Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 241432 AAB

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
932 AM CDT Thu May 24 2018

.NEAR TERM...(Rest of Today)
Issued at 932 AM CDT Thu May 24 2018

Visible imagery indicates a clear sky for nearly all of the forecast
area as of 1430Z. Mid to high level dry air was noted on the OHX
sounding this morning, with a sharp contrast to deeper moisture at
BMX near a weak frontal boundary. The boundary stretched from
southern MO through northern MS to the Birmingham and Atlanta areas.
This will be the focus for showers and thunderstorms this afternoon
just to our south and southwest. We will maintain a SChc PoP in our
far south/southwest areas this afternoon. The HRRR and NAM suggest
that isolated development may occur early this evening along the
Cumberland Plateau/Sand/Lookout Mtn areas, so will keep a SChC going
in these areas from 00-03Z. Otherwise, another warm day is ahead, but
dew points may dip into lower 60s for a time this afternoon during
peak mixing.

.SHORT TERM...(Friday through Saturday)
Issued at 350 AM CDT Thu May 24 2018

The dry airmass should retreat to the north as a warm front lifts
northward on Friday. This also occurs as a weak shortwave trough
moves north from central GA/AL to north AL and another weak shortwave
trough moving across the MO/mid MS Valley. What this could translate
to is a couple of separate waves of storms on Friday afternoon and
evening which with a saturating profile would act to increase
coverage of storms, and heavy rainfall potential. Retained the
ongoing 40-50% POPs for that period given this potential and also due
to the fact that there is some uncertainty on whether the shortwave
trough materializes from central GA to northern AL. Weak steering
flow should translate to slow moving storms and isolated flash
flooding. Daytime highs should still register in the low 90s. Outflow
boundaries may result in additional storms developing through the
evening, but this is strongly dependent on extent of daytime
convective activity.

Meanwhile, the models continues to depict a tropical wave moving into
the Gulf of Mexico at differing trajectories and strength. More
indirect effects could occur (saturation of the column) on Saturday.
However, expecting that moisture transport across the TN Valley will
continue to increase PWATs and saturate the boundary layer. The slow
moving shortwave trough across the MS River Valley should provide the
lift necessary for numerous showers and thunderstorms on Saturday
especially during the afternoon hours. With the moist adiabatic
profile, any storms should remain ordinary in nature but could pose a
flash flooding threat. Increased cloud cover should keep daytime
high temperatures in the mid 80s.

.LONG TERM...(Saturday night through Wednesday)
Issued at 350 AM CDT Thu May 24 2018

Soggy conditions will continue over the "official" start of summer
holiday weekend and early next week. On a big picture, a blocky
mid/upper flow pattern was in place over the northern hemisphere.
An Omega-type block was situated over the Europe and western Asia
region at the start of extended period. This block will break down
somewhat early next week; but Rex-type blocking will become
established over the NW North American domain early next week. This
will maintain a generalized troughing west and ridging west pattern.

To complicate matters even further, a moist tropical-like atmosphere
will be firmly ensconced over the southeastern CONUS. This unstable
airmass, along with daytime heating will result in a diurnal setup of
better rain chances in the daytime hours for the extended.

The main feature concerning the extended forecast - is what will
become of a system trying to form over the western Caribbean Sea and
Yucatan Penn. Per model output, this system will continue moving
northward into the central Gulf, and possibility develop into a
subtropical or tropical low during the Saturday and Sunday time
frame. Water temperatures in the favored "path" of this system were
into the lower 80s, warm enough for TC formation. The path this
system takes were complicated by a few items. The 00Z Canadian was
not available this morning (which was the most bullish models
regarding its development potential in comparison to the others).
Also, the 00Z GFS has come around to the previous Canadian, and
current ECMWF and end of the NAM`s run, with a more western track.
The GFS/ECMWF were in reasonable agreement regarding the eventual
path of this system into Sunday. Afterward, the EC takes a decidedly
more NW track, moving it slowly over southern/central Louisiana on
Mon/Tue. The GFS, though more westward than its previous tracks, has
it moving across the Florida panhandle east of Panama City, then
tracking along the AL/GA border into the middle of next week. The
GFS-Ensemble was close to the deterministic model, then took a more
NW trend sort of following the EC. Went with a more westward

As stated above, this will bring wet weather across the Tennessee
Valley into the first half of next week. Rain chance wise, scaled
back rain chances somewhat on Sunday, as the models did suggest
somewhat of a delay in showers affecting the region. Shower chances
will pick up more Sunday night and Memorial Day onward. Precipitable
water amounts in the atmosphere Saturday night were in the 1.8 inch
range, which should decrease into the 1.5 inch range on Sunday. PW
amounts increase towards the 2" range on Mon/Tue as the tropical
system nears the region. Widespread showers/thunderstorms with
torrential tropical rains could start causing hydrologic issues after
Memorial day, with flash and areal flooding a possibility, along with
rises in creek, stream, and river levels towards the middle of next
week. With more clouds than sun, daily high temperatures into early
next week should trend a tad below seasonable levels -- mostly in the
lower 80s.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 624 AM CDT Thu May 24 2018

VFR conditions are expected to continue across north Alabama/southern
middle Tennessee for the next 24 hours. Isolated thunderstorms are
expected this afternoon mainly south of the Tennessee River. As a
result of the isolated nature of the storms, have not included in the
TAFs. Otherwise, there is the potential for fog formation late in the
period due to increased humidity levels and light winds.





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