Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 232050
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
350 PM CDT Sun Jul 23 2017

.UPDATE...
Evening Update.

&&

.SHORT TERM...

This Afternoon into Monday. Will hold onto the likely pops for
northeastern counties the remainder of the afternoon. This area
has plenty of boundaries, lift and moisture and radar is becoming
a bit more activity in the Tennessee Valley. Elsewhere, scattered
activity is expected. The convection will be extended into the
7-8pm time frame and then dissipating most areas. The mid to upper
trough approaching Central Alabama will provide some upward
motion and lift overnight and there are also numerous outflow
boundaries around. Therefore, will keep pops going overnight
north.

On Monday, the mid to upper level trough moves in. Numerous
smaller scale surface boundaries will be around and some surface
reflection of the low aloft will probably be seen as a trough.
This will add some convergence and focus for shower and
thunderstorm development through the day. Due to this, mentioned
likely rain chances for the entire area. High temperatures will
only reach the upper 80 to lower 90s with similar dew points. Heat
Indices will only reach the 100 degree mark and an advisory will
not be necessary.

75

Previous short-term discussion:
Overall forecast trend remains relatively intact this morning. We
are starting to see some changes in the overall flow over Central
Alabama. The upper ridge is being pinched off right overhead. An
upper trough is working its way southeastward toward Central
Alabama, while the relatively weak upper low sits over Louisiana.
Therefore, there is a transition zone of upper wind directions
but all these winds are relatively light anyway. But this will
still affect the over steering of storms and only somewhat in
propagation of storms. The northeast areas will experience the
most convection today with the highest overall mean moisture, high
PWATS, orographic influences, several boundaries moving into the
area from the north, and some subtle short wave energy moving
overhead. The driest area seems to be the southwest this afternoon
and have the lowest chance pops mentioned. All other areas about
normal for a summer time day (40 pop). Made some changes to the
hourly temps and dew points per the latest observations and newest
short term, hi-res model guidance. Surface dew points appear to
be running lower than the past few days and do not see a chance
for an increase during the day today. Also bumped high up a degree
and this will affect the overall heat indices. The heat indices
will generally range from around 101 to 105 area wide. The hottest
conditions will be west and will keep the current advisory going.
Not all areas may see the 105 or better and it may be for a short
duration, but it is close and hot and humid.

The wet microburst index is in the moderate category this
afternoon. This indicates some potential of gusty winds with any
strong thunderstorm. Although an isolated severe storm is possible
anytime this index is moderate or above, expect mainly gusty
winds of 40-45 mph, brief heavy rain and deadly lightning with
storms that do develop.

75

.LONG TERM...
Monday through Sunday.

The weak frontal boundary continues to push southward Monday into
Tuesday, increasing rain/storm chances to 50-60% during the
afternoons and 30-40% for the overnight hours as we lose peak
heating. The boundary stalls across South-Central AL and GA Tuesday
night into Wednesday before getting pushed south of the area on
Thursday. Therefore, have roughly 50% PoPs in the forecast for
the southerly half of the area Wednesday, decreasing to 30% by
Thursday afternoon. Due to the front passing on Tuesday/Wednesday,
humidity builds back in and we could see another round of
hot/humid weather by Thursday afternoon. Have added a low
confidence mention in the HWO for heat indices reaching or
exceeding 105 Thursday afternoon. This could be adjusted if future
model runs slow the front down resulting in a higher coverage of
rain/storms.

Another upper-level low pressure system and trough digs through the
Great Lake region Friday and pushes a cold front down through the
region Friday afternoon and into the weekend, therefore have
increase PoPs for Friday and Saturday afternoons.

25/Owen

&&

.AVIATION...
18Z TAF Discussion.

Rather stable looking cumulus field at the present time. Will hold
on to the scattered thunderstorms north at this time and mention
VCTS for a few hours during the maximum heating. Otherwise, still
mostly a VFR forecast. There will be a period of MVFR BR HZ toward
daybreak again Monday morning. Some isentropic lift, around 300K,
will also be present in the morning. This may allow some patches
of IFR/MVFR clouds to develop. Will re-evaluate in later forecast
for possible inclusion if necessary.

Better rain chances appear on tap for Monday as an upper trough
approaches from the northwest.

75


&&

.FIRE WEATHER...

Rain chances remain high Monday and Tuesday as a mid to upper
level trough enters the region. Plenty of moisture, instability
and convergence/focus will aide in this development. Temperatures
not quite as hot as the previous several days as heat indices only
reach around 100. Dispersion values will be low due to relatively
light surface winds and light transport winds.

75

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     72  88  72  88  71 /  40  60  30  50  20
Anniston    73  88  72  88  72 /  30  60  30  50  20
Birmingham  75  90  75  89  74 /  20  60  30  50  20
Tuscaloosa  74  91  74  89  74 /  20  60  40  60  20
Calera      73  92  74  88  73 /  20  60  30  60  20
Auburn      72  89  73  88  72 /  30  60  30  60  40
Montgomery  74  92  76  90  75 /  30  60  40  60  30
Troy        72  89  73  89  72 /  30  60  40  60  40

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Heat Advisory until 9 PM CDT this evening for the following
counties: Autauga...Bibb...Blount...Chilton...Dallas...Fayette...
Greene...Hale...Jefferson...Lamar...Lowndes...Marengo...Marion...
Montgomery...Perry...Pickens...Shelby...Sumter...Tuscaloosa...
Walker...Winston.

&&

$$



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