HSV AFD was a good discussion
An
active period awaits us in the short term. The cold
front from
earlier today will stall over central Alabama. Its parent low and
associated
upper level cutoff low will continue to track northeast as
they rotate around a second
shortwave trough over the High Plains. A
split
flow jet structure aloft creates a complex synoptic pattern
with an even more complex severe weather setup.
On Friday, the subtropical
jet will push a
shortwave trough from
California into the Southern Plains in more
zonal flow aloft. As this
trough tracks toward the Southeast, a surface low will generate over
Texas. With the tightening low level pressure
gradient, southerly
surface winds will
gust near 20 mph tomorrow afternoon. Additionally,
the northern stream
jet will push the High Plains
shortwave trough
into the Central Plains and intensify it into another
cutoff low. The
trickiest part of this forecast is how this new
cutoff low interacts
with the
shortwave trough, and unfortunately, there is not very high
confidence in a solution at this time.
What we are able to identify at this time is that the environment
will be primed for severe weather over the TN Valley. In spite of
lacking deep southerly
flow,
moisture availability will be more than
sufficient for deep, moist
convection. Additionally, an elevated
mixed layer (
EML) will accompany the Texas-originated surface low as
it tracks into the Southeast, providing steep mid-level lapse rates,
large values of
CAPE, and introducing a threat for large
hail.
Lastly, an intensifying southwesterly
low level jet on the magnitude
of 50-60
kts will precede the surface low and its attendant cold
front. This will elongate hodographs and allow the large
CAPE from
the
EML to be surface-based.
An initial and smaller
MCS will track through the Deep South based
on the track of the
shortwave trough on Friday evening. This is
currently our greatest source of uncertainty, because global
guidance,
ensemble guidance, and CAMs show this line tracking
anywhere from along the Gulf Coast to directly over the TN Valley.
Threats with this first round will be large
hail, damaging winds, and
tornadoes.
There is higher confidence in a second, much larger QLCS tracking
through the Southeast late Friday night into Saturday morning.
Regardless of the track of the initial
MCS, this second line will be
severe with damaging winds, potentially over 70 mph, large
hail, and
again tornadoes will be possible. This line of storms looks to enter
Northwest AL as early as midnight Saturday, but most
likely around 4
AM, and then not exit northeast AL until Saturday around mid-morning,
but potentially as late as noon. Showers will be possible through
Saturday evening, as the cold
front itself does not push through
until Saturday night. With multiple rounds of heavy
rainfall, most
areas will see at least 2" of
rainfall, with some areas seeing higher
amounts. Due to mostly saturated soils,
flash flooding will be
possible.
Now for the important part: what could go wrong? It is worth noting
that any guidance that pushes that initial
MCS south of our area has
a weaker, yet still severe, second QLCS that tracks through the TN
Valley. Additionally, it is possible that should the initial
MCS
track through the TN Valley, it creates a rain-cooled environment
that is not surface-based and reduces the severity of the second
round QLCS. Regardless, it is important to treat this as though both
rounds will be severe. Careful
mesoscale analysis of corfidi vectors,
steering
flow, and any theta e ridges will be essential as we try to
predict with a few hours` notice where the
MCS may track. As we have
seen multiple instances of in the summer,
MCS tracks are often not
well represented in CAMs and will need to be relied upon more with
careful observational analysis.