Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 190509

Area Forecast Discussion
National Weather Service Birmingham AL
1209 AM CDT Mon Mar 19 2018

For 06Z Aviation.


Through Tonight.

An area of rain with embedded thunderstorms is moving across
Central Alabama this afternoon. This activity is tracking along a
stalled frontal boundary that will lift back to the north
overnight. Within the warm, moist airmass south of the now lifting
warm front, scattered to numerous showers are possible. This
activity is not expected to be strong or severe. Temperatures
tonight will be rather muggy, with lows in the mid 50s north to
mid 60s south.

Looking toward tomorrow, not much has changed from what was
discussed in the previous long term discussion. There remains
some uncertainty regarding the extent of rain coverage early
Monday morning (either just to our north or along the Gulf coast),
and how that affects the airmass through the morning hours.
There is good model agreement regarding the development of a
broken line of storms occurring just to our west or over NW
Central Alabama by late afternoon, and pushing eastward through
the evening hours. The expected impacts and coverage areas have
not changed. We made slight adjustment to the timing this
afternoon and updated graphics will be out shortly. We will
continue to refine timing tonight and tomorrow morning.


Monday through Saturday.

Confidence that the area will see significant severe
thunderstorms continues to increase for Monday, as models are
coming into better agreement on the synoptic pattern. As usual,
there are some mesoscale details that will determine the exact
timing and type of threat. Models are coming into better agreement
with the timing and placement of a compact upper low that will
eject out of the western CONUS trough into northeast New Mexico
this afternoon. It will deamplify slightly as it moves eastward to
the TN/KY border region as it approaches confluent flow
associated with the eastern Canadian trough, but still remain a
potent system with strong height falls and a neutral tilt. A
strong WSW mid and upper level jet max (~90kt at 250 mb, ~70 kts
at 500 mb, ~65 kts at 700mb) will be located to its south, which
will move over Central Alabama. A ~35-40kt southwesterly LLJ at
850mb will accompany a 998mb surface low moving across Tennessee.
This surface low will be close to steady-state in strength during
the afternoon, before effectively splitting into two pieces as it
encounters the southern Appalachians Monday night. A trailing
pre-frontal trough/dry line feature will move into West Alabama
from Mississippi Monday afternoon.

Starting off Monday morning, not expecting to see a whole lot of
activity on the radar due to dry air and capping aloft associated
with an elevated mixed layer and weak shortwave ridging aloft.
Showers and storms associated with Sunday night`s shortwave will
likely be mainly north and east of the forecast area. But with
low-level isentropic lift there could be some light showers, and
perhaps some isolated leftover thunderstorms from Sunday night.
There will probably be some low stratus as well. One possible
wrench in the forecast is a secondary LLJ in the NAM and some of
the global models that these models show causing an increase in
moisture and isentropic lift across southeast Alabama and the
Florida panhandle during the late morning with associated
precipitation. This could affect destabilization in the afternoon,
or if accompanied by thunderstorms could result in an isolated
severe threat developing earlier than expected in the far
southeast counties. However, the CAMs are much further east with
this activity, closer to the Florida Big Bend and South Georgia.
These may have a better handle on the situation if they are
handling Sunday night`s preceding convection better than the
global models. Also, the models that show this feature all still
indicate very strong instability developing by Monday afternoon
across the entire area, so it may not matter much.

Most of the day on Monday may be a nice warm sunny spring day, but
folks should not let their guard down. This daytime heating will
allow CAPE values of 2000 to 3000 J/kg to develop due to
temperatures in the upper 70s to low 80s. The 00Z NAM did show an
unlikely situation where low stratus clouds never mix out all and
surface temperatures remaining much cooler, preventing convective
initiation. But this seemed extremely unlikely given the strong
southwesterly flow and dry air aloft. The 6Z NAM has already
trended back in line with strong daytime heating, but is slower
with the storms than the guidance consensus as is its typical bias.
The WRF-ARW, NSSL WRF, and WRF-NMM were in good agreement in
explosive thunderstorm development along the pre-frontal trough/
dry line as upper- level forcing arrives, between about 4PM and
7PM roughly along/northwest of the I-59 corridor. These storms
will probably initiate right over Central Alabama rather than
moving in from Mississippi. The convective mode will be a broken
line of semi- discrete supercells, due to westerly 55-60kt 0-6 km
bulk shear vectors orthogonal to the initiating boundary. Dry air
aloft/capping will likely inhibit storms from developing ahead of
this line, except perhaps in the far northeast counties, so
confidence in timing out this line is increasing. The environment
will be very favorable for large to very large hail to develop due
to the rotating updrafts and fat CAPE profiles more typical of
the Plains than the Southeast, with significant hail >2" diameter
appearing likely with some of the storms. Damaging winds will also
occur with the hail-enhanced downdrafts due to high DCAPE.

