Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 191131
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
631 AM CDT Fri Jul 19 2019

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 500 AM CDT Fri Jul 19 2019

Mid and high cloud cover has broken up considerably across a good
portion of northern Alabama and into Tennessee. However, a
convergence boundary and a more concentrated area of cloud cover
still can be seen in observations stretching from far eastern TN
southwest into DeKalb and possibly into Cullman County, AL. North of
this boundary, some drier air is advecting southward. Also, behind it
some subsidence looks to be in place courtesy of a 1018 mb meso-high
over southeastern Kentucky and north central TN. Most of the better
instability is in central and southern Alabama south of this
boundary. The subsidence behind this boundary and weak nature of the
lift near it should keep any showers from developing through
daybreak.

However, with light/calm winds expected through daybreak and
dewpoint depressions already of 2 degrees or less, patchy/areas of
fog were introduced into the forecast through daybreak. Believe the
most widespread fog will occur in locations that see the least cloud
cover. Based on current satellite imagery and forecast RH fields,
that should be northern Lawrence...eastern Colbert counties northeast
into southern middle TN. With slightly drier air pushing south
behind this boundary, expected lows to be in the 69 to 73 range
primarily around daybreak.

As this boundary hangs up during the morning hours or very slowly
edges southward, expect isolated to scattered showers and
thunderstorms to develop after 10 AM, especially near and east of
I-65. Most forecast soundings show very little dry air in the mid and
upper levels of the atmosphere today and DCAPE values mainly between
600 and 700 J/KG in the afternoon. However, a good amount of
instability will still be present (between 2000 and 3000 J/KG).
Frequent lightning and some gusty winds around 45 mph will still be
possible with this much instability in stronger storms. However,
heavy rainfall and isolated flooding concerns will be the main
threats in the afternoon hours with stronger convection east of I-65
(PWATS around 2.00 inches).

Expect more prevalent mostly cloudy conditions also near and east of
I-65 with higher precipitation coverage. This should only allow
highs to climb into the upper 80s to around 90 degrees in these
locations. Further west, expect highs to climb into the 92 to 94
degree range with more breaks in cloud cover and less precipitation
possible. Between the advection of drier air southward and possible
mixing this afternoon, believe lower dewpoints should keep heat index
values mainly between 100 and 103 degrees. Near the AL/MS border,
there may be a few 105 degree heat index values, but do not believe
they will be widespread enough to warrant a heat advisory at this
time. If highs end up a few degrees warmer than expected (~ 96
degrees) in northwestern Alabama, then a heat advisory may be needed.

.SHORT TERM...(Tonight through Sunday)
Issued at 500 AM CDT Fri Jul 19 2019

Most guidance continues to push this boundary further southeast
later this afternoon and into the evening hours, likely ending
precipitation chances briefly tonight. However, as a weak surface low
with this boundary moves back to the northwest towards daybreak on
Saturday, isolated to scattered showers and thunderstorms look to
return to the forecast across northern Alabama and southern middle
Tennessee. This coverage should increase during the morning hours and
continue into the early evening hours before decreasing in coverage.
This will continue to be produced by the weak surface low as it
continues to move from central AL into northeastern MS. Similar
threats are expected on Saturday as well with this activity, though
instability may be a bit less (~ 1000 to 2000 J/KG). Again the main
threats will be frequent lightning and isolated flooding concerns
with this activity. Temperatures should remain in the upper 80s to
around 90 degree with more widespread cloud cover and precipitation
expected across northern AL and southern middle TN. With the more
moist conditions associated with the weak surface low moving
northward towards the area, warmer lows in the 72 to 75 degree range
are expected on Saturday morning.

Sunday looks about as wet, as the surface low moves northeast and
across northern Alabama. With little shear, ample instability, high
PWATS, and little dry air aloft frequent lightning and locally heavy
rainfall look to remain the biggest threats with convection on Sunday
as well. Temperatures will remain cooler in the 87 to 92 degree
range, keeping heat index values well below 105 degrees. Lows should
remain a bit warmer (only dropping into the 72 to 74 degree range)
with the more moist conditions associated with the surface low moving
across the area.

.LONG TERM...(Sunday night through Thursday)
Issued at 500 AM CDT Fri Jul 19 2019

Extended range guidance is in good agreement that the TN Valley will
be located beneath a broad weakness in the subtropical ridge aloft
at the beginning of the extended period. Well to the north of the
region, a strong mid/upper-level cyclone is predicted to drop
southeastward into northern Quebec on Sunday night before shifting
slowly eastward on Monday-Monday night. As this occurs, several
smaller but significant shortwave disturbances will travel
southeastward within the broader circulation around the cyclone,
resulting in amplification of a longwave trough, which could
potentially extend as far to the southwest as the lower MS Valley by
early Tuesday morning. A modest increase in west-northwest flow aloft
is expected to occur as a result of this evolution, and this will
subsequently lead to higher chances for showers and thunderstorms in
the moist and moderately unstable airmass to the south of an
approaching cold front.

At this time, it appears as if the initial episode of more
widespread convection to impact the area will develop along the cold
front as it pushes into the lower-OH Valley. Presuming that showers
and thunderstorms are as numerous as indicated in guidance from the
various global models, this activity should track steadily
southeastward driven by development of a large cold pool at the
surface and strengthening northwest flow aloft. Although typical
timing differences exist at this range, we currently feel that this
round of convection will impact the region Monday morning--perhaps as
early as 12Z across the far northwestern portion of the CWFA. A
second widespread area of convection is expected to develop along the
actual synoptic cold front, as it drops slowly southward through the
region Monday night/Tuesday morning. Weak bulk shear and only modest
instability depicted in forecast soundings suggests that this
activity should largely remain below severe thresholds. However,
given the expected mode of convection and degree of low-level
convergence, a few strong storms producing gusty outflow winds will
certainly be possible during this period.

During the second half of the week, guidance suggests that the slow-
moving southern portion of the longwave trough will become detached
from the parent cyclone and drift into the southern Gulf coast
states, creating another weakness in the subtropical ridge. However,
at this point, it appears as if our region will remain within a
light northerly flow regime, as we should experience slightly more
influence from the longwave trough to our northeast. With the surface
cold front expected to stall to the immediate southeast of the CWFA,
dewpoints may fall into the u50s-l60s for much of the period from
Tuesday night-Thursday. This, along with clear skies and light
boundary layer flow, will set the stage for a couple of cool nights
featuring lows in the lower 60s and seasonably warm days with highs
in the m-u 80s. Although most of the forecast area will be dry during
this period, we have maintained a slight chance POP for the
southeastern counties due to some uncertainty regarding the precise
location of the stalled front.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 630 AM CDT Fri Jul 19 2019

VFR conditions are expected through much of the TAF period. Expect
winds to become westerly between 5 and 10 knots after 15Z. Although
isolated to widely scattered -TSRA are possible, the confidence that
any of this activity will directly impact either terminal is very
low. Therefore, leaving this out of the forecast for now. If any do,
some MVFR CIGS or VSBYS may need to be added. A better possibility
of -TSRA may occur towards daybreak on Saturday. But being so far
out on the end of the forecast period left out for now. Do expect
some clearing towards midnight tonight. This and light winds should
allow VSBYS to drop to at least MVFR criteria (possibly lower)
towards daybreak on Saturday.

&&

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

$$

NEAR TERM...KTW
SHORT TERM...KTW
LONG TERM...70/DD
AVIATION...KTW


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