Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 242044
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
344 PM CDT Mon Jul 24 2017

.UPDATE...
Evening Update.

&&

.SHORT TERM...

Chaotic conditions across Central Alabama this afternoon. Even
more mechanisms out there now than we previously discussed. The
boundary from near Double Springs to Birmingham to Wedowee has
been the most active boundary so far. It has also maintained its
relative position just a tad north than I had originally thought.
An outflow has pushed out south of the this area and expect
additional development south and east as we progress through the
afternoon. Some of the rainfall has had some high rain rates and
localized flooding remains possible with the high moisture
content in the atmosphere. Wet microburst indices are in the
Moderate range so a few wind gusts into the 40 to 45 mph range are
also possible but will be very localized.

I`m not giving up on the south, as a few boundaries are noted
moving into the southern areas from the south and southwest.
SBCape values are around 2500 and Li`s are only -3, indicating
some warm air aloft. At any rate, still enough for some thunder
development and will keep the rain chances above normal summer
time activity south too.

Overnight will be quite tricky. Believe we will experience a few
showers and thunderstorms as upper trough approaches from the
northwest and a mid level low intrudes from the south. We will
lose heating of the day, but scattered activity should be around.

Much the same scenario on Tuesday as the airmass does not change,
boundaries around, and plenty of moisture and instability to
ignite some showers and storms. Kept the highest chances south and
this was once again above the normal rain chances for a summer
day.

75

Previous short-term discussion:
Wow, there are numerous mechanisms out there today that may
enhance shower and thunderstorm coverage. The challenge will be
where to place the highest chances. Overall, an above normal rain
chance for this summer day.

First off, we have a surface trough which is a reflection of the
mid to upper trough approaching the area from the northwest. This
subtle boundary was in the neighborhood of the southern Tennessee
Valley Counties. The actual synoptic front was a few hundred miles
behind this trough and therefore no airmass change expected. Some
early morning convection along this boundary sent out a few
outflow boundaries that were generally located from near Hamilton
to Calera to Sylacauga. This was also very close to the north and
east extent of the morning low stratus deck. This low stratus
deck also produced a differential heating zone which just enhanced
the boundary zone. Two additional differential heating zones,
most likely from leftover convective boundaries, were located near
Auburn and Phenix City. On the latest Water Vapor Imagery, a
"relative" jet streak was observed from northern Arkansas to
northern Kentucky which may enhance the lift over northern
Alabama. It appears this jet streak was only 45-50kts, but OK for
summer time in the south. Some of the low stratus was also still
depicted on the least satellite imagery, and therefore have the
convective initiation zone located near these smaller scale
boundaries. Some activity already beginning in the higher
orographic zones northeast and along the aforementioned boundary
zone.

There is a large rain mass across southern Mississippi that the
models are not handling too well. There is a mid level low moving
slowly eastward. Enhanced 850 theta e field and some better 850
winds near this feature. It appears that this feature will move
slightly north of east today and eventually enhance the rain
potential across south Alabama. Upper level winds become more
westerly oriented area wide by this afternoon and expect a
convergence of these focusing mechanisms across the south half of
central Alabama late this afternoon.

The forecast retains its same general flavor from the previous
ones, but did increase rain chances this afternoon south and into
this evening. With PWATS close to 2 inches, some of the rainfall
may be locally heavy. At this time, it will be too local for any
mention of minor flooding or other concerns. The wet microburst
potential is Low to Moderate today, so once again, some of the
storms may produce gusty winds and deadly lightning, but an
overall low threat strong to severe is not warranted.

Temperatures today will be below what we have experienced the
past several days. Although low level moisture remains the same,
heat indices will not reach 105 degrees to due clouds and rain.

Thought of the day...it was in the 30s in central and northern Quebec
this morning.

75

.LONG TERM...
Tuesday through Monday.

The main trough axis and frontal boundary push southward through
Central AL on Tuesday. This will increase coverage of thunderstorms
across the area with the best chances being south of the I-20
corridor Tuesday afternoon. The boundary slides into Southern AL
overnight Tuesday into Wednesday. Low level ridging builds in behind
the front Wednesday afternoon, keeping rain chances lower for the
northern portions of Central AL. Have kept 40 PoPs in across the
southern third of the area.

Southerly flow returns on Thursday as low level ridging builds in
across the TX and LA coasts. This will increase moisture across the
area leading to dewpoints in the low to mid 70s. Models still
hint that we could see another hot/humid afternoon on Thursday;
however, another trough begins to dig southward as a surface low
slides eastward through the Midwest. This will lead to height
falls across Central AL, so there`s some uncertainty on just how
warm it can get Thursday before the southerly flow becomes more
westerly and dries out, lowering the afternoon heat indices. I`ll
keep a low confidence mention in the HWO for Thursday afternoon
because it`s still possible some areas might reach 105 degree heat
index briefly. Will continue to follow trends in models to see if
this needs to be adjusted or removed.

There is some model disagreement on timing, but we could see a more
substantial cold front push into Central AL Friday and through the
weekend. Thunderstorm chances will increase, but due to timing
uncertainty, have left 40-50 PoPs in the forecast Friday evening and
Saturday. Temperatures expected to be in the mid to upper 80s behind
the front for the second half of the weekend and into early next
week as the northerly flow continues with troughing in the Eastern
US.

25/Owen

&&

.AVIATION...
18Z TAF Discussion.

Chaotic conditions across Central Alabama today. As mentioned in
the short term discussion, numerous small and some larger scale
boundaries and lifting mechanisms around the region this period.
Mentioned thunderstorms at all terminals this afternoon as VCTS
and Tempo. Will be updated for prevailing conditions as necessary
and will affect BHM/EET/ASN first. This convective activity may
last into the evening hours and have kept some mention of VCSH
into the early evening. This may need to be extending when and if
a trend establishes itself.

Low stratus has lifted for the most part but there are a few
lingering areas of MVFR ceilings. These clouds will continue
lifting into VFR ceilings in a few hours outside rain areas. There
will be no airmass change and low level conditions will be very
moist overnight. Models are hinting at some weak isentropic lift
again around daybreak near the area that had the clouds this
morning. Will keep the mention of IFR ceilings starting just
before daybreak and slowly mixing out through the morning. Showers
and thunderstorms are expected again tomorrow most numerous south.

75


&&

.FIRE WEATHER...

Rain chances remain above normal for a summer day and night
through 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  90  72  91  73 /  30  50  20  30  20
Anniston    73  89  73  91  73 /  30  50  20  30  20
Birmingham  75  89  74  93  75 /  30  50  20  30  20
Tuscaloosa  74  89  74  94  75 /  50  50  20  30  20
Calera      74  89  73  92  74 /  40  50  20  30  20
Auburn      73  87  73  88  73 /  50  60  30  40  20
Montgomery  75  90  75  92  75 /  50  60  30  40  20
Troy        72  88  73  90  73 /  50  60  30  40  20

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
None.

&&

$$


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