Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 221338
AFDBMX

Area Forecast Discussion
National Weather Service Birmingham AL
838 AM CDT Sun Apr 22 2018

.UPDATE...
MESOSCALE UPDATE.

&&

.NEAR TERM...

...Severe Storms Possible This Afternoon and Tonight...

General Discussion:
Cloud cover was abundant over the region this morning as an upper
level low located over northeast Oklahoma and associated surface
low pressure areas over northwest Mississippi and southwest
Louisiana were moving generally northeast. Radar imagery indicated
isolated showers across our area with showers and embedded
thunderstorms across portions of our far northern counties and
further north into the Tennessee Valley region. To the southwest,
more showers were detected by NEXRAD radar across west Tennessee,
north-central Mississippi and into southwest Alabama.

Expect isolated showers with potential for a few thunderstorms to
increase over the next few hours with higher coverage north and
west. The sampled environment contains sufficient wind shear to
support severe storms with a damaging wind and isolated tornado
risk. Primary limitation in severe storm formation and sustainment
is limited surface-based instability currently in place across the
area. Surface warm advection and breaks in the cloud cover will be
closely monitored as severe storm potential will be more closely
correlated with available low-level instability.

Technical Analysis:
Upper-Level Features:
GOES-R Multi-channel imagery depicts an
upper level low located across northeast Oklahoma. Global
numerical models depict the upper low elongated in a negatively
tilted fashion.

Surface Features:
Objective surface analyses reveal two surface low pressure areas,
once near Greenwood, MS and another across southwestern
Louisiana. Both surface lows are fairly diffuse but the one in
Louisiana is slightly deeper at this hour.

An examination of surface Theta-E and Mixing Ratio objective
analyses indicates a diffuse moisture gradient present across the
southern third of Mississippi and extending east-southeast across
southwest Alabama and bisecting the far western Florida Panhandle.

Sounding Interpretation:
22/12Z BMX sounding indicates significant moistening has occurred
since 00Z, especially above around 760 mb. The vertical profile is
nearly saturated from near the surface aloft to near 200 mb. An
inversion exists from around 945 mb to around 920 mb, likely
influenced by the more easterly flow near the surface. This more
stable layer is eliminating much of the convective available
potential energy (CAPE) and is supporting low cloud bases
generally less than 2500 ft AGL across the area. With the
increased moisture in the profile precipitable water (PWAT) values
have substantially increased to 1.34 inches.

The wind profile veers with height with strong directional shear
from the east-southeast near the surface quickly shifting from the
south towards 900 mb and becoming southwest near 630 mb. Wind
speeds generally increase with height from around 5 kts at the
surface to around 35 kts around 925 mb. Winds slacken briefly
above this level then intensify further aloft with speeds of 40
kts near 550 mb to near 50 kts around 280 mb. The strong
directional shear and moderate speed shear are resulting in 0-6 km
bulk shear values around 49 kts with 0-1 km storm-relative
helicity (SRH) values around 254 m2/s2 with 0-3 km SRH values of
239 m2/s2.

05

.SHORT TERM...
Today and Tonight.

Summary:

Models have trended a bit further north with the position
of the warm front this afternoon, resulting in a little bit more
instability across parts of our southern/western counties.
Uncertainty remains due to rain moving in from the west, but a low
confidence/marginal threat of brief tornadoes/damaging winds has
been added to the HWO. Gradient winds will also strengthen across
parts of north-central/northeast Alabama this afternoon and tonight,
and a wind advisory has been issued for these areas.

Details:

Water vapor imagery indicates a strong upper-level closed low
spinning near Ft. Smith AR. Lift and moisture transport ahead of the
upper low has resulted in a large area of rain and a couple embedded
thunderstorms across the Lower Mississippi Valley. A small MCS is
also noted near the Louisiana Gulf Coast. Some mid-level radar
echoes are located over West Alabama and probably not reaching the
ground, while lower-level isentropic lift has resulted in a few
showers developing across northern portions of Central Alabama east
of I-65. At the surface, a broad area of disorganized low pressure
is located over the Arklatex, with one mesoscale are of slightly
lower pressure over northwest AR and another one near Lake Charles
LA near the small MCS. A cold front is currently pushing towards the
Texas Gulf Coast, with a warm front extending along the rest of the
northern Gulf Coast.

A very messy pattern is expected on the mesoscale that will impact
today`s forecast. Multiple models are suggesting that the southern
meso-low near Lake Charles will strengthen a bit and lift northward
across MS and into West AL. This results in greater low-level
southerly flow, allowing the warm front to push further north as it
tries to dislodge the weak wedge air mass over the area. Models are
now indicating mid 60s dew points making it into our southern and
perhaps far western counties, and some associated surface-based
instability (though only around 500 J/kg). Backed surface winds and
strong low-level shear results in potentially favorable SRH, curved
hodographs, and near-orthogonal critical angles for brief tornadoes
to develop with any low-topped supercells that can maintain strong
enough updrafts. Several CAMs develop isolated updraft helicity
tracks indicating the potential for rotating storms. Damaging winds
would also be possible with any stronger bowing segments. This all
remains very conditional, however, as the large mass of rain
continues to advance from the west and would act as a stabilization
agent, and the small MCS near the Louisiana Gulf Coast would need to
not race too far east along the Gulf Coast. Enough models are
indicating that the southern part of the precipitation may slow
down/break up/lift far enough north just enough to warrant mention
of this threat. There could be two waves of activity, one lifting
across the south-central and southeast counties, and a conditional
wave developing later over MS near the strong dynamics associated
with the upper low and the meso-low at low-levels. Given these two
waves, will indicate the same start time for each portion of the
threat area, with the ending times progressing from west to east.

