FXUS64 KOHX 272348

Area Forecast Discussion
National Weather Service Nashville TN
648 PM CDT Thu Apr 27 2017




Models have not changed much in regards to the active pattern over
the area through the next few days. Initial focus is for Friday
afternoon/evening, then again for Sunday into Monday morning.
Before that, conditions look to be dry with the potential for some
patchy fog this evening, especially east of I-65. Models want to
bring in some cloud cover overnight in the northwest and that may
help prevent some fog development, as well as some increased winds
near sunrise. For now though, tossed in mention of patchy fog
early tomorrow morning.

Friday afternoon/evening continues to be a difficult forecast. The
potential is definitely there for severe thunderstorms with MUCAPE
values between 2000-3000 J/kg depending on which model you choose,
and 0-6km shear will be around 40 to 50 knots. The troubling thing
is the CIN, which is quite strong looking at model soundings. The
NAM is showing the strongest cap over the mid state, and has
little to no precip developing over the area either. The GFS and
ECMWF have a weaker cap and may allow for some thunderstorm
development later in the afternoon and evening, especially if they
develop out west and move into the area. The GFS has become a bit
drier and leaning towards the NAM, but the ECMWF still has the
northwest zones seeing development late afternoon and evening
hours Friday. With such high CAPE and great shear, primary
threats will be damaging winds and large hail thanks to a nice dry
EML and good mid level lapse rates. With 0-1km shear over 30
knots, great low level curvature on hodographs, and 0-3km helicity
over 300 m2/s2, isolated tornadoes will also be possible with
stronger convection. The EML is also responsible for the strong
cap, so it will be interesting to see how this event develops
should a few cells get through the strong CIN. For now, trended
just a little drier than the previous forecast since short term
models also want to keep most of the precip north and west, but,
the potential is there should storms develop to not only become
severe, but also put down heavy rain. With already saturated
ground conditions, flash flooding may also be a concern with
training of heavy precip.

With the strong upper high over the SE CONUS, Saturday looks to
primarily be dry. The upper high will also be a culprit in keeping
precip from developing on Friday as well, in addition to the
strong cap. A few models develop some isolated cells Saturday
afternoon, so kept in schc mention for now. Long term models keep
the afternoon dry as the upper high continues to strengthen a bit
and keeps the stronger band of lift/precip off to the northwest.

On Sunday, the upper high slides eastward, and allows the Plains
trough to move into the region. This will bring precip back to the
mid state ahead of the cold front starting Sunday mid day and
continuing through Monday morning. With the slow progression of
the cold front as the upper closed low tracks NE towards the Great
Lakes. Severe parameters are a bit weaker, but potential exists
for some strong to severe storms ahead of the cold front. Main
threat with this event will be heavy rain and potential flooding
as model soundings show PWAT values approaching 1.5 inches across
the area. Also, freezing levels are above 13kft, and instability
is spread nicely through the atmosphere. This will also support a
heavy rain threat and will continue to monitor for possible flash
flood risks. The good thing is storms will be fast moving,
although again training of showers and storms over a local area
will pose a higher flood threat.

Tuesday looks to be mainly dry as we will be in the wake of the
upper low and fropa from earlier Monday, but a few models have
some lingering precip chances so kept schc pops for now. Upper
flow becomes more zonal for a few days before another Plains
trough moves in for Thursday. So for Wednesday have mainly chc
pops for afternoon thunderstorm potential, but a better chance for
rain will be on Thursday with the next upper trough.



Upper level ridge builds westward from the southeast atlantic
seaboard, across the southeastern United States, during the
forecast period. Surface pressure gradient, along western
peripheryof high, tightens during the daylight hours on Friday,
creating gusty surface flow across the Mid-State. A few isolated
thunderstorms are possible over western parts of Middle Tennessee
late Friday afternoon.

VFR conditions are expected to prevail at area terminals
throughout the forecast period. It's possible that a stray
thunderstorm could impact CKV and/or BNA late in the forecast
period, but confidence in such an event is still low at this





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