Concerning the tornado potential, the 850mb-925mb flow will be a
bit veered to the southwest and not extremely strong. This veered
flow will probably mix down to the surface; the NAM is more backed
but does not seem to be mixing the boundary layer properly as
mentioned related to the spurious looking low stratus mentioned
above. The surface low will also not be deepening, so isallobaric
flow will not be a factor. So near-surface streamwise vorticity
does not appear to be enhanced, with the critical angles between
the 0-0.5km shear vector and 0-0.5km storm-relative flow being
mainly well below the optimal 90 degree angle. The presence of
strong rotating updrafts in a high CAPE environment with around
200m2/s2 0-1km SRH can compensate somewhat for the less than
favorable wind directions. Therefore, think there is a threat for
a couple to maybe a few tornadoes, especially the further north
you go closer to the surface low, and especially right around/just
after sunset when the nocturnal LLJ begins to strengthen. One
potential area of concern for strong tornadoes, however, is the
far northeast counties where flow could remain a bit more backed,
near a lingering wedge front over north Georgia. Confidence
remains too low to mention the potential for a strong/significant
tornado in the HWO/graphics, but we will continue to monitor
closely as we get closer. Either way, these will be dangerous
storms that everyone needs to pay attention to. Did opt to lower
some of the far southern counties from an enhanced risk to a
slight risk in our local forecast. There is still certainly a
severe threat in those areas that people need to be aware of, with
plenty of instability present, but storm coverage is expected to
be more isolated down there due to weaker forcing.

Looking briefly at the extended, another shortwave will bring a
chance of showers Tuesday, but nothing severe is expected. Lows
may drop to near freezing Wednesday night. Models are not in good
agreement with a shortwave/possible frontal passage in the
Friday/Saturday timeframe, but generally indicate a lack of
significant moisture return ahead of it.



06Z TAF Discussion.

An areas of rain showers along with a few claps of thunder will
continue to swing east-northeast/eastward across the state. Except
for some brief activity in the south which should mainly scrape
MGM/TOI, activity should be light during the overnight hours. The
morning could have a some scattered showers, but the main show
will come late in the afternoon and into the evening with TSRA as
the boundary moves into the area. MVFR cigs will be around
tonight with some IFR as well. Storms may be strong/severe during
the later part of the forecast as the boundary moves in.




Several rounds of showers and thunderstorms are possible through
Tuesday. Best rain chances this afternoon and evening will be
across the southern counties. Scattered to numerous showers are
possible overnight tonight, with a drier period Monday morning.
Severe thunderstorms are expected during the late afternoon and
evening hours as a line of storms moves through the area from west
to east. Light showers are possible Tuesday, before dry
conditions arrive by Wednesday. RH values will be elevated the
next couple days. Low clouds and patchy fog are possible tonight
and possibly again Monday night. There are no fire weather


Gadsden     74  51  63  37  56 /  50  80  40  30   0
Anniston    75  53  67  37  57 /  50  80  40  30   0
Birmingham  77  52  66  38  58 /  70  80  40  20   0
Tuscaloosa  79  51  63  38  62 /  80  70  30  10   0
Calera      77  53  66  39  59 /  50  80  30  20   0
Auburn      73  56  69  39  58 /  40  80  30  20   0
Montgomery  80  56  70  41  63 /  40  60  30  10   0
Troy        80  58  71  42  62 /  40  60  20  10   0





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