Scientific Forecaster Discussion
fxus64 kbmx 251138
Area forecast discussion
National Weather Service Birmingham Alabama
638 am CDT Tue Jul 25 2017
for 12z aviation.
today and tonight.
A mid level trof axis extends from Louisiana northeast across south
central Alabama and into the mid Atlantic states. This feature will
be the trigger for scattered to numerous showers and thunderstorms
today, mainly south of I-20 and in the afternoon. Drier air was
located north of the trof axis, and the trof axis is forecast to
shift slowly southward this afternoon. Rain chances will be highest
across South Alabama where deeper moisture and better forcing will
co-exist. Also, satellite imagery early this morning shows an mesoscale convective vortex
over South Alabama which was producing showers on the north side of
the mesoscale convective vortex. Steering level winds will be less than 10 knots near the
upper trof axis. With pwat's around 2 inches, slow moving
thunderstorms could produce localized flooding. Scattered showers
and thunderstorms will continue through the evening hours, but most
of the rain will likely dissipate by 9 PM. The warmest temperatures
today will be north of I-20 where lower 90s are forecast, with highs
in the upper 80s across the southern counties due to rain and lower
1000-850mb thickness values.
Wednesday through Tuesday.
Upper level ridging builds in behind the departing trough axis early
Wednesday. Latest model runs have this building in quicker,
increasing 1000-850mb thickness to 1430-1435m, which could support
temperatures in the mid 90s. Have therefore increased afternoon
temperatures area-wide. Winds are generally out of the west instead
of being southerly, which might limit dewpoints to the low 70s. Some
areas west of the I-65 corridor could exceed 100 degree heat
indices. Currently, the forecast has some localized areas hitting
105 heat index briefly Wednesday afternoon. Therefore, I have added
a low confidence mention of heat impacts in the severe weather potential statement. While we might
not reach heat advisory criteria, we're still likely to see triple
digit heat indices across the western counties.
The 1000-850mb thickness values increase to 1440m across the western
half of central Alabama on Thursday, which would support temps in the mid
to upper 90s. Models are hinting at weak low level convergence in
the eastern half of central Alabama Thursday afternoon, so have kept
chance pops in the forecast for those areas. This could also help
moderate heat indices east of the I-65 corridor. Will leave mention
of heat impacts in the severe weather potential statement Thursday afternoon and monitor model
trends. At this time, expect many areas to approach or exceed 100
degree heat indices with areas in the western counties hitting 105.
Meanwhile on Thursday, and upper trough begins to dig through the
Great Lakes region and a surface low moves through the Ohio River
valley. A cold front will stretch through the Tennessee Valley overnight
Thursday into Friday morning. This cold front will slowly push
southward through central Alabama Friday and into early Saturday morning.
Pwats associated with this system are above the maximum
climatologically at around 2.3 - 2.5". This could lead to heavy
rainfall-producing storms along the frontal boundary as it pushes
Behind the front, expect cooler and drier conditions for the
weekend. Have left mention for scattered showers since we'll still
be near the trough axis and could see some extra lift for afternoon
convection. This trough axis essentially stays in place across the
southeastern states through Tuesday, so expect northerly flow,
cooler than normal temperatures, and scattered afternoon storms into
early next week.
12z taf discussion.
Widespread IFR cigs developed overnight, mainly for areas along
and south of I-20. All central Alabama taf sites except ktcl were
reporting cigs aob 1000 ft agl. The cigs will generally lift or
dissipate by 15z. An upper level trof was located just north of
I-85. Scattered showers and a few thunderstorms were already
occurring south of this trof axis this morning. The activity will
increase in areal coverage during the day, mainly impacting kmgm
and ktoi. Drier air aloft will push southward across north Alabama
this afternoon, and limit convective potential. For now left out
any tstms for northern taf sites, but isolated showers or tstms
could still impact these sites. The convection will diminish
quickly after sunset. IFR cigs will likely develop at kmgm and
ktoi after 09z.
Rain chances remain above normal for a Summer day and night
through today, 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
Preliminary point temps/pops...
Gadsden 91 73 92 73 92 / 30 20 20 10 30
Anniston 90 73 92 73 92 / 30 20 20 10 30
Birmingham 91 75 94 76 94 / 30 20 20 10 20
Tuscaloosa 90 74 95 75 95 / 40 20 20 10 20
Calera 89 74 93 74 93 / 40 20 20 10 20
Auburn 86 73 90 73 90 / 50 20 30 20 30
Montgomery 89 75 93 75 94 / 60 30 30 10 30
Troy 86 72 91 73 93 / 80 30 30 20 30