4/7/21 - 4/10/21 Severe Weather Discussion

What is the timing for Friday’s weather? Late?

Today's Enhanced Risk has expanded a little.



Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0751 AM CDT Wed Apr 07 2021

Valid 071300Z - 081200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
OF ARKANSAS...NORTHERN LOUISIANA...WESTERN/NORTHERN
MISSISSIPPI...AND SOUTHWESTERN TENNESSEE...

...SUMMARY...
Scattered severe thunderstorms are expected from this afternoon into
the evening across parts of the lower Mississippi Valley, Mid-South,
Arklatex and Ozarks. Wind damage, hail and tornadoes will be
possible.

...Synopsis...
In mid/upper levels, a split-flow pattern will persist across the
central CONUS, downstream from a strong trough digging southeastward
from the AK Panhandle across BC and the Pacific Northwest. The
feature responsible for that split is a meridionally elongated
cyclone -- now apparent in moisture-channel imagery over the central
High Plains -- with a basal vorticity max over southwestern KS. The
primary 500-mb low should proceed east-northeastward across KS to
the EMP/TOP area by 00Z, then eastward to near COU by 12Z.

At the surface, 11Z analysis showed an elongated area of low
pressure along a frontal zone from western IA to eastern KS, with
cold front southwestward across south-central OK and west-central/
southwest TX. A primary surface low should consolidate and deepen,
reaching southwestern/south-central IA by 00Z, with cold front
arching across eastern MO, northern to southwestern AR, and
northeast to south-central TX. The low should move little overnight
while becoming more deeply occluded. The cold front should reach
western KY, middle TN, eastern MS/western AL, and southeastern LA by
12Z.

...Arklatex/Ozarks to lower/mid Mississippi Valley...
See SPC mesoscale discussion 320 for coverage of a marginal,
near-term hail concern ongoing in the Ozarks and vicinity.

Scattered to numerous thunderstorms