Area Forecast Discussion
Issued by NWS Huntsville, AL

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745
FXUS64 KHUN 261131 AAA
AFDHUN

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
631 AM CDT TUE JUL 26 2016

.UPDATE...
For 12Z TAFs.

&&

.DISCUSSION...
(Issued 450 AM CDT TUE JUL 26 2016)
Water vapor satellite data confirms that a weak 500-mb inverted
trough currently across the western Gulf of Mexico continues to
shift slowly westward this morning. Short term model guidance
suggests that the retrogression of this wave should persist today and
tonight, as a subtropical high centered off the coast of NC/SC begins
to expand westward into the central TN valley. A weak vorticity max
pivoting nwwd across the Gulf coast is likely providing the mid-level
ascent necessary for the generation of isolated shower activity
around the region this morning -- as the northern extent of this
feature continues to graze our region -- but the impact of this vort
max is expected to diminish later this morning, as it advances into
the lower Miss valley and anticyclonic flow strengthens aloft. At the
surface, a weak/diffuse cold front remains nearly stationary this
morning near the Ohio river, and this boundary is expected to move
little -- if at all -- today and tonight. However, it does appear
that very weak low-level convergence will exist across the forecast
area today, provided by low-level ssely flow strengthening from
south-to-north across the region. Model consensus indicates that the
strongest mass convergence will exist near and north of the state
line late this afternoon and early this evening -- where scattered
convective cells should evolve as surface temps warm into the mid
90s. In spite of weak vertical shear, MLCAPE values approaching 2000
J/kg by late afternoon will lead to a threat for strong wind gusts
with the most intense storms.

Although the position of the Atlantic subtropical ridge will remain
virtually unchanged during the period from Wednesday-Thursday, a
series of lower amplitude disturbances embedded within the mid-
latitude westerlies will spread esewd from Montana across the Dakotas
and into the upper Miss valley. This will effectively amplify a
longwave trough across the central Plains, resulting in stronger and
deeper southwesterly flow across our region -- which will ultimately
bring much needed relief from the heat as well as at least a short
term improvement in drought conditions. There is still a bit of
uncertainty regarding when the onset of the pattern featuring more
widespread precipitation and cooler temperatures will occur, but
convection may begin to increase as early as sunrise tomorrow morning
-- especially to the s & w of Huntsville -- as strengthening
southerly flow around the retreating subtropical ridge forces a
rapid return of tropical moisture /PWAT values rising up to around 2
inches/. A much greater areal coverage of convection is expected
Wednesday, as remnants of Gulf inverted trough become absorbed in the
strengthening flow aloft -- enhancing large scale ascent in the very
moist /PWATs 2-2.25 inches/ and unstable /CAPE 1500-2500 J/kg/
airmass. Deep-layer flow will increase further and veer to the
southwest Wednesday night-Thursday as central Plains longwave trough
axis slowly spreads eastward toward the Miss valley, and this will
maintain a favorable combination of rich moisture and ascent for the
generation of several episodes of much needed rainfall. Although
winds through the lower-middle troposphere will be stronger during
this period, nearly moist diabatic lapse rates aloft will limit
destabilization and the threat for strong-severe storms. Due to
clouds and precip, expect highs on Wednesday-Thursday to only reach
the 85-90 range.

Global models suggest that the synoptic pattern previously described
will persist through at least the first half of the upcoming
weekend, with episodes of showers and thunderstorms possible both
during the day and at night. Although the coverage of convection
during any one particular period may be enhanced by the passage of
weak disturbances in the southwest flow aloft, these features are
impossible to predict this far in advance and 50-60% POPs will be
carried through the entire period. There are indications that a
gradual reduction in the magnitude of the swly flow aloft may occur
during the second half of the weekend and early next week, which
would likely translate to a diurnal precipitation pattern more
typical of late July/early August. POPS have been slightly lowered
next Sunday-Tuesday to reflect this, with only a gradual increase in
temperatures expected based on position of residual 500-mb trough
axis across the region and moist ground conditions expected by this
point.

70/DD

&&

.AVIATION...
For 12Z TAFs:

VFR conditions will prevail through the next 24 hours at the KMSL and
KHSV terminals. Isolated showers and thunderstorms are possible this
afternoon, however confidence/coverage of storms is too low to add to
the TAFs. Otherwise, partly cloudy skies and light winds are
expected.

73

&&

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

$$

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



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