morning with winds initially going
southerly but then shift back southeasterly briefly overnight as the
upper ridge shifts east. Even though the deep layer moisture isn`t
quite there on Wednesday, the warm front will bring around 1500 J/KG
of CAPE with it so wouldn`t be surprised to see a line of
thunderstorms moving south to north with it. The shear is basically
nonexistent so getting anything severe will be difficult. Wednesday
will be warm behind the front, even with a SE flow, with highs in
the upper 70s to near 80. We then start to get back into the
southerly flow by Thursday morning as the cold front and precip
approaches.

GFS/ECMWF/CMC are in reasonably good agreement at least from a
synoptic stand point with the 12z GFS coming into better alignment
with the ECMWF (00z and 12z are similar) and a slightly further north
surface low placement. The timing of the line is still uncertain and
will play a role in how strong the storms end up being. Instability
will also be a major limiting factor with the best instability in MS
and to the south of us. All three models are showing the possibility
of a surface low forming along the gulf coast and cutting off our
moisture and instability. However, given the strong upper dynamics,
strong shear (40-60kt at 0-6 km and 20- 40kts at 0-1km,depending on
the mode) and decent mid level lapse rates, we may not need much more
instability to produce strong to severe storms. Think that the
greatest severe threat will be west and then south but there is still
a decent chance at a few severe storms with damaging winds being the
greatest threat at this time. Stay tuned! It will, however, be
pretty warm again on Thursday with highs in the upper 70s.

The cold front moves through by Friday night with winds shifting
around to the NW. Lingering showers with maybe a thunderstorm or two
will linger on Friday as moisture remains and weak waves rotate
around the base of the departing upper trough. Temps will start to
get cooler with highs around 70 degrees. So far it looks like Friday
night through Saturday night should be dry as one trough departs the
east coast and another starts to dig into the desert southwest. The
local area remains under a weak mid level ridge until it shifts
offshore Sunday with a southerly return flow building back in. The
next system may start to impact us on Sunday but have kept thunder
out at this time due to timing uncertainties. Temps will be warming
again with highs Sat and Sun in the lower 70s.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 539 PM CDT Sun Mar 26 2017

VFR flight weather conditions will last through this evening and
possibly overnight. Southerly flow will bring scattered clouds at
~025agl by ~08Z, with some potential for MVFR ceilings as well. The
better chance that ceilings will arrive will be ~13Z. The clouds will
lift into a scattered cumulus deck by 19Z with south flow gusting at
18-20kt during the afternoon hours. Scattered to numerous showers and
thunderstorms will develop in northwest AL during the afternoon, so
have included a VCTS at KMSL.


&&

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

$$

NEAR TERM...17
SHORT TERM...SL.77
LONG TERM...LN
AVIATION...17


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