FXUS64 KBMX 191910

Area Forecast Discussion
National Weather Service Birmingham AL
210 PM CDT Thu Jul 19 2018

Through Tonight.

Previous convection from the leftover MCV that moved into western
Alabama has since diminished. In its wake, high clouds have
streamed across most of the northern half of Central Alabama.
These clouds have suppressed convective development as well as
daytime heating. Therefore, PoPs have been lowered for areas that
were in or near the previous MCV for the rest of the afternoon. We
can't rule out some isolated or scattered development, but that
may not happen until later on in the afternoon or even early
evening hours.

Meanwhile, with the leftover stalled frontal
boundary still hanging around, numerous showers and storms have
developed across eastern and southeastern counties. These storms
have been very slow-moving, but a more organized outflow boundary
is starting to move quickly off to the west within low-level
easterly flow. That boundary will help to trigger additional
storms farther west later on this afternoon and evening.
Torrential downpours will accompany the storms, along with gusty
winds. As is always the case this time of the year, convection
could last a bit longer than anticipated later this evening due to any
outflow boundary interaction that occurs.


Friday through Thursday.

Ridging strengthens across the South-Central and Southwest CONUS on
Friday as a low pressure system slides through the Upper Plains and
into the Midwestern States. The upper level trough deepens as the
low pressure system strengthens later Friday afternoon and evening.
As this takes place, warm air advection in the mid-levels will
increase the capping inversion over Central AL, suppressing
convection and cloud cover, which will allow temperatures to
increase into mid 90s, especially in the southwestern counties.
Dewpoints will also be in the mid 70s, so expect heat indices to
reach or exceed 105 by peak heating on Friday afternoon. A heat
advisory may be necessary, but for now, I'll continue mentioning in
the HWO.

A few shortwave impulses will likely push through the base of the
trough Friday night and clip into the northern portions of Central
AL, which could lead to one or more MCSs across the area. The
difficulty will be forecasting the timing and location of where
these MCSs develop and move. With the ridge to our west, warm air
advection maintains a fairly strong capping inversion that only
strengthens overnight. The question will be which shortwave (if any)
will provide enough forcing to overcome the inversion. If
convective initiation can take place, the environment will be well
primed for strong to severe storms across much of the area. CAPE
values are forecast to exceed 3000 J/kg, with the NAM even
painting some areas approaching 5000 J/kg, which wouldn't be out
of the question considering the high temps and dewpoints coupled
with the upper level height falls. An upper level jet starts
sliding into the area on the backside of the trough thanks to the
deepening upper level low. This will help increase the
environmental shear with 0-6km Bulk Shear reaching 40-50 kts. The
GFS has slightly lower values thanks to the absence of a
significant 850mb jet that the NAM brings in; however, even the
GFS has sufficient shear to maintain an MCS propagation.
Therefore, I'll add continue for severe weather in the HWO, mainly
for the northeastern two-thirds of Central AL. Our southwestern
counties will likely have a stronger capping inversion in place,
plus the strongest impulses and synoptic height falls are
currently forecast to be a little further north and east. Some
uncertainty remains as global and regional models typically won't
handle mesoscale processes that well. Our higher resolution
convective-allowing models don't go out far enough right now to
get a feel for how the event will play out, but given the
environmental thermodynamics and the synoptic pattern, I would
expect at least some storms to push through our northeastern areas
later Friday night into Saturday morning leading to the threat
for damaging winds. The curved hodographs would normally lead to
concerns for tornado development; however, I'm not sold just yet
on whether the convective mode (MCS) would be conducive for
tornadogenesis in this environment. I won't ignore the
possibility, but will continue mentioning damaging winds as the
main threat for now.

For Saturday, the upper level trough digs further south across the
Southeastern and Eastern CONUS, which allows the ridge to the west
continue to amplify. This will mark a shift to mainly northerly flow
across Central AL. The GFS tries to push another weak shortwave
through Northern GA, which slides yet another MCS along the eastern
counties Saturday afternoon. This really isn't supported by the NAM
or ECMWF, and given that by this point, the main trough axis is just
to our east, I wouldn't expect an MCS to push through like that.
I'll maintain at least chance PoPs in the east Saturday
afternoon/evening, but I think most of the severe potential will
remain to the east of our area.

The synoptic pattern continues to amplify Sunday into early next
week with the upper trough digging in across the East Coast and the
Ridge remaining strong in the Southwest US. Central AL will remain
caught between the two synoptic features Sunday through at least
Monday afternoon, leading to the best rain chances being in the
eastern half of Central AL where the better synoptic uplift will be.
Areas in the western half would likely see more synoptic
subsidence on the backside of the trough, leading to relatively
lower rain chances. By Monday afternoon, the upper level low
becomes cut-off and drops across Central AL and hangs around the
Gulf States through at least the middle of next week. More
southerly flow will return to the region as this takes place, and
with the added lift of the upper level low , we'll see an increase
in diurnal showers/storms for Tuesday through Thursday.



18Z TAF Discussion.

The large mass of showers and storms that moved into western
Alabama this morning has dissipated, and only light rain showers
remain over KTCL. High clouds associated with that previous
convection have moved over a majority of northern counties within
Central Alabama. Those clouds are helping to suppress convective
development, and any mention of VCTS or TSRA have been removed
from the TAF at KEET, KBHM, KANB, and KASN. However, ample
sunshine has allowed numerous storms to develop across the south
and east, and VCTS will remain in the TAF, with high likelihood
of amendments for TSRA through the afternoon, with rapid
reductions in vis and ceilings and wind gusts above 20 knots

For the overnight forecast, model guidance is hinting at the
possibility of low stratus moving westward from Georgia overnight
tonight. Best chance of low clouds will be at KTOI, but based on
guidance trends and observations this evening we may have to add
at least MVFR ceilings at KMGM as well. Otherwise, VFR conditions
are expected during the overnight hours elsewhere.



Scattered showers and storms are expected today with the most
coverage south and east. Localized fog will be possible each morning
over the next couple of days where rainfall occurs during the
previous afternoon and evening. There are no fire weather concerns.


Gadsden 71 92 72 91 70 / 20 20 50 50 40
Anniston 71 92 72 90 71 / 40 20 40 50 40
Birmingham 73 93 75 92 73 / 20 20 40 50 30
Tuscaloosa 74 95 76 94 74 / 20 20 30 50 20
Calera 72 92 73 91 72 / 20 20 30 50 30
Auburn 72 89 72 90 71 / 50 30 30 50 40
Montgomery 73 94 75 94 74 / 40 20 20 50 40
Troy 73 92 74 92 73 / 50 40 10 50 40




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