Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 202333
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
633 PM CDT Sun Aug 20 2017

.UPDATE...
For 00Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 300 PM CDT Sun Aug 20 2017

Ridge of high pressure has built into the Tennessee Valley and is
promoting a mostly sunny and hot day across the region. Some drier
air filtered in yesterday behind a very weak front, resulting many
locations across the Tennessee valley with some slightly lower
dewpoints in the mid to upper 60s this afternoon. However, a few
areas along the Tennessee River have not mixed out, with Huntsville
(74) and Decatur (76) as of 19z. The result is some locally
oppressive heat index values of 103 degrees. As mentioned, these
conditions are confined to in/around the river. Scattered Cu has
developed once again, especially along the Cumberland Plateau where
coverage is slightly higher. In addition, some isolated showers have
developed over parts of Cullman and Marshall Counties due to a very
weak convergence zone and moisture underneath the ridge. Thus, have
included an isolated storm in the forecast through 00z. For tonight,
the sky will quickly clear out as we lose heating. Good radiational
cooling will allow temperatures to drop quickly by late evening,
before bottoming out in the upper 60s to lower 70s. Given the lack of
fog last night and the dry air near the surface, have left a mention
of it out of the forecast tonight (though some guidance suggests
light, patchy fog may be possible). We will need to watch trends
overnight, especially in areas that did see a little rainfall today.

.SHORT TERM...(Monday through Tuesday)
Issued at 300 PM CDT Sun Aug 20 2017

Attention quickly turns to the historical Total Solar Eclipse event
for Monday. The good news for folks in the Tennessee Valley is that
the upper-level ridge should help to promote fairly decent viewing
conditions. However, a look at model soundings suggests a fair bit
of moisture present at 850 mb, which may support a higher CU field
than previously thought. Additionally, would not be shocked to see a
few isolated showers along the plateau later in the afternoon (though
was not confident enough to mention in the forecast at this time).
Still, viewing conditions should be fairly good, with scattered CU
potentially causing some visibility issues, locally, at the peak
obscuration. The other interesting trend to watch will be
temperatures will would normally peak in the low to mid 90s, but may
drop 3-5 degrees during this event due to the interruption of
normalized heating. Have reflected this in the forecast. Thus, we
may see our actual high around 5 PM local time due to this event.

Southerly flow around the ridge will continue to push more Gulf
moisture underneath it on Tuesday. With this ridge slowly weakening
and shifting further south, a weaker CAP may allow for isolated to
scattered convection to form in the afternoon. The main interest,
though, will be late Tuesday night as a prefrontal trough is progged
to trigger a line of storms over Middle Tennessee. Latest model
guidance suggests this could approach and move into the Southern
Middle Tennessee and far Northern Alabama 09-12z Wednesday. Most of
this activity should weaken and remain below severe limits given the
lack of appreciable instability. However, have adjusted rain/thunder
chances up during this period to account for the possibility of an
earlier arrival time. The NAM is more bullish than the GFS, CMC, and
ECMWF -- but all show some convective, QPF signals at our doorstep by
12z Wednesday. We will continue to monitor this over the coming
days.

.LONG TERM...(Tuesday night through Saturday)
Issued at 300 PM CDT Sun Aug 20 2017

On Wednesday, the upper-level trough will continue to dig through
the Great Lakes and into Quebec with the sfc low slightly ahead of
it. Models are hinting that the trailing cold front will now move
into the TN Valley earlier than what previous runs were depicting due
to a short wave rounding the base of the trough. This would bring
the front in closer to Wednesday afternoon, developing numerous
showers and thunderstorms across the TN Valley. Daytime heating will
provide additional instability, and with bulk shear ~20kts and PWATs
hovering just under 2 inches, will not rule out a strong storm or two
with gusty winds and locally heavy rainfall as the primary threats.
Precip chances will taper off as the front pushes southeast heading
into Wednesday night. Cooler and drier air will filter in behind the
front where both the highs and lows will be slightly below normal.
Thursday will be the coolest day of the week, literally, with highs
in the low/mid 80s and overnight lows in the lower 60s with possibly
some upper 50s in southern middle TN! High pressure across the Great
Lakes will build into the region on Thursday and Friday keeping us
dry. Isolated showers and storms are possible again on Saturday as
the pattern deamplifies and wedging along the Appalachians develops.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 633 PM CDT Sun Aug 20 2017

With high pressure layered across the mid/srn Atlantic Coast, VFR
conds are xpcted thru the TAF period.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...AMP.24
SHORT TERM...AMP.24
LONG TERM...JMS
AVIATION...09


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