Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 290821

Area Forecast Discussion
National Weather Service Huntsville AL
321 AM CDT Thu Jun 29 2017

.NEAR TERM...(Today)
Issued at 312 AM CDT Thu Jun 29 2017

Clouds will quickly be on the increase early this morning as a weak
upper-level disturbance along the northern Gulf Coast helps advect
moisture (and some light isolated showers) into the southern
Tennessee Valley between 12-15z. Latest water vapor imagery shows a
plume of drier air wedged between this coastal low to the south and
a front/MCS feature to the NW. Model guidance suggests these light
showers will continue to overspread the region throughout the day,
though the drier air aloft may help to erode some of it initially.
Still, with a southerly flow off the Gulf of Mexico in place, think
the boundary layer will continue to moisten and support better
coverage and rainfall potential for the afternoon/evening. This can
be see in model soundings, which show this -- with progged PWAT
values increasing from 1.3 inches at 06z this morning to close to 2.0
inches by 00z this evening. Given this, have felt confident enough
to go 50-60 percent in the afternoon, with the higher chances
favoring NE Alabama and Cullman County. Additionally, a few peaks of
sunshine will help destablize the atmosphere enough to warrant
prevailing thunder -- though any storms will be pulse-like and not be
organized enough to get too strong. Cloud cover will keep highs a
little bit cooler, generally in the low to mid 80s.

.SHORT TERM...(Tonight through Saturday)
Issued at 321 AM CDT Thu Jun 29 2017

Models want to lift the aforementioned upper-level disturbance into
Central and eventually Northern Alabama tonight. As it does so, the
added lift may trigger some additional rounds/clusters of showers and
thunderstorms. Given the very moist environment, it won`t take much
to generate a shower/storm capable of producing locally heavy
downpours. For this reason, have trended higher with the QPF and gone
likely with PoPs after midnight. As this feature lifts NE into
Tennessee Friday morning, models indicate an opportunity for greater
coverage as well in the 09-18z time period, especially across NE
Alabama -- where I have introduced categorical PoPs. If some pockets
of heating can occur, instability parameters may get high enough to
support a couple strong storms in the afternoon.

This activity will taper off somewhat overnight as the wave shifts
NE into Tennessee. However, another broad upper-trough over the
Northern Plains and Great Lakes will force a cold front SE into the
Mid-South and in the general vicinity of the Tennessee Valley. While
not directly over us, this feature will serve as a focus in this very
moist environment for additional showers and thunderstorms on
Saturday -- that could drift toward the area during the day,
especially by the afternoon. Cloud cover may thin out a little bit as
well on Saturday, allowing for a more heating. Model progs indicate
some better instability, with ML CAPE values around 2000 J/kg. Though
the degree of breaks in the cloud cover is still uncertain, this
does suggest the potential for a few strong to perhaps marginally
severe storms -- capable of damaging winds and heavy rainfall. These
storms will be outflow driven and in the form of scattered pulse
cells and/or some linear clusters that could develop along the
boundary to the NW. Will need to continue to watch trends in the
coming days -- as localized damaging winds and flooding will be
possible with this.

.LONG TERM...(Saturday night through Wednesday)
Issued at 312 AM CDT Thu Jun 29 2017

The extended forecast could be characterized by the word messy.
To our south a subtropical high is forecast to slide westward into
the central Gulf of Mexico Saturday into Sunday. To our north a
trough will be moving across the Great Lakes. The resulting mid-
upper level flow between these two features will be close to zonal
with a slight NW component. At the surface though, it is likely that
some form of weak front/convergence axis will be near the region for
much of the period. Models are in decent agreement in moving a few
shortwaves through the zonal flow and across the area Sunday to
Tuesday. The lift tied to these weak waves will combine with
afternoon temperatures each day in the upper 80s to lower 90s and
scattered thunderstorms are possible afternoon. Instability will
generally range between 1,000-2,000 J/kg with shear values < 20 kts.
One interesting thing to note is that with winds having a slight NW
component models are suggesting the possibility of some drier air
being advected into the area. The result in the sounding when
combined with good afternoon mixing is larger Theta-E depressions in
the low/mid levels, which would be very favorable for damaging wind

There is some uncertainty regarding the exact movement of the
shortwaves heading into the Fourth of July holiday but would expect
some afternoon thunderstorms might ruin some outdoor plans
unfortunately. Beyond Tuesday model solutions diverge significantly
as they try to figure out what to do with a ridge building across
the western US. Ensemble spread is also very high and have continued
to go with a blend of guidance for the forecast beyond Tuesday.
Typical summer-time thunderstorms chances are expected with highs in
the upper 80s to lower 90s.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1237 AM CDT Thu Jun 29 2017

Clouds will increase across the southern Tennessee Valley this
morning as an a weak upper-level disturbance near the Gulf helps to
bring appreciable moisture back into the region. Scattered SHRA/TSRA
will develop by the afternoon, though the extend of the coverage is
still uncertain at this time. For now, have maintained a VCSH at each
terminal from the late morning through the early evening. This
activity should wane after sunset, but lowering MVFR ceilings will
set in for the remainder of the period.




LONG TERM...Stumpf

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