Area Forecast Discussion
Issued by NWS Huntsville, AL

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703
FXUS64 KHUN 201115
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
615 AM CDT Thu Jul 20 2017

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 412 AM CDT Thu Jul 20 2017

Today should start out fairly warm with lows in the 70 to 75 degree
range in most locations. A few locations in northeastern Alabama
could see lows dip into the upper 60s. Patchy fog has already formed
east of a Cullman to Meridianville line and in western Tennessee.
At this time, visibilities are around 5 miles in northern Alabama.
Not thinking pockets of dense fog will form looking at current
temperatures and dewpoints (even in locations that are already seeing
lower visibilities). This fog should lift fairly quickly around 8
AM.

Most model guidance, especially the short-term guidance, shows deeper
moisture advecting southward into northern Alabama after sunrise.
Likely associated with some weak convergence/energy aloft moving
into the area. Coupled with plenty of instability, this should
produce isolated to scattered showers and thunderstorms, mainly
southeast of a line from Moulton to Winchester. Abundant low level
moisture through 700 mb should make it hard for dewpoints to mix out
this morning into early afternoon hours. Thus, keeping surface
dewpoints in the 70 to 75 degree range through about 2 or 3 pm.
However, there should be enough dry air well aloft around 500 mb for
strong thunderstorms to develop capable of producing winds gusts to
around 50 mph. This is mainly based on forecast soundings, which show
DCAPE values over 1000 J/KG between 10 AM and 1 PM. Into the early
afternoon hours, temperatures of 95 to 97 degrees should combine with
dewpoints in the lower 70s to produce heat index values of 105 to
106 degrees. This is expected to occur in Franklin, Limestone,
Lauderdale, Colbert, and Lawrence counties in Alabama. Thus, issuing
a heat advisory for these counties valid from noon through 7 PM.

Between 1 PM and 3 PM, models show this energy/deeper moisture
pushing mainly south of the area (except maybe Cullman Marshall, and
Dekalb counties). Much drier air aloft above 700 mb is advected into
the area by then. This should lower dewpoints in the mid to late
afternoon hours a good bit. Even though drier air will push into the
area, temperatures as high as 96 to 98 degrees look possible the
Huntsville and Muscle Shoals area. Thus, the 105 to 106 heat index
values could still last through the mid afternoon hours. Further
southeast of these counties, cloud cover and scattered precipitation
should keep heat index conditions between 95 and 103 degrees.

.SHORT TERM...(Tonight through Saturday)
Issued at 412 AM CDT Thu Jul 20 2017

A few isolated showers or thunderstorms could linger in Cullman
northeast through Dekalb counties this evening, before dissipating.
Depending on how widespread the rainfall, more and possibly denser
fog could form late Thursday night into Friday morning. Lows in the
70 to 75 degree range should continue, low level moisture recovers
Thursday night (as we lose mixing of drier air aloft).

Friday could see a few isolated showers or storms move northward.
However, overall much more sunshine should allow temperatures to
reach the 95 to 99 degree range over most of the area. With
temperatures still around 70 degrees, heat index values may climb
into the 105 to 107 degree over a larger area of northern Alabama and
Southern Middle Tennessee, before dewpoints mix out later in the
afternoon. Thus, a heat advisory may be needed for a larger portion
of the area on Friday.

By Saturday, although it will be more humid models show an inverted
trough axis extending northward from the western Gulf of Mexico. This
should increase shower/storm chances to scattered again and keep high
temperatures in the 90 to 95 degree range, keeping heat index values
lower around 100 degrees. Little shear with this inverted trough axis
so although instability will be there, only gusty winds and frequent
lightning are expected in stronger storms.

.LONG TERM...(Saturday night through Wednesday)
Issued at 412 AM CDT Thu Jul 20 2017

The inverted trough axis holds over the southeast, producing enough
lift for isolated showers and storms to continue overnight on
Saturday.

By Sunday, models increase forcing over the area, as a cold front
approaches from the north. Likely chances of rain were included in
the forecast. Although not much shear is present, dry air aloft
should allow for the development of stronger thunderstorms capable of
producing gusty winds. There is a bit better shear and ample
instability with this feature as it pushes southeast through the area
Monday into Tuesday. Thus have highest chances of showers and
thunderstorms during this period. At this point, strong storms
capable of producing gusty winds look like the main threat Monday and
Tuesday as well, with dry air entrainment aloft and better
shear. There is some uncertainty concerning how quickly this
boundary will move south. However, general consensus seems to be
Tuesday through early Wednesday. Regardless, this will be a welcome
respite from the hot temperatures, with highs returning back to the
85 to 90 degree range mostly Sunday through Tuesday.

A dry and warm period returns to the forecast by Wednesday, as high
climb back into the upper 80s to lower 90s.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 615 AM CDT Thu Jul 20 2017

VFR conditions are expected through most of the TAF period. BKN/SCT
Cigs around 5000 feet are expected to develop this morning at both
terminals and continue into the mid-afternoon hours. Isolated to
scattered -TSRA are expected to develop also near KHSV. For now left
TSRA or VCTS out of the KHSV TAF, since coverage is expected to be so
low. If a TSRA does directly affect KHSV, cigs and vsbys could
become MVFR (1500-3000 feet) or IFR(2 SM). Otherwise, VFR cigs will
return this evening, as fog drops vbsbys to MVFR (4SM) late tonight
through daybreak on Friday.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Heat Advisory from noon today to 7 PM CDT this evening for
     ALZ001>005.

TN...NONE.
&&

$$

NEAR TERM...KTW
SHORT TERM...KTW
LONG TERM...AMP.24
AVIATION...KTW


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