Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 251116 AAA
AFDHUN

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
616 AM CDT MON JUL 25 2016

.UPDATE...
For 12Z TAFs.

&&

.DISCUSSION...
(Issued 518 AM CDT MON JUL 25 2016)
The forecast area remains located beneath a broad weakness in the
subtropical ridge over the southern CONUS, with stronger
anticyclones centered across the Four Corners region and immediately
offshore the coast of the Carolinas. An inverted 500-mb trough --
currently across the eastern Gulf of Mexico -- will continue to
retrograde around the southern periphery of the subtropical ridge,
but is expected to decelerate as it approaches the weakness in the
subtropical ridge later today. The westward progression of the
inverted trough will effectively induce a southeasterly flow aloft
today, and this will allow a plume of drier mid-level air -- noted on
water vapor imagery across GA -- to spread into the region. At the
surface, a very weak cold front is expected to drift southeastward
across the Ohio valley, and although the primary boundary will
remain well to the north of our forecast area weak low-level
streamline confluence will be sufficient to support the development
of isolated- widely scattered afternoon/early evening convection.
However, a lower areal coverage of precipitation is anticipated
today, primarily due to the advection of drier air from the southeast
and slightly lower PWAT values. Less cloud cover and precipitation
will translate to very warm afternoon temps once again, with highs
rising into the m/u 90s and heat indices briefly peaking in the
105-107 degree range for portions of the area.

For the remainder of the short term period (Tuesday-Wednesday), short
term model guidance suggests that the inverted mid-level trough
will continue to push very slowly westward across the western Gulf
and adjacent portions of the nw Gulf coast. The subtropical high cell
centered off the coast of the Carolinas is forecast to expand
westward across the region, providing a continuation of hot
temperatures and no mid-level support for the development of
convection. However, it appears that the remnants of the weak frontal
boundary to the north will gradually shift southward into the TN
valley and lead to a gradual increase in the coverage of convection.
Although this process will likely begin Tuesday afternoon, a more
noticeable increase in shower/thunderstorm activity will be seen on
Wednesday as a deeper southerly flow develops...allowing PWAT values
to increase into the 1.7-1.9 inch range once again. Forecast
soundings from the NAM/GFS/ECMWF depict higher low-level dewpoints
and a greater amount of potential instability on both Tuesday and
Wednesday, and this should translate to stronger thunderstorms in
spite of very weak vertical wind shear /5-10 knots or less/.

A stronger/deeper southwest flow regime is predicted to evolve by
Wednesday night, as a series of weak vort lobes track east-
southeastward from the northern Rockies into the northern Plains --
amplifying a trough over the central Plains and forcing the Atlantic
coast subtropical high cell to retreat to the east. This trend will
continue on Thursday and strengthen Thursday night, as a stronger
shortwave trough moving across the northern Plains/upper Mississippi
valley amplifies the trough even further and increases the height
gradient along the northwestern periphery of the ridge. This change
in the synoptic pattern will become more favorable for several
episodes of convection to bring beneficial rainfall to the region --
beginning Thursday night and continuing through at least
Saturday/Sunday, as several low amplitude disturbances may spread
northeastward across the region during this period. Although forecast
soundings suggest weaker lapse rates aloft and slightly lower CAPE by
the end of the week, stronger lower-mid tropospheric flow/shear may
still yield organized convection. Temperatures will respond to the
increase in cloud cover and precipitation, with highs in the u80s and
lows in the l70s.

70/DD

&&

.AVIATION...
For 12Z TAFs:

VFR conditions will prevail at the KMSL and KHSV terminals through
the forecast period. An isolated thunderstorm will be possible this
afternoon, however coverage is too low to add mention to the TAFs.
Additionally, BR may develop early Tuesday morning and reduce
visibilities into the MVFR category. However this will greatly depend
on if precip occurs at the TAF site and therefore will not include in
this issuance.

73

&&

.HUN Watches/Warnings/Advisories...
AL...NONE.
TN...NONE.
&&

$$

For more information please visit our website
at weather.gov/huntsville.



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