Area Forecast Discussion

FXUS64 KBMX 181144
Area Forecast Discussion
National Weather Service Birmingham AL
644 AM CDT Sun Mar 18 2018
For 12Z Aviation.
Today and Tonight.
A weak cold front has pushed into north Alabama early this morning, 
roughly along a Gadsden, Al, to Columbus, MS line. The front will 
likely drift southward this morning and becoming stationary just 
south of I-20 later this morning. Rain free conditions will continue 
across central Alabama through mid morning as the air mass remains 
relatively dry in the 850-500mb layer. An 850mb warm front will lift 
northeast across Mississippi and southwest Alabama this afternoon. 
Numerous showers and thunderstorms will develop along this warm 
front. Instability will increase in this warm sector with SB CAPE 
values forecast to climb to around 1000 j/kg this afternoon, roughly 
along and south of the Highway 80/I-85 corridor. Limiting factor 
will be weak mid level lapse rates (5.5-6 C/Km), and will keep 
threat level at marginal for southern counties for this afternoon 
and early evening, with damaging winds the primary threat. Broad-
scale lift overnight will produce widespread showers and 
thunderstorms. Despite destabilization in the mid levels later 
tonight, there does not appear to be any areas of concentrated 
forcing to produce organized severe thunderstorms, but an isolated
storm still possible.
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. This could be a
limiting factor for the strong (EF2+) tornado potential. Still, 
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, even if
they may be EF0/EF1 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.
12Z TAF Discussion.
A weak cold front was located near the I-59 corridor. Sfc winds
were from the north at 3-5 kts north of the front, with west to
southwest winds 5-7 kts ahead of the front. Low cigs, blo 3000 ft
agl, were mainly confined to south Alabama, with lcl IFR cigs at
KTOI. Drier air across north Alabama should keep cigs aoa 3000
feet agl thru 06z, with a lowering of the cigs after 06z. Sct
shwrs will reach KTCL by 19z and spread rapidly eastward thru the
afternoon. The intensities do not look particular strong, so 
tstms chances will remain very low thru 19z. Moisture and 
instability increase across south Alabama tonight, and chances 
will be higher for isolated TSRA. Better chances for rain tonight 
across the northern TAF sites as lift increases area wide. 
Instabilities also increase this evening for northern TAF sites as
cold air aloft moves over the area, and included prob30 TSRA. 
Several rounds of showers and thunderstorms are possible through 
Tuesday. Best rain chances this afternoon will be across the 
southern and western counties this afternoon. Widespread showers and 
thunderstorms are expected tonight. A good portion of the day Monday 
will be dry before severe thunderstorms develop during the late 
afternoon and evening hours. Lighter showers are possible for 
Tuesday, before dry conditions arrive by Wednesday. RH values will 
be elevated the next couple days. Low clouds and patchy fog are 
possible this morning and possibly again Sunday night. There are no 
fire weather concerns.
Gadsden     71  56  75  51  63 /  20  80  70  70  40 
Anniston    73  58  76  53  67 /  30  80  60  70  40 
Birmingham  72  59  77  52  66 /  30  80  70  70  40 
Tuscaloosa  73  61  80  51  63 /  60  80  60  40  30 
Calera      74  60  78  53  66 /  40  80  60  70  30 
Auburn      75  59  75  56  69 /  40  80  40  70  30 
Montgomery  77  63  81  56  70 /  60  80  40  60  30 
Troy        77  62  81  58  71 /  70  80  40  50  20