Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 201811

Area Forecast Discussion
National Weather Service Birmingham AL
1211 PM CST Fri Jan 20 2017

For 18Z Aviation.


Today through Saturday night.

The mid-level dry slot associated with yesterday`s system has moved
over the area, putting an end to the bulk of the precipitation,
but lingering low-level moisture continues to result in a few
showers in East Alabama. Winds have remained elevated enough for
low stratus to be more prevalent than fog but patchy low
visibilities will remain possible through this morning. Today only
isolated to scattered showers at most are expected with some low-
level moisture but also dry air and ridging aloft. A stray
thunderstorm will also be possible in the far southeast.
Temperatures will continue to be very warm.

Overall there isn`t much change to the forecast thinking from the
previous discussion for this weekend. All of the parameters
necessary for a significant severe weather event are present at
one time or another; the question is if and when they will overlap
in space and time. Multiple rounds of strong to severe convection
result in the confidence decreasing in the details of each
successive threat even as the potential impacts increase. The
overall synoptic setup is a deep cold core upper low developing
over the Southern Plains and moving eastward across the Deep South,
with very steep mid- level lapse rates, a strong jet rounding its
base, and warm moist air ahead of it. The cyclonic flow at low-
levels, however, is somewhat broad resulting in the low-level
jet/warm conveyor being displaced well to the east of the upper
low itself.

The first threat is still set to arrive after 3 AM Saturday morning.
An initial upper-level jet streak and lead shortwave will move
towards the area in southwest flow aloft, loosely associated with a
system moving through the Central Plains. This will be
accompanied by a southwesterly low level jet. Thunderstorms will
develop by late evening over Louisiana and the Gulf on the nose of
the low level jet and move northeastward through Mississippi,
entering our southwest counties after 3 AM. A mix of supercells
and line segments are expected, possibly organizing into one or
more MCSs. Bulk shear, lapse rates, and SRH will be conducive for
a threat of tornadoes, damaging winds, and large hail. Instability
will be on the low side initially, but will be quickly increasing
during the morning. The speed of this initial convection will
determine the magnitude of the threat. More cold pool dominant,
more progressive convection, similar to the NCEP and NSSL WRF-
ARWs, would result in less instability than a slower progression
similar to the WRF-NMM and 3/4 km NAM runs. IF the slower
progression was realized then greater instability will develop
ahead of it and a strong tornado or two would be possible, but
this remains conditional at this time. Also, if convection closer
to the Gulf Coast becomes dominant this would lessen the threat,
but southwesterly oriented Corfidi vectors argue for convection to
lift away from the Gulf Coast and into the forecast area. Overall
the current elevated threat area across the southern half for this
morning threat looks on track, with confidence being too low for
an upgrade to a significant threat at this time.

During the day on Saturday instability continues to increase, while
the LLJ decreases by afternoon. Models generally indicate a lull in
activity by afternoon, and afternoon redevelopment is uncertain
with weak capping present in some forecast soundings. However,
CAPE values will be increasing to around 2000 J/kg with 60 kts of
bulk shear, with weak impulses moving through in cyclonic flow
aloft. Therefore there will be a conditional risk of isolated
supercells developing. The main threat with these appear to be
large, potentially significant, hail, as well as damaging winds.
The tornado threat with the afternoon activity is more uncertain
due to weaker LLJ and veering surface winds, however the high CAPE
and bulk shear values may be able to partially compensate, as well
as any boundary interactions.

Upper-level forcing will increase Saturday evening and overnight as
the upper low and another jet streak approaches. Expect an uptick
from any lull in convection during the afternoon. It should be noted
that the NAM is an outlier compared to the GFS/ECMWF/SREF as it does
not close off the upper low, rather making it a more progressive
open wave. Therefore the NAM and its higher res counterparts must be
considered untrustworthy at this time. During this time period, the
magnitude of instability and low level jet response as well as what
occurs upstream will determine whether or not a significant threat
evolves. The GFS/ECMWF indicate the best LLJ will be across or just
to the south and east of our southeast counties, where an elevated
threat will be maintained. Adjustments to the forecast for this
period are likely as the event approaches. All interests in Central
Alabama need to closely monitor and remain weather aware from late
tonight through Sunday. Given the nocturnal (after 3 AM) nature of
the initial threat tonight, it is imperative to have a weather radio
or other means of waking yourself up to receive warnings.


Sunday through Friday.

For Sunday and Monday, the strong low pressure system moves through
the Southeastern US bringing rain for much of Central AL. On Sunday,
enough instability remains that some thunderstorms could develop as
the main 500mb vort max moves across, but at this time, expecting
much of the severe threat to be to our south and east where better
support from the upper level and low level jets exists.

By Tuesday, the low pressure system has shifted eastward over the
Mid-Atlantic states and ridging begins to build in over the Deep
South ahead of deepening trough over the Western US. This brief
period of ridging should leave skies clear to partly cloudy for
Tuesday afternoon.

Models are in good agreement with the synoptic setup for the second
half of the work-week, but differ on timing. Overall, an upper low
develops on the lee side of the Rockies within a positively-
tilted longwave trough. The low moves northeastward into the Great
Lakes region by Wednesday, draping an elongated nearly east/west-
oriented cold front across Central Plains and Midwest. The ECMWF
has this cold front moving through Central AL Wednesday into
Thursday bringing chances for rain showers, while the GFS has the
front hanging back a little and moving through Thursday into
Friday. For now, have mention for "chances" of rain (25-35%
chance) for the entire second half of the work-week to reflect the
model differences. Will trim this back to more specific timing as
models trend one way or the other. Behind the front, we can expect
cooler, more seasonable, temperatures towards the end of the week.



18Z TAF Discussion.

Cigs will improve this afternoon, with broken decks rising to VFR
category heights. Southwesterly winds will increase to 10-12kts this
afternoon with an expected decrease this evening. There could be an
occasional stronger gust this afternoon. As southerly flow
increases ahead of an approaching squall line tonight, cigs will
lower to MVFR. Current timing of TSRA into west Central Alabama and
TCL is approx 21/11Z. This line will likely produce severe storms
with strong wind gusts, and timing will likely need to be refined as
storms develop to our west.




A break in the rain today as the area is in between storm systems.
Afternoon relative humidity values will generally remain above 50
percent through Monday due to the moist and unseasonably warm
conditions. Some strong to severe storms can be expected Saturday
and Sunday, along with periods of heavy rainfall. Rainfall amounts
through Sunday will average 1 to 2 inches.


Gadsden     71  58  69  55  63 /  20  40  70  70  80
Anniston    72  60  72  56  67 /  20  50  70  70  80
Birmingham  73  61  71  58  65 /  20  60  70  70  80
Tuscaloosa  74  61  72  55  64 /  20  80  70  70  80
Calera      73  61  72  56  64 /  20  70  70  70  80
Auburn      74  63  72  59  68 /  20  70  90  70  80
Montgomery  79  64  76  58  67 /  20  90  80  70  70
Troy        78  65  75  57  69 /  30  80  90  70  70




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