Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 201806

Area Forecast Discussion
National Weather Service Birmingham AL
1206 PM CST Wed Feb 20 2019

For 18Z Aviation.


Today and Tonight.

Satellite and water vapor imagery indicate a shortwave trough
lifting through the Central CONUS, with an apparent baroclinic leaf
present along the Mississippi River. Meanwhile, another vort max
within the longwave trough over the western CONUS is digging into
the Pacific Northwest, while the subtropical ridge remains centered
near the Bahamas. Radar indicates a slow-moving but narrow band of
showers and thunderstorms along a surface trough/developing cold
front crossing the Mississippi River. One warm front responsible
for yesterday`s thunderstorms has lifted into Tennesse, while a
marine warm front is currently located near the Gulf Coast, where
dew points in the upper 60s to around 70 are currently reported. A
strong cold air damming wedge remains in place over Central Alabama,
associated with a 1037 mb parent high near New York City. This is
resulting in continued cool easterly winds and low stratus. A few
wind gusts to advisory criteria remain possible especially at
higher elevations. A surface low is in the process of developing
near the ArkLaMiss intersection. Tonight`s 06z special BMX
sounding launched in support of the VORTEX-SE MESO18-19 field
project indicated pronounced drying aloft and a notable
subsidence inversion at 660mb above the frontal inversion at
900mb. This is due to subsidence in the wake of yesterday`s
convection and convective overturning, which was more robust than
previously thought. This results in poor 700-500mb lapse rates,
though there are steep lapse rates above the inversion. This does
throw a wrench into today`s severe weather forecast, though it is
preventing any shower/thunderstorm ahead of the front other than
light low-level isentropic lift showers/drizzle. The lack of
precipitation will remove one of the limiting factors preventing
the warm front from lifting northward. Looking upstream, this
inversion is present on other operational and research soundings
such as in Slidell LA and Jackson and Starkville, MS. Looking
further upstream however, it is not present in other sites such as
Memphis, Little Rock, Monroe LA, and Shreveport. So as height
falls from the shortwave occur, it may begin to erode. Will
continue to monitor this on the 12z soundings.

Model trends from yesterday have continued, indicating that the
the CAD wedge will indeed erode and the warm front will lift up into
the southern half of the area. Still have some concerns that this
will happen given the strength of the wedge, but SPC mesoanalysis
does indicate 4-8F 3 hr dew point changes in SW Mississippi and dew
points are fairly impressive, so this does begin to increase
confidence that this will happen. CAMs are in good agreement that the
current line of showers and thunderstorms will continue to slowly
move eastward through the day. Embedded low-topped supercells
combined with favorable SRH will result in a potential for a tornado
or two where surface-based CAPE can develop, as the line moves into
our southwest counties during peak heating with some weak updraft
helicity tracks noted in some HREF members. Some uncertainty
regarding exactly how far north surface- based CAPE will extend,
but the HREF indicates the threat should stay south of I-20. Did
nudge the threat a bit further east of I-65 given HRRR trends.
Threat should diminish by evening further east as the line loses
steam as the front stalls and the low level jet lifts out of the
area. While areas south of I-85 will heat up this afternoon, and a
very conditional threat of severe storms exists if a storm
develops, it looks like the cap/ridge aloft should win out there.
Further north, models indicate MUCAPE of 1000 J/kg, strong bulk
shear, and mid- lapse rates around 6.5 C/km suggest that while
there will not be a tornado threat, elevated storms will be
capable of producing hail with isolated quarter size hail possible
similar to yesterday. Given that elevated instability has
generally overperfomed with this system, will mention a marginal
threat of hail for much of the area north of I-85. Tried to
delineate the tornado/hail vs. hail only threat in the graphics as
best as I could. Not enough confidence to mention a slight risk
in the far southwest, but we will monitor mesoscale trends
through the day.

Moving on to the flooding threat, there are several competing
factors. Negative factors include the small overall size of the
precipitation area and limited large-scale forcing other than some
upper-level divergence. Positive factors remain the PWATs near
February all-time highs, potential for training, slow-moving nature
of the convection, and the stalling front. The primary risk of true
flash flooding appears to have trended upward for the northwest
counties where a band of 2-3+ inches fell yesterday. This is where
WPC has a moderate risk of excessive rainfall. Elsewhere, overall
rainfall amounts are forecast to be 1-1.5 inches. While this would
mainly cause areal flooding of creeks, stream, low-lying areas, and
poor drainage areas, still think there is a risk of at least
isolated flash flooding given yesterday`s rainfall of 1-1.5 inches
and the tendency for rainfall to over-perform lately. Many rivers
are already forecast to rise above flood stage. The current flood
watch looks good with no change necessary. Mainly post-frontal,
lighter rain will linger through the night in a southwest to
northeast band as the front stalls, which will continue to cause
some potential flooding issues. Meanwhile some drier air will
begin to make it into just the far northwest.


