4/3/18-4/4/18: Severe Weather Threat Discussion Thread

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1247 AM CDT Mon Apr 02 2018

...SUMMARY...
Thunderstorms associated with wind damage, large hail and a few
tornadoes will be possible across parts of the Ohio Valley
southwestward into the lower to mid Mississippi Valley and southern
Plains. An enhanced threat for wind damage may occur across parts of
the lower Ohio Valley.

...Ohio Valley/Mid Mississippi Valley/Southern Great Lakes/Central
Appalachians...
A fast moving shortwave trough is forecast to move
east-southeastward across the mid Missouri Valley and southern to
central Plains on Tuesday. Ahead of the trough, a cold front will
advance quickly across the mid Mississippi Valley. Strong moisture
advection will take place ahead of the front with a tongue of lower
to mid 60s F surface dewpoints setting up during the afternoon from
northeast Arkansas to central Indiana. Surface heating and
increasing low-level convergence should result in convective
initiation along the front and in parts of the warm sector during
the afternoon. The current thinking is that a line of strong to
severe thunderstorms will quickly organize across the mid
Mississippi Valley and lower Ohio Valley during the late afternoon.
This line is forecast to move eastward into the foothills of the
central Appalachians by early to mid evening.

Concerning the severe potential, a 75 to 90 kt mid-level jet will
move through the base of the shortwave trough. The eastern side of
the jet will overspread the mid Mississippi and lower Ohio Valley
Tuesday afternoon helping to create moderate to strong deep-layer
shear. GFS forecast soundings for Louisville and Cincinnati at
00Z/Thursday show 0-6 km shear of 50 to 55 kt with mostly
unidirectional flow in the low to mid-levels. In addition, model
forecasts are now developing moderate instability ahead of the front
suggesting the environment will be favorable for severe storms. As a
squall line moves across the instability corridor, wind damage will
be likely especially in the enhanced risk area from northwest
Tennessee northeastward across western Kentucky into southern
Indiana. Isolated large hail and a few tornadoes will also be
possible with supercells ahead of or embedded in the line. The
severe threat should become marginal with northward extent into the
southern Great Lakes where instability is forecast to remain weak.
The severe threat should lessen Tuesday evening as the line moves
into weaker instability across the central Appalachian foothills.

...Southern Plains/Lower Mississippi Valley...
A shortwave trough will move quickly eastward across the southern
Plains on Tuesday. At the surface, a cold front is forecast to
advance southeastward into the southern Plains and Arklatex. Ahead
of the front, a moist airmass will be in place with surface
dewpoints in the mid to upper 60s F. In response, moderate
instability should develop by afternoon with convective initiation
taking place along the front. The current thinking is that a line
segment or two will organize and move eastward across east Texas and
northern Louisiana during the late afternoon and early evening.
Forecast soundings across this area suggest that moderate to strong
deep-layer shear will be in place. This combined with steep
low-level lapse rates should support a wind-damage threat. Isolated
large hail and perhaps a tornado or two may also occur with rotating
storms that develop across the warm sector or in the line itself.
Moderate instability is currently forecast to develop as far
southeast as New Orleans suggesting that the line could retain a
severe threat during part of the overnight period into parts of
southern Louisiana and south-central Mississippi.

and over some of the areas and water ways we dont need more water in.

LIX agreeing with the SPC and most model guidance that the highest threat area will remain WEST of southeast Louisiana:


Area Forecast Discussion
National Weather Service New Orleans LA
428 PM CDT Mon Apr 2 2018

.SHORT TERM...Clouds finally scattered out across the region this
afternoon with highs getting into the 80s just about everywhere.
Only hold outs by 20z was coastal MS which were in the upper 70s.


Main forecast concern in the next 60 hours will be the cold front
coming through tomorrow night and the severe potential with it.
So prior to our showers and thunderstorms tuesday night the sfc
high that has been so dominant the last few days will continue to
slide east while cyclogenesis will occur along the lee of the
Rockies near the Colorado/Kansas border. This is in response to a
strong s/w currently working east-southeast out of the PAC nw
states. At the same time a system over the Baja is lifting out to
the northeast and should phase with the northern stream s/w as it
takes over. The trough axis will move across the region overnight
Tuesday but it is a rather broad trough over our area(almost
flat). As for the surface low it will quickly track from western
KS to the ene into the eastern Great Lakes by Tue evening. A
trailing cold will also swiftly move through the Mid MS Valley
and into the Lower MS Valley. This cold front may actually push
out ahead of the mid level forcing and could already be into the
coastal waters before sunrise. With this cold front and trough we
will see showers and embedded thunderstorms the question though
is how much if any severe weather will we see.

Once again it looks like things are just not going to quite come
together in our area. There is some instability to work with out
ahead of the cold front which looks to enter our cwa around the
4-7z time frame but again is expected to quickly move through.
Behind the cold front we really lose that instability which was
mainly sfc based and with very shallow lapse rates in the mid
levels the atmosphere will be conditionally unstable at best. The
kinematic field ahead of the front is not impressive but is
sufficient. After frontal passage mode definitely doesn't favor
tornadoes. There is some bulk shear to work with with 0-6km
around 40-50kts. As for dynamics much of the best forcing remains
well north of the region. In fact when we actually start to get
some forcing into the area it appears to be post frontal. One
aspect that is favorable is we appear to have a window with
enhanced divergence aloft. We may have the benefit of a coupled
jet as the weakening subtropical jet will remain just south of our
area and run through FL/GA and the jet associated with the
northern branch begins to intensify over the Mid MS Valley and
OH/TN valleys. Other than that there really isn't much that is
overly impressive with respect to strong/severe storms especially
since it appears these storms will likely be elevated. Hail does
not appear to be an issue with the anemic lapse rates. The
biggest concern is can we get any storms with the cold front. If
storms are not post frontal then there is a pretty good risk of
gusty to damaging straight-line winds and even a few tornadoes
again these storms need to be along the front otherwise they will
be elevated and it will be very difficult to get any winds down to
the surface.

HRRR for 1am:

Severe thunderstorm watch in effect until midnight for SE Texas and SW La up to just east of the Atchafalaya.