Special Weather Statement, NWS Birmingham, AL

Area Forecast Discussion
National Weather Service Birmingham AL
1251 PM CDT Fri Jun 1 2018

.UPDATE...
For 18Z Aviation.

&&

.SHORT TERM...
Today and Tonight.

An MCS is currently pushing eastward across Tennessee with an
outflow boundary moving southward into northern Mississippi. A few
showers and thunderstorms will be possible across the far northern
counties through the morning hours along the outflow boundary, but
the stronger storms this morning are expected to remain north of the
area. This boundary is expected to stall out across the northern
counties, and then may drift northward slightly due to southerly flow
at the surface. This outflow boundary will likely serve as the key
player for convective initiation this afternoon, serving as the
forcing mechanism that was missing yesterday. The MCS is associated
with a shortwave trough in the northwest flow aloft between an upper-
level ridge over Mexico and the Southern Plains and a trough along
the East Coast. This shortwave with some convective/MCV enhancement
will provide some upper-level forcing today. 00z CAMs and the HRRR
are in fairly good agreement on convective initiation along the
boundary across our far northern counties by afternoon with storms
moving from the WNW to the ESE with 20-25 kts of 700-500mb mean flow.

Area 00z soundings sampled steep mid-level lapse rates associated
with an elevated mixed layer across the area and a pocket of 500mb
temperatures around -10C. The 00z BMX sounding indicated 700-500mb
lapse rates of 7.2 C/km, while the 00z JAN sounding had 700-500mb
lapse rates of 7.9 C/km. These steep lapse rates aloft combined with
daytime heating and a humid air mass at the surface will result in
SBCAPE values of 3000+ J/kg and MLCAPE values over 2000 J/kg with
fat CAPE profiles. This will be favorable for strong updrafts, while
DCAPE values over 1200 J/kg will be favorable for strong downdrafts
and strong to damaging winds. Microburst parameters are also in the
high to very high category. The limiting factor preventing a more
widespread/significant damaging wind threat appears to be 850-700mb
flow oriented parallel to the outflow boundary rather than
perpendicular, and capping further south, but this will be closely
monitored. Expect several clusters and bowing segments to develop.
Quarter size hail will be possible as well. Can't rule out hail up
to golf ball size as well given the lapse rates/instability, but
limiting factors will be hot air at the surface and weak bulk shear,
so will stick with just quarter size hail in the graphics at this
time.

All in all, the SPC upgrade to a slight risk looks very reasonable
and will indicate this in our severe graphics. There's some question
as to whether any severe storms will make it as far south as the I-
85 corridor given warmer 700 mb temperatures down there and the
westerly component to the flow. Will hold off on a threat in these
areas but monitor trends. One thing to keep an eye on will be how
far south the outflow boundary makes it this morning. If it makes it
further south than forecast, this could cause the threat to shift
further south, and perhaps decrease the threat in the Highway 278
corridor. Overall flooding threat does not look very high compared
to previous days based on PWATs and the speed of storm motions.
However, some localized flooding will be possible given saturated
soils and uni-directional 850-700mb flow which could result in areas
of training. The PoP forecast for tonight will be dependent on how
far south outflow from the afternoon convection makes it. There are
some signals for scattered showers and isolated thunderstorms
lingering through the overnight hours along an outflow boundary.

32/Davis

.LONG TERM...
Saturday through Thursday.

On Saturday, any minimal upper ridging over the Caribbean weakens
as the Eastern Conus upper trough pushes further east over the
Atlantic Seaboard. Lingering activity from Friday's system moving
southward in the morning across Central Alabama may likely squash
our instability for during the day on Saturday. It all depends
how quickly Friday's system deteriorates, if there are lingering
boundaries in its wake, and if we can heat up enough early on in
the day Saturday. However, at present, guidance is hinting at the
first scenario.

Step forward to late Saturday night into the pre-dawn hours on
Sunday. Another convective complex ahead of the front will move
south/southwest through the upper flow. Indications are that it
would be deteriorating in the late night hours. Also, guidance is
trending a little slower which could put the system barely entering
the northwest counties between 4 AM and sunrise depending on
model timing differences.

Along the front in the south Sunday afternoon, we could see enough
forcing with our moist environment in place for a few strong
storms and some damaging wind gusts during the heat of the day.
After the front, we dry out and cool down some for the first half
of the week. With the front stalling out along the coast, we could
see an isolated storm in the far south by Wednesday. However, for
the most part our extended is dry.

08

&&

.AVIATION...
18Z TAF Discussion.

Numerous showers and thunderstorms will continue to develop
through the afternoon hours, some possibly severe, especially
across the north. Expect brief heavy rain and frequent cloud-to-
ground lightning with these storms. Stronger storms may produce
damaging wind gusts and/or large hail. Most of the convection
should diminish after sunset, but some storms could remain across
the south. We could see some redevelopment overnight, but
confidence in location and coverage is too low to mention at this
time. IFR/MVFR cigs expected again late tonight as a stratus deck
returns.

19

&&

.FIRE WEATHER...

A northwest flow pattern will keep rain chances fairly elevated
through the weekend with scattered to numerous showers and
thunderstorms expected. The best chances will be in the
afternoon. Rain free conditions return on Monday and through the
middle of next week. No fire weather concerns at this time.

08

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     67  90  70  87  61 /  40  30  20  50  10
Anniston    68  88  71  88  62 /  40  40  20  50  10
Birmingham  71  90  73  89  64 /  40  40  20  40  10
Tuscaloosa  72  92  74  90  64 /  40  40  20  40  10
Calera      71  90  73  88  64 /  40  50  20  50  10
Auburn      71  87  72  89  65 /  50  50  20  50  10
Montgomery  73  90  74  90  66 /  50  50  20  50  10
Troy        72  89  73  91  65 /  50  50  20  50  10

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
None.

&&

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