Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 170040

Area Forecast Discussion
National Weather Service Birmingham AL
740 PM CDT Fri Mar 16 2018

For 00Z Aviation.


Through Tonight...

An area of light to moderate rain has spread into west Central
Alabama this afternoon. Current radar trends show the activity
diminishing as it moves eastward and encounters drier air. The
best rain chances through sunset will be across the west.

This activity is being aided by a weak upper level impulse that
will move to the east, and expect a break in much of the shower
activity this evening. The next impulse will cross the area late
tonight, and rain showers will return to the area after midnight
tonight, mainly across the north. Low clouds are possible but with
boundary winds remaining elevated, do not expect fog overnight or
early Saturday morning.

With increased moisture and southerly flow, temperatures tonight
will be in the mid 50s in the east to around 60 in the west.


Updated for the severe weather threat on Monday.

NAM/GFS/ECMWF have taken steps toward a more ominous setup for
Monday afternoon and evening. There appears to be less emphasis on a
potentially disruptive convective episode Sunday night into Monday
morning. This is probably due to a stronger elevated mixed layer
spreading in across the region Sunday night which would allow the
warm sector to move inland more rapidly. Disagreement remains
regarding the finer but crucial details for the evolution of the
surface to 850mb response to the upper-level trough. This will make
or break the potential for tornadoes. Currently the solutions range
from a tornado outbreak, shown by the NAM, to a lower tornado threat
on the GFS/ECMWF. However, confidence continues to increase in the
potential for large to very large hail due to steep mid-level lapse
rates and strong deep-layer shear. Our updated products will reflect
an increase in the severe weather threat, particularly for large
hail, with an acknowledgement of an upward trending but still
uncertain tornado potential.


Previous Long Term Discussion from 405 AM CDT Fri Mar 16 2018.


A weakening surface low quickly slides eastward and pushes a front
southward through Central AL. This is a fairly broad front without
much forcing, but should be enough to produce a line of showers.
Coverage of showers along this line will likely be more scattered,
thus I will not go much higher than 50% PoPs through Saturday
afternoon. Can`t rule out a thunderstorm or two with these as
instability will be 1000-2000J/kg. However, veered surface winds
should limit any significant severe weather threat. By Saturday
evening and into the overnight hours, models pick up on another
frontal boundary sliding southward, but doesn`t show much in the way
of showers/storms developing along it. I would expect at least
scattered showers and maybe an isolated thunderstorm to develop
along this front in the overnight hours, even though models aren`t
really picking up on it yet, so I`ve included slight chance PoPs
with mention of isolated thunderstorms.

Sunday and Monday.

The previously-mentioned front is still pushing through early Sunday
morning and will stall just south of us by Sunday afternoon.
Troughing digs into the Central and Southern Plains later Sunday
afternoon and develops a strong upper level low. The GFS still
develops a pre-frontal shortwave trough that ejects out of the base
of this deeper longwave trough and helps lift that stalled front
northward as an effective warm front. Lift along the frontal
boundary, coupled with the shortwave will likely lead to a MCS
sliding eastward late Sunday night into Monday morning. Ample
environmental shear exists to help organize and propagate this MCS
like models are showing, but instability is in question given the
overnight timing. Furthermore, GFS limits the moisture rebound, so
I`m not 100% sold on a significant severe threat with this MCS,
though some gusty winds can`t be ruled out. We`ll need to continue
to monitor the thermodynamics of this Sunday night/Monday system to
assess the severe potential. The biggest concern right now would be
for heavy rain and the possibility of training storms along the
frontal boundary. This could lead to some localized flooding,
highlighted by WPC`s slight risk area for excessive rainfall, but
confidence isn`t high enough to mention in our HWO at this time.
It`s worth noting that the 00z NAM has this MCS moving through
Northern AL rather than Central/Southern AL, but this is at the
fringe of NAM output, so I wouldn`t much stock in it just yet.

Later Monday morning into the early afternoon, the upper low
intensifies and the surface low deepens. This will bring a dryline
boundary across the MS River Valley and through MS by Monday
afternoon. Ahead of this dryline, models are in decent agreement
right now with the moisture rebound and afternoon destabilization
across Central AL. MUCAPE values in both the GFS and ECMWF are 1500-
2000 J/kg with surface based CAPE exceeding 1500. Surface winds are
backed to the south for most of the area, with some spots forecast
to see more southeasterly winds according to the GFS. These backed
surface winds, coupled with the flow around the deepening upper low
will result in a fairly deep shear. 0-6km Bulk shear is around 55-
65kts with 0-3km being 40-50kts. Essentially, the forecast
hodographs have good curvature and SRH values are greater than 300,
which would support rotating updrafts. I`m not overly impressed with
the strength of the LLJ that both the GFS and EC are showing, but
that wouldn`t prevent a severe threat. The biggest uncertainty right
now is the Sunday night/Monday morning MCS. Timing and location of
that MCS will almost fully determine where and what kind of severe
threat we`ll see on Monday afternoon/evening. The MCS could very
well cut us off and limit any destabilization, but for now the
models show moisture and instability building back in. Because of
that, and with the models now showing more backed surface winds and
better low level shear, I will change the severe thunderstorm threat
in the HWO to a tornado threat and increase the confidence slightly
(to a 2). At this time, all modes of severe weather look possible
(damaging hail, wind, and tornadoes) for Monday afternoon/evening.
We`ll need to monitor model trends in the next couple of days to
determine specifics on timing and location.

Tuesday through Friday.

The surface low continues to move eastward into the Appalachians and
Coastal Mid Atlantic Tuesday into Wednesday. Models pick up on
another shortwave coming around the base of the trough that will
kick the actual cold front through Central AL on Tuesday, but
moisture return and instability look limited with this. Could see
some showers with it, so have carried slight chance PoPs through the
evening hours. Wednesday through Friday look to be cooler and dry as
surface high pressure builds in on the backside of the departing



00Z TAF Discussion.

Rain shower activity is on the decrease during the evening with a
small break with another round of showers expected during the late
night hours after 6z moving across the state through midday on
Saturday. Cigs are expected to start out as MVFR for most during
the late evening except for the west (TCL) who is already IFR.
Cigs for remainder of TAFs are expected to lower to IFR during
the late night/pre dawn hours as next round of convection moves in
from west with more upper level disturbances. Conditions are
expected to go back to VFR during the afternoon after rains stop
and take a break before more rounds of storms for Sunday into




Several rounds of showers and thunderstorms are possible this
weekend. The first is occurring this afternoon with the best rain
chances in the west through early evening. The second will bring
an increase in shower coverage to the area after midnight tonight,
with rain chances decreasing from west to east through the day
Saturday. The better chance for widespread rainfall and thunderstorms
will be Sunday night and Monday. In addition, to this wet pattern,
temperatures will be warmer and RH values increased. Low clouds
are possible tonight, although fog is not expected. There are no
fire weather concerns.


Gadsden     56  75  52  71  55 /  80  40  20  10  80
Anniston    56  74  53  72  57 /  80  40  20  20  80
Birmingham  58  76  55  72  60 /  60  40  20  20  80
Tuscaloosa  59  79  57  74  61 /  60  40  20  30  80
Calera      58  76  57  73  61 /  60  40  20  30  80
Auburn      56  75  58  75  61 /  50  50  20  30  70
Montgomery  58  78  60  77  64 /  50  50  20  50  70
Troy        58  77  61  78  64 /  40  50  20  50  60





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