Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 300401
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
1101 PM CDT Sat Apr 29 2017

.UPDATE...
For 06Z Aviation.

&&

.DISCUSSION...

A widespread cumulus field has developed over Central Alabama this
afternoon, as high clouds move by overhead. This has produced a
mix of sun and clouds across the area. A few showers, and possible
a thunderstorm, are possible for the eastern half of the area the
remainder of the afternoon. Much of this activity will dissipate
as the sun goes down. It is a bit breezy out there with southerly
winds 15-20mph with a few gusts 30mph or more. These winds will
also subside after sunset. Variable clouds this evening as the
cumulus field will deteriorate, but this will be somewhat short
lived. Low clouds and patchy fog will develop from south to north
overnight with most places experiencing clouds at sunrise. The
building humidity will keep the low temperatures rather mild from
the mid 60s to lower 70s.

Southerly winds will pick up rather quickly Sunday morning as the
pressure gradient remains high with the developing storm system to
out north and west. Southerly winds of 15-20 mph with gusts of 30
mph or more are expected area wide. Most areas will remain dry in
the morning but thunderstorms are expected by early afternoon near
the Mississippi state line and slowly advancing eastward. The
latest convective allowing models have a chance of multiple rounds
thunderstorms, some possibly severe. Therefore, have adjusted the
timing on the severe storm potential to cover a larger period.
Western areas generally 1pm until 1 am and eastern areas 6pm to 10
am Monday. No significant changes to the severe weather parameters
associated with this system. But entering a bit early would
provide more instability but out ahead of the upper system. Precip
water values climb above two inches and this is somewhere close to
twice the normal for late April. Some will add mention of locally
heavy rain and watch closely for multiple rounds of heavier rain.

The previous discussion below on the severe weather threats is
still valid.

75


&&

.AVIATION...
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.

75

&&

.FIRE WEATHER...

Strong gusty winds possible today and tomorrow ahead of an
approaching cold front. Rain chances remain low through Sunday
night as the cold front moves through. There are no major fire
weather concerns aside from the gusty winds. RHs will remain high.

&&

.PREV DISCUSSION...
/Issued 333 AM CDT Sat Apr 29 2017/
The main stories today will be the gusty winds and high temperatures
across Central AL. A 593 decameter 500mb high is centered just off
the Carolina coast as a 552 decameter low moves into the Texas
Panhandle through the day. This will help set up a fairly tight
surface gradient with south-southeasterly winds gusting to 20-30 mph
with locally higher gusts possible. Have issued a wind advisory
valid through the afternoon hours during peak heating when the
greatest mixing will occur. Winds will weaken during the overnight
hours and could increase again tomorrow back to advisory conditions.
Will mention this in the HWO for Sunday afternoon.

With the strong gradient and southerly winds, expect continued warm
air advection and temperatures rising to the upper 80s today with
some areas breaking the 90 degree mark by late afternoon. It`s
worth noting that the strong southerly flow will also be bringing
quite a bit of moisture with it, making those 90 degree
temperatures feel every bit of 90s degrees or higher with the heat
index. A few pop-up showers are possible across the area, with
the best chances being in the eastern half of the area. So, in
summary, today will be warm and windy, with low chance of some
isolated pop-up thunderstorms.

Taking a closer look at the system coming in on Sunday, a surface
low moves northeastward through the Central Plains and becomes
occluded as we go into Sunday afternoon. A cold front stretches down
through the Mississippi River Valley and progresses eastward into
Central AL by later in the evening and through the overnight hours
into Monday morning. Much of the convection should remain along the
frontal boundary as the 0-6km bulk shear values are roughly at 10-15
degrees relative to the orientation of the front. A strong low level
jet enhances the 0-6km bulk shear to over 40kts, while the upper
level jet remains off to the north and west. Forecast soundings show
a thin, deep layer of instability and a largely saturated profile.
The hodographs are quite impressive due to the combination of the
backed surface winds and the strong 850mb jet that becomes
southwesterly as the storms move through. This setup supports a
convective line of storms moving through that could possibly have
some breaks or bowing segments. The main threat will be damaging
winds, but brief tornadoes can`t be ruled out given the wind
profiles. Will highlight a slight risk across our entire area for
Sunday night into Monday morning. SPC has the slight risk generally
for the western two-thirds of the area with the eastern counties
under a marginal risk. Can`t argue with this either, as the exact
timing of the storms is still uncertain. However, with forecast
soundings showing the wind profiles and instability remaining
favorable into Monday morning, and a narrow corridor of higher
dewpoints building in just along and ahead of the cold front, will
carry the slight risk for the eastern counties as well. The big
question will be the timing and whether or not the front moves
out of the area before sunrise. Along with any severe potential,
heavy rain can be expected with these storms, as PWATs are
forecast to be above the maximum climatologically.

The cold front passes and surface ridging builds in Monday evening
through Wednesday. Temperatures are moderated slightly and shouldn`t
reach as warm as the past few days. Rain chances increase late
Wednesday ahead of the next system moving in. An elongated
positively- tilted trough slides south and models hint at a
surface low developing over eastern Texas and moving through the
Ozarks and into Northern Mississippi Wednesday night into
Thursday. There is still quite a bit of uncertainty with this
system as models disagree on the track of the surface low and the
orientation of the upper trough. Will continue to monitor this
system for any changes in the severe threat. Rain chances decrease
by Friday afternoon as the upper-level low moves east of the area
and cold air advection moves in on the back side of the low
bringing daytime highs Friday and Saturday down into the upper 60s
to low 70s.

25/Owen

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     66  85  59  74  50 /  10  30  90  40   0
Anniston    68  85  60  74  51 /  10  20  90  50   0
Birmingham  71  86  58  75  54 /  10  40  90  20   0
Tuscaloosa  69  85  56  76  53 /  10  80 100  10   0
Calera      69  86  59  76  54 /  10  30  90  30   0
Auburn      69  85  64  75  55 /  10  10  90  80   0
Montgomery  70  88  62  78  54 /  10  20  90  50   0
Troy        69  86  64  78  54 /  10  10  90  70   0

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Wind Advisory from 8 AM to 10 PM CDT Sunday for the following
counties: Autauga...Barbour...Bibb...Blount...Bullock...
Calhoun...Chambers...Cherokee...Chilton...Clay...Cleburne...
Coosa...Dallas...Elmore...Etowah...Fayette...Greene...Hale...
Jefferson...Lamar...Lee...Lowndes...Macon...Marengo...Marion...
Montgomery...Perry...Pickens...Pike...Randolph...Russell...
Shelby...St. Clair...Sumter...Talladega...Tallapoosa...
Tuscaloosa...Walker...Winston.

&&

$$


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