Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 300028 AAC

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
728 PM CDT Sat Apr 29 2017

.NEAR TERM...(Tonight)
Issued at 722 PM CDT Sat Apr 29 2017

Regional radar imagery shows isolated convection still ongoing across
portions of northern Georgia and middle/eastern Tennessee early this
evening. Across our area, the earlier shower/thunderstorms
development has either dissipated or lifted to the north leaving
mostly clear skies in its wake. Looking at the soundings from the RAP
indicates a fairly unstable airmass, though with the loss of daytime
heating and no discernible trigger expect a fairly quiet remainder
of the evening.

We are monitoring a couple of small convective clusters/complexes
that have not been handled that well by the high res models today.
One was located across the MS Delta Region with another across
southern Louisiana. The mean flow should keep this activity west of
our region tonight but it may have some impact on the evolution and
timing of the line moving across on Sunday.

For the overnight, have not made too many changes overall. Have made
some minor tweak to the cloud cover based on the IR/VIS trends and
also adjusted the temps based on current trends and the shortblend
guidance. No other changes planned.

.SHORT TERM...(Sunday through Monday)
Issued at 300 PM CDT Sat Apr 29 2017

Current satellite imagery shows a mesoscale convective complex
developing over northern Louisiana. This is south of the main frontal
boundary over Oklahoma northeast into Missouri and northern Indiana.
As this front develops and pushes slowly east toward the Tennessee
Valley, this may affect the evolution of the pre-frontal trough axis
on Sunday and how far east it initially develops. Most synoptic
operational models do not show this feature, including the NAM12.
These operational models hold off shower and thunderstorm activity
with the prefrontal trough axis until between 4 PM and 8 PM in
northwestern Alabama and continue this activity through the overnight
hours. However, after midnight with lessening instability the severe
storms do not look as likely east of I-65. Either way the timing
works out, the threats remain similar to previous forecasts. The main
threat will be damaging winds. A tornado or two cannot be ruled out
(mainly west of I-65). Heavy rainfall between 1 and 2 inches with
locally higher amount up to 3 inches look possible given strong
forcing and high instability. A few showers could linger into Monday
as drier and cooler air moves into the region.

.LONG TERM...(Monday night through Friday)
Issued at 300 PM CDT Sat Apr 29 2017

The cold front responsible for much of the active wx ending
the weekend period should be well e of the TN Valley into
the Atlantic Basin going into Tue, as high pressure out of the
srn Plains quickly works its way into the SE region. Other than
perhaps a brief shot of cooler/drier air during the first half
of the new work week, little relief in overall temps is xpcted with
this frontal passage thru mid week, as the low level wind field veers
back to the se/s and the flow pattern aloft turns more toward the
wsw. Afternoon temps both Tue/Wed look to return close to 80F, as
another upper trough pattern drops well into the srn Plains states.

Unfortunately, there is a large disconnect with the progression of
this upper trough axis and its associated sfc wave between the latest
12Z guidance and the previous 00Z model runs. The latest model suites
are now hinting at a more vigorous upper low/trough system gradually
traversing enewd into the wrn/cntrl Gulf states for the second half
of the work week, compared to the faster 00z solutions. This scenario
does at least maintain showers/tstms developing Wed/Thu, as the
initial sfc wave and its associated cold front translate ewd across
the cntrl TN Valley. However, the prob may now exists for secondary
sfc waves to develop along the upper low/trough axis lingering back
to the w. This now results in iso/sct showers into Fri as the upper
low/trough gradually moves ewd across the cntrl Gulf region.


.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 635 PM CDT Sat Apr 29 2017

VFR conditions should continue through the evening and much of the
overnight hours as the area remains sandwiched between a high off the
southeast US coast and a strengthening system across the Southern
Plains. The pressure gradient between the two systems will keep
rather stout southerly winds through the TAF period, and have made
only minor changes to the previous issuance.

Conditions will be marginal for LLWS at both sites late tonight into
early Sunday but did not include at this time. Will monitor trends
before next TAF issuance. Expect a fairly quick influx of low clouds
due to isentropic upglide and warm advection toward morning with a
period of MVFR ceilings likely. For the time being, we capped the
maximum gusts at each site at 30 knots though a few higher gusts are
likely given the tightening gradient. Model guidance has some notable
discrepancies with the timing of the approaching line/convection
Sunday afternoon so only included Prob40 for the time being.


AL...Wind Advisory from 10 AM to 10 PM CDT Sunday for ALZ001>010-016.

TN...Wind Advisory from 10 AM to 10 PM CDT Sunday for TNZ076-096-097.




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