Thursday through Tuesday.

Height rises will occur aloft on Thursday as ridging temporarily
builds in over the eastern CONUS ahead of a closed low near Las
Vegas. The surface front will remained stalled out near the I-85
corridor during the day. However, southwest flow returns aloft ahead
of the trough out west, causing the 925/850mb front to lift
northward. Associated frontogenesis/isentropic lift/overrunning will
maintain the post-frontal precipitation area and cause it to begin to
shift northward. While overall rainfall amounts should be lower, the
presence of some weak elevated instability suggests any ongoing
flooding could be aggravated, and the flood watch will continue to be
in effect. CAMs also suggest scattered thunderstorm development
south of the front with daytime heating of an unstable air mass.
This could pose a conditional strong/isolated severe storm risk over
our far southeast counties where hail/gusty winds would be possible
given sufficient deep layer shear, while the stronger low-level
shear stays north of the boundary.

On Friday, continued southerly low-level flow should cause the
surface front northward as a warm front. The front will continue to
serve as a focus for precipitation along and north of it. The
NAM/ECMWF push the front north of the area while the GFS is slower
to push it north and is wetter. If the wetter solution wins out,
then an extension may be needed to the watch. A similar
conditional risk for a strong/isolated severe risk to Thursday
will exist over a greater portion of the area, but if the
NAM/ECMWF solution wins out then we may remain capped on Friday
afternoon with warm/dry air aloft. Also of note will be the
warming trend in temperatures with readings near 80 in our
southern counties.

Attention then turns to the potent shortwave trough and associated
deepening surface low tracking from the Southern Plains to the Great
Lakes, and the associated cold front. There remains some model-to-
model and run to run differences in the tilt/amplitude/timing of
this shortwave that will be key in determining the magnitude of the
event. The stronger upper-level forcing remain well northwest of the
area but there are some decent height falls. The somewhat weaker
upper-level forcing could help storms stay discrete longer, but
could also limit the areal coverage of severe storms or prevent them
from occurring at all this far south given capping concerns/warmer
700mb temperatures. A stout warm sector will be present with this
system, with mid to upper 60s dew points and temperatures in the mid
to upper 70s, resulting in potential CAPE values of 1000-1500 J/kg
with favorable lapse rates. 70kt southwesterlies at 500mb and a 50
kt 850 mb jet would suggest the potential for supercells to form
ahead of a QLCS. Latest model trends are a bit quicker with this
system, suggesting storms may move through at a time when the
atmosphere is more unstable. SRH is certainly supportive of a
tornado threat. SPC has expanded the 15%/slight risk southeastward
to include areas along and north of I-59, and we will follow suit
in our graphics and begin to trend the HWO confidence number
upward. Again, the possible forcing issues remain one potential
limiting factor, but looking at the moisture and QPF depicted by
the models suggests thunderstorms should develop this far south.
This rainfall could also cause additional flooding concerns.

Conditions finally dry out Sunday afternoon through Monday. Rain
chances return to the forecast Monday night and Tuesday, however.
Rainfall amounts currently look to be on the lighter side with an
overrunning situation as a weak wave of low pressure moves along the
Gulf Coast.



18Z TAF Discussion.

The line of showers and thunderstorms has reached TCL as the front
continues progressing eastward. Ceilings will generally remain in
in IFR with impacts to visibility through the period with surface
winds veering during the frontal passage. As the front stalls
south of our northern sites, sufficient instability values remain
steady through the evening with lightning possible at most
terminals. MGM and TOI may be spared from most of the thunderstorm
activity depending on how quickly the front stalls, but will
continue to monitor. The front will begin to gradually lift
northward during the morning tomorrow with southerly surface winds




A prolonged period of rainfall will continue through the end of the
week, with a strong storm system expected over the weekend. Very wet
conditions are expected, especially across the northern half of the
area. There are no fire weather concerns at this time.


Gadsden     58  51  61  51  67 / 100 100  80  80  80
Anniston    64  54  67  54  72 / 100 100  80  80  70
Birmingham  64  53  64  55  71 / 100 100  80  80  70
Tuscaloosa  66  53  64  55  72 / 100  90  80  80  70
Calera      66  55  68  56  73 / 100 100  80  80  70
Auburn      67  61  75  61  75 /  50  70  70  70  40
Montgomery  73  63  77  63  80 /  80  80  80  70  50
Troy        73  66  79  65  80 /  50  50  60  50  30


Flood Watch through late Thursday night for the following
counties: Bibb...Blount...Calhoun...Cherokee...Clay...Cleburne...
Shelby...St. Clair...Talladega...Tuscaloosa...Walker...Winston.


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