Have included a marginal risk in our impact graphics along and south
of a Reform to Clanton to Phenix City line. This represents the best
potential for destabilization based on HREF mean CAPE values, but
confidence in destabilization decreases the farther northwest you
go.

The gradient wind response to the meso-low will result in breezy
conditions this afternoon and tonight. Models are indicating strong
925 mb SE winds of 45-50 kts developing over the northeast quadrant
of Central Alabama by 00z. A lingering stable layer near the surface
due to the weak CAD wedge will be a limiting factor for mixing down
any higher gusts. But low-levels will be breezy enough to warrant a
wind advisory for this part of the area. The highest winds gusts
will occur at higher elevations, and if any areas of stronger winds
can get trapped underneath the stable layer.

Plentiful moisture will result in the potential for locally heavy
rainfall amounts, with amounts averaging around 2" across the area
with pockets of 3" or more. Flash flood guidance has largely
recovered across the area in the past week, with streamflows
returning to near normal, with reading below normal in parts of East
Alabama. It appears that the areas of higher rainfall rates will be
progressive, and the heaviest amounts will be over the eastern
counties where antecedent conditions are drier. Therefore, will not
be issuing a flash flood watch at this time, as the potential for
organized flash flooding looks low. But minor flooding of poor
drainage and urban areas and some rises on creeks and rivers is
expected.

32/Davis

.LONG TERM...
Monday through Saturday.

On Monday, our rain maker surface low pressure system is expected
to be near MEM with a front extending out from it into North and
Far Eastern Alabama. Rain activity will continue Monday across the
north and eastern counties with moisture slowly getting cut off
from the south. A few light rain showers will be possible for
Tuesday with the passage of the upper low to our north across
Kentucky and Tennessee. We have a break for late Tuesday as
system one exits. Behind the first front, temperatures should be
rather seasonable for mid April with 50s/70s for lows/highs
respectively. Not expecting a big drop as this Pacific system will
not have any decent cold air intrusion over Alabama.

An upper trough will dig into Eastern Conus and become a closed
low over the Great Lakes on Wednesday. As it does, a shortwave
will swing around it into the Deep South from the Central Plains.
In doing so, it will usher in front number two on Wednesday and
bring an end to our short rain break. With little recovery time of
our air mass, only scattered rain showers are expected with this
system through Thursday with the front (Wed) and then upper
shortwave (Thu). The air mass will be reinforced but likely only
a few degrees cooler behind system 2.

As if two were not enough, system three approaches right as two
exits with only a brief rain break for Thursday night. This third
system does have lower confidence as guidance solutions differ by
6-12 hours on timing. However, low rain chances are expected for
Friday for now and possibly could be added to Saturday if the
slower solution wins out. This system does have a rather large
upper low that takes its time to move across the Great Lakes and
New England over next weekend. This would mean a longer time with
a slightly cooler upper flow to allow for readings a little lower
as we round out the week into next weekend.

08

&&

.AVIATION...
12Z TAF Discussion.

MVFR cigs are currently present at all TAF sites due to moist
easterly flow north of a warm front. These will generally stick
around all day, though cannot rule out some improvement at MGM/TOI
after the warm front moves through. Scattered showers will
continue early this morning, before widespread showers and
embedded thunderstorms move in from the west associated with a low
pressure system. Tried to tempo/prob30 in when the best chance of
thunder/significant visibility reductions will be at the TAF
sites, but amendments are likely with multiple rounds moving
through. Rain ends from west to east overnight after the cold
front moves through, but low clouds remain. Winds will be breezy
through the day today and tonight, with gusts of 20-30 kts
possible.

32/Davis


&&

.FIRE WEATHER...

Our pattern changes today as widespread wetting rains expected
through Monday. Some thunderstorms will be possible this afternoon
and evening along with heavy rain at times. A few storms could
become severe across the southern counties this afternoon and
evening. A moist air mass will linger over the area for the first
half of the upcoming week. Afternoon minimum relative humidity
values will remain above 50 percent through Thursday. Critical
fire weather conditions are not expected to be met.

08

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     68  57  70  54  70 / 100 100  70  40  40
Anniston    68  58  71  55  71 / 100 100  60  40  40
Birmingham  69  59  70  55  70 / 100 100  50  40  40
Tuscaloosa  70  57  71  55  72 / 100  80  40  20  30
Calera      69  58  71  55  71 / 100 100  40  30  30
Auburn      68  60  73  56  72 / 100 100  50  20  30
Montgomery  74  60  75  56  73 / 100 100  30  20  30
Troy        74  62  75  56  74 / 100 100  30  10  30

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Wind Advisory from 1 PM this afternoon to 4 AM CDT Monday for
the following counties: Blount...Calhoun...Cherokee...Clay...
Cleburne...Etowah...Jefferson...Randolph...Shelby...St. Clair...
Talladega.

&&

$$


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