Area Forecast Discussion
Issued by NWS Birmingham, AL
FXUS64 KBMX 300850
Area Forecast Discussion
National Weather Service Birmingham AL
350 AM CDT Sun Apr 30 2017
Another windy day in store across Central AL. A strong low pressure
system over Kansas/Oklahoma is becoming occluded this morning and
has stretched a cold front down through Central Arkansas and
Louisiana. Rain and storms will push eastward with this front
through the day and approach Central Alabama by the afternoon hours.
There`s still quite a bit of uncertainty with the timing, which will
impact the severity. This is the current thinking, but stay tuned
through the day for updates:
Latest model runs haven`t really helped narrow down the timing of
these storms too much. The HRRR and ARW are certainly the quickest
with the storms entering our western counties around or just after
1:00pm. Some of the other guidance varies from around 4:00pm to
6:00pm for our western counties. Guidance is also hinting at
possibly two waves, but again the timing is still very uncertain.
I`m leaning closer to the ARW solution at this time, which has the
initial line getting into our western counties roughly 1:00pm to
4:00pm and reaching the I-65 corridor around 3:00pm to 6:00pm before
slowing down or possibly stalling. The 2nd line will be getting into
the western counties by the 3:00pm to 6:00pm timeframe, push into
the I-65 corridor around 9:00pm to 12:00am, and will push the whole
system through Montgomery and the eastern counties after midnight
and through 4:00-6:00am Monday morning.
The main threat with these lines of storms will be damaging winds.
With the quicker trends in the timing for our western counties,
models hint at surface based instability reaching into the 1500-2500
J/kg range. 0-6km bulk shear is on the lower end with the best shear
off to our west, but is still around 35-45kts, which will promote
the organized updrafts with the storms along the QLCS. 0-3km SRH is
above 300 considering the backed surface winds and the strong 850mb
jet with winds >50kts. Therefore, would expect a line of strong
storms that are capable of producing damaging winds. Given the low
level wind profile, can`t rule out brief spin-up tornadoes with any
bowing segments or breaks in the line. These threats are greatest in
the western counties as the line potentially could move through
during the peak heating of the day when there is the greatest
instability. As the line pushes east of the I-65 corridor during the
evening and overnight hours we have lost a lot of the surface based
instability, but values remain around 400 - 800 J/kg along the line
through 7am in our southeastern counties, which is sufficient for
sustaining strong to severe storms with a QLCS given the wind
profiles are still favorable.
Another concern with this system will be the heavy rainfall
across the western two-thirds of Central AL. PWATs are above the
maximum climatologically, and the first line of storms has the
potential to stall out during the evening hours. Current forecast
is for 1.5 - 2.5 inches for most areas, with some counties in the
west seeing close to 3 inches. Streamflows across Central AL are
at or well below average due to the recent drought, so widespread
flooding isn`t expected. However, some localized small streams or
urban areas might see isolated flooding issues.
This system moves out of Central AL Monday morning and low level
ridging builds in across the area clearing out the skies for Monday
and Tuesday. On Wednesday, a positively tilted trough digs southward
into the Southeastern US and develops a surface low over the Gulf
States. Models are still in disagreement with the exact
track/development of this low, so nailing down any specific severe
threats at this point is difficult. Will continue to monitor trends
in the models concerning the severe potential, but at the least,
expect widespread rain Wednesday night into Thursday. Some light
showers may linger into Friday, but confidence is low at this time
considering the differences in the models. For next weekend, models
show ridging building in across much of the Central and Southern US.
06Z TAF Discussion.
Setup very similar to previous nights as high pressure
off to the east and potent storm system getting going to our west.
The pressure gradient and wind aloft increase overnight and have
added LLWS to the terminals generally from 6-12z. Expect the low
level winds below 2k ft to increase out of the south to around
45kts. This combined with a decrease in the surface winds
overnight may yield some LLWS. Have a hard time adding in fog even
though have been burned a few times the past few nights. Will go
with MVFR ceilings. The latest satellite imagery has some MVFR
ceilings just off to the southwest, but not advecting quickly our
way. These ceilings should develop by 7-11z time frame. It
appears that the isentropic fields are more parallel to the
surfaces tonight, but the atmosphere is still saturated and
plenty of wind. Ceilings improve through the morning hours and
surface winds increase. Southerly winds will be gusty all day.
Thunderstorm chances increase after 18z in western Alabama have
added some mention VCTS/Prob30 depending on the length of the
forecast. Thunderstorm chances by late afternoon into the evening
are high. This will most likely be expanded in the next forecast.
Strong gusty winds possible today ahead of an approaching cold
front. Rain chances remain low through this afternoon as the cold
front moves through. There are no major fire weather concerns aside
from the gusty winds. RHs will remain high.
.PRELIMINARY POINT TEMPS/POPS...
Gadsden 85 59 73 50 79 / 40 100 50 10 10
Anniston 86 60 73 51 79 / 20 100 60 10 10
Birmingham 86 58 74 54 81 / 40 100 30 10 10
Tuscaloosa 84 56 75 53 83 / 80 100 20 10 10
Calera 85 59 75 54 81 / 40 100 30 10 10
Auburn 86 64 74 55 80 / 20 100 80 10 10
Montgomery 88 62 77 54 84 / 20 100 50 10 10
Troy 87 64 77 54 83 / 20 100 80 10 10
Wind Advisory from 8 AM this morning to 10 PM CDT this evening
for the following counties: Autauga...Barbour...Bibb...Blount...