Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 261116

Area Forecast Discussion
National Weather Service Huntsville AL
616 AM CDT Wed Jun 26 2019

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 330 AM CDT Wed Jun 26 2019

High clouds continue to stream northeast along a weak convergence
boundary extending from central Louisiana into northwestern Alabama.
An area of light rain and virga is moving from southeastern
Mississippi into central Alabama at this time. Although cloud cover
is primarily high in nature, it should be thick enough to keep
temperatures from dropping below the lower 70s in many areas near and
west of the I-65 corridor (due to the very moist airmass in place
near the boundary). The exceptions to this would be southern middle
Tennessee and portions of northeastern Alabama, which saw some clear
skies earlier and have already dropped into the 66 to 68 degree range
(with a tad drier air in place). Cloud cover should keep even patchy
fog development at bay through daybreak, despite light winds.

As some weak upper level energy moves northeast across the area
today along this quasi-stationary boundary, guidance forecasts an
unstable airmass to be in place (SBCAPE between 2500 and 3500 J/KG).
Luckily, forcing will be weak and shear is forecast to be on the
order of 10 to 15 knots. So pulse convection at best is expected.
DCAPE values between 800 and 900 J/KG are not very impressive, but
with instability forecast some storms could produce gusty winds to
around 45 mph and frequent lightning. Highs in the upper 80s to lower
90s look reasonable given 925 mb temperatures and higher cloud cover
expected most of the morning. Heat index values should top off
around 96 degrees.

.SHORT TERM...(Tonight through Friday)
Issued at 330 AM CDT Wed Jun 26 2019

Expect this thunderstorm activity to become isolated by the early
evening hours, as the atmosphere stabilizes with the loss of daytime
heating. Another warm night in store tonight, as the boundary
changes orientation, but remains somewhere over northeastern MS or
Alabama. Lows in the lower 70s in northwestern Alabama look
reasonable with slightly cooler overnight lows further east in the
mid to upper 60s.

Despite a large area of surface high pressure over the southeast in
general, a weak convergence boundary remains in most model output
stretching from the Appalachians southwestward into Alabama or
Mississippi Thursday into Friday. Forcing does not look very strong
with this boundary. However, a very moist airmass and temperatures
in the lower to mid 90s will continue to keep instability very high
during the late morning/afternoon hours. With this weak boundary near
the area, cannot rule out isolated to scattered showers or thunderstorms
in the afternoon and early evening periods both days. Similar shear
and wind aloft look to be in place, so pulse convection at best
continues to be expected. A few stronger storms could produce gusty
winds to around 45 mph and frequent lightning given the strong
instability shown in guidance. An afternoon cumulus field could help
to keep temperatures slightly cooler than they could be, especially
on Thursday. However, it will be even more hot and humid
Thursday/Friday with heat index values climbing into the 96 to 101
degree range roughly both days. So make sure to stay hydrated if you
are working outside late this week. Cannot rule out isolated showers
or thunderstorms lingering into the night both Thursday and Friday
night. Lows Friday night will remain warm, only dropping into the
lower 70s in many locations as well.

.LONG TERM...(saturday through Tuesday)
Issued at 330 AM CDT Wed Jun 26 2019

At the start of the weekend, an Omega block pattern will be present
across the CONUS, with troughs on the West and East Coasts, and a
high amplitude ridge in between. Despite the large scale pattern, a
weak, sheared upper short wave will continue to be present in the
lower Mississippi Valley. This feature will help to instigate
showers/storms in the region, but the best dynamic forcing will
likely be to our south and west. With the overall weak forcing, and the
region placed well south of the main upper jet, a typical diurnal
pattern of summertime convection will result. Surface flow actually
appears to be a bit diffluent during most afternoons into early next
week, which will act to limit convection somewhat, but the flow is
rather weak. Showers and thunderstorm development may be aided a bit
by the weak and departing upper trough to our west during the
weekend, but differential heating and mesoscale outflow boundaries
will likely be the main drivers behind convection. Sufficient daytime
heating may result in low-lvl lapse rates exceeding 7 C/km, and
combined with PWs generally in excess of 1.6 inches could cause some
brief, strong wind gusts in any storms. However, just pulse storm
activity is expected given the continued lack of deep layer shear.

Good consistency among the suite of ensemble and operational
guidance indicates that the upper ridge is likely to build farther
into the East and Southeast early next week. Mainly diurnal
convection is likely to take place with the ridge nearly overhead
the area, with generally low POPs in the forecast for now. However, a
moderate upper jet is depicted to develop across the region on its
western flank by Tuesday/Wednesday. This could produce extra upper-
lvl dynamic support for showers/storms by early next week in the
region. The exact placement of this feature remains uncertain, but
appears more favored to our west in the latest 00Z guidance suite.
Barring any significant cloud cover or showers, the combination of
temperatures in the 90s and high humidity will lead to heat index
temperatures topping 100F at some locations potentially early next


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 616 AM CDT Wed Jun 26 2019

VFR conditions should hold on at both terminals through much of the
TAF period. Light winds this morning will become southwesterly around
8 knots after 15Z at both terminals. Isolated to scattered -SHRA or
-TSRA could develop near either terminal later this afternoon.
However, due to the uncertainty of coverage, left any precipitation
out of the terminals. If a SHRA or TSRA does directly affect either
terminal later this afternoon, MVFR TO IFR CIGS or VSBYS could occur.
Will have to watch for fog late tonight if precipitation directly
affects either terminal.





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