Scientific Forecaster Discussion
fxus64 kbmx 181138
Area forecast discussion
National Weather Service Birmingham Alabama
638 am CDT Tue Jun 18 2019
for 12z aviation.
Today and tonight.
Widespread showers and thunderstorms expected today as a short wave
trof passes through the area. A large area of mostly light rain was
currently located over eastern Mississippi due to forcing ahead of
the trof axis. Showers and a few embedded thunderstorms will spread
into West Alabama before sunrise. The activity will spread over the
remainder of central Alabama during the afternoon hours. The cloud
cover and rain will keep high temperatures in the middle to upper
80s, which will limit instability. Saturated soundings will also
create poor mid-level lapse rates, so no severe storms expected
today. Steering level winds near 20 knots will keep heavier rainfall
rates associated with individual cells from staying over one location
too long for flooding issues. The short wave trof axis will shift
into Georgia early this evening, and combined with the loss of
daytime heating, will result in a rapid reduction in showers and
thunderstorms during the evening hours.
* marginal risk for severe weather on Thursday.
Wednesday and Thursday.
After a good coverage of showers and thunderstorms on Tuesday, a
lull in convective activity is forecast on Wednesday as the upper-
level pattern begins to shift upstream. Have placed chance pops
across the area with better coverage expected in the afternoon.
Highs in the upper 80s to low 90s are expected, which will be the
overall theme for the long-term.
A more potent shortwave trough is progged to advance from the mid-
Mississippi River valley eastward into the Ohio Valley with an axis
extending into the Gulf Coast by Thursday morning. Increasing
westerly flow aloft will result as a 40-50 kt mid-level jet Max
moving over central Alabama, with ~30 kts of 850 mb west-
southwesterly flow. As such, forecast hodographs depict vertical
wind profiles that are generally unidirectional throughout the day
as better crossover remains to our northeast (downstream sector of
the shortwave). However, eff. Bulk shear will increase to 30-40 kts
as a relatively unstable airmass builds in the afternoon & evening,
with expectation that mid-level lapse rates as high as ~7.5 c/km and
rich low-level moisture increase MLCAPE upwards of 3,000-3,500 j/kg.
Severe thunderstorms will be possible as a result, with dry air
aloft and inverted-v forecast soundings suggestive that dcape may
range 1,200-1,500 j/kg, particularly high and indicative of strong
convective downdrafts. With cooling temperatures aloft (-9 to -10 c
at 500 mb) and robust updrafts possible, large hail is also a threat
with this event.
Given indications the trough axis will be to our east by the early
afternoon, subsidence and other unforeseen mesoscale factors still
remain a negative factor for higher confidence in severe coverage,
as well as specific timing/placement of these threats. It does
appear that a tornado threat is not likely as low-level directional
shear is not favorable and relatively high surface-based LCLs (above
1,100 m) are currently expected. Will maintain a low confidence
threat of severe weather in the severe weather potential statement until higher confidence is
achieved (such as specific threat areas) in upcoming forecast
Friday through Monday.
In wake of the exiting trough (now across the East Coast by Friday
morning), the upper-level pattern will begin to evolve across the
Continental U.S. With a longwave trough building across the intermountain west
and ridging developing across a good portion of the eastern conus,
including central Alabama. As a result, rain chances will decrease
overall with warming expected in our temperatures during the latter
half of the long-term. Most rain chances at this point will reside
along boundaries and/or pooling of better moisture quality,
generally in a slight chance or chance pop fashion through the
weekend. This activity will be diurnal in nature as peak heating
takes place in the afternoon and convective temperatures are
approached amidst our humid Gulf airmass. Have adjusted high
temperatures up a degree or two for the weekend with southern areas
making a run for mid 90s. With hot temperatures and a humid airmass
in place, heat indices will likely exceed 100 degrees in places.
12z taf discussion.
Widespread showers and a few embedded tstms have moved into West
Alabama ahead of a slow moving upper trof. The low cigs have not
developed as much as expected, with only LCL MVFR cigs along the
I-65 corridor. Expect numerous showers and isold tstms to spread
across all of central Alabama today, with LCL heavy downpours.
Cigs will likely remain above 3000 feet agl thru 08z, with LCL
cigs blo 3000 feet agl in heavier downpours. The activity will
decrease in intensity and coverage after 01z due to loss of
daytime heating. MVFR cigs will likely develop across most of the
area after 06z, along with vcsh.
Widespread showers and thunderstorms are expected through sunset
today. Southerly flow and tropical air will keep relative humidity
values at or above 60% percent with further recovery after peak
heating subsides. Kbdi values remain high, but increased low level
moisture content will keep US from reaching critical fire weather
criteria. We will have elevated chances for wetting rains for
much of this week.
Preliminary point temps/pops...
Gadsden 84 69 88 70 86 / 80 50 40 60 70
Anniston 85 69 88 71 88 / 80 50 50 50 80
Birmingham 85 71 90 74 89 / 80 40 40 50 70
Tuscaloosa 87 71 92 74 91 / 80 30 40 50 80
Calera 85 71 89 72 88 / 80 40 40 40 80
Auburn 85 70 87 73 88 / 70 40 50 30 80
Montgomery 87 71 91 74 92 / 80 40 40 20 80
Troy 87 71 89 73 90 / 70 40 50 10 70