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000
FXUS64 KMEG 182233
AFDMEG

Area Forecast Discussion
National Weather Service Memphis TN
533 PM CDT Thu Apr 18 2024

...New MESOSCALE UPDATE...

.MESOSCALE UPDATE FOR SEVERE WEATHER...
Issued at 533 PM CDT Thu Apr 18 2024

We`re finally beginning to see more robust storm development north
of Interstate 40. This is where the greatest instability is
located. Farther south, the cap is stronger and showers/storms are
really struggling to maintain/gain intensity despite an
approaching shortwave trough near the Arklamiss. The latest
analysis indicates MLCAPE (instability) on the order of 2000 J/kg
from near Memphis into southeast Missouri (with 0-3 km MLCAPE near
or above 100 J/kg). Mid-level lapse rates are fairly steep across
portions of AR/MO/TN by Mid-South standards (near 7 C/km) which
will support stronger mid-level updrafts and hail growth. One of
the negatives is the lack of stronger deep-layer shear. Bulk shear
isn`t all that bad at 35 kts or so, but that is more marginal for
supercell development, especially with 0-3 km SRH at or below 100
m2/s2. This suggests more in the way of multicells with weak
supercell characteristics in general. Large hail and localized
damaging wind are the primary concerns. Weak low-level wind shear
will keep the tornado threat low for now.

Storms upstream are expected to grow upscale into a linear mode,
moving into the Mid-South later this evening. The greatest
uncertainty will be whether or not the air mass preceding this
line of storms is able to remain surface-based instability. If so,
damaging winds will be a larger concern. If not, storms may remain
elevated which will limit the severe weather potential.

MJ

&&

.SYNOPSIS...
Issued at 333 PM CDT Thu Apr 18 2024

Scattered showers and thunderstorms will continue over the
Midsouth tonight, in advance of an upper level trough lifting
through the mid-Mississippi River Valley. Severe thunderstorms
will be possible, especially north of a Jonesboro Arkansas to
Jackson Tennessee line. Damaging winds and large hail will be the
primary threats. A few short-lived tornadoes will also be possible
through the early overnight hours, mainly to north and west of the
Memphis metro area.

Additional showers are expected across the Midsouth through early
Sunday, with isolated thunderstorms over north Mississippi. Severe
thunderstorm chances will be low. A reinforcing cold front will
push through the Midsouth Saturday night and early Sunday,
bringing low humidity and cool conditions for Sunday night through
early Tuesday.

&&

.DISCUSSION...
(This evening through next Wednesday)
Issued at 333 PM CDT Thu Apr 18 2024

Primary forecast concern remains severe thunderstorm chances
through tonight. SPC/RAP mesoanalysis showed mixed-layer CAPE of
1500-2000 J/kg along and west of the MS River. Mixed layer CINH
was estimated at zero across the Midsouth, but early afternoon
convective trends suggest consequential CINH remains. 12Z
soundings from LZK, SGF and JAN showed a stout elevated mixed
layer (EML) and underlying inversion. 17Z aircraft sounding from
LIT showed this EML remained strong, and based at 750mb. GOES
visible imagery showed multiple standing wave clouds over
northwest AR, suggestive of an overlying capping inversion.
Immediately west, frontal convergence was evident over far
northeast OK and southwest MO, but deep convection was limited to
areas further north along the front, over central MO.

To our south, thunderstorms persisted through most of the morning
and early afternoon. This convection initiated on an elevated
boundary, near the base of a relatively low (900mb) EML. This
convection was not initialized on early morning runs of the HRRR,
despite a pronounced upstream shortwave evident on GOES water
vapor imagery. The Arklamiss/central MS storms persisted into
central MS at midafternoon and have spread considerable high
cloudiness over north MS. This has limited surface heating and
ability to sustain deep convection - at least to this point.

Should a storm or two manage to break through the low level
capping inversion, robust CAPE and steep midlevel lapse rates will
support large hail over any portion of the Midsouth. Any of three
following features may aid in the breaking the convective cap: height
falls associated with aforementioned shortwave exiting the Arklatex,
strong surface heating, and general height falls associated with
the longwave trough passage over the mid-MS River Valley. None of
these are a sure bet, hence the lower typical Day 1 confidence
level in severe storms.

Should severe storms fail to materialize this afternoon, it
doesn`t mean we`re out of the woods. NAM and HRRR depict a
pronounced strengthening of 925mb SSW winds late afternoon and
evening. This is likely a contributing factor the looping model-
derived hodographs over eastern AR late this evening. If storms
manage to become surface-based, a mid to late evening QLCS
tornado threat may be realized over eastern AR. After midnight,
severe threat should diminish with strengthening low level
inversion, weakening elevated instability and weakening/veering of
the low level jet.

Comparatively tranquil weather will prevail Friday and Saturday,
as a surface cold front stalls over north MS, parallel to the
upper level flow. An open southern branch trough will lift into TX
on Saturday, with downstream moisture bringing showers mainly
south of I-40. A few thunderstorms will be possible over north MS,
but similar to yesterday, NBM thunder probabilities appear too
high/too far north Friday night through Sunday morning.

A reinforcing cold frontal passage Sunday night will bring dry and
cool conditions, with temperatures Sunday and Monday more typical
of early March.

PWB

&&

.AVIATION...
(18Z TAFS)
Issued at 1239 PM CDT Thu Apr 18 2024

A busy period is forecast this TAF cycle as multiple rounds of
storms and IFR CIGs occur. Expect the first round of storms to
develop this afternoon, with potential to impact all terminals. A
few of these storms may become strong to severe, so kept TSRAGS in
MEM TAF. Another round of storms will develop late this evening
into the overnight hours, with the greatest TS chances at JBR.
Expect precipitation to clear the Mid-South just after sunrise.

In addition to precipitation concerns, IFR CIGs are forecast to
develop overnight as the second round of storms moves into the
area. IFR should stick around until midmorning, when a cold front
allows for MVFR CIGs to return. Behind this boundary, expect gusty
north winds through the rest of the period.

ANS

&&

.MEG WATCHES/WARNINGS/ADVISORIES...
AR...None.
MO...None.
MS...None.
TN...None.
&&

$$

PUBLIC FORECAST...PWB
AVIATION...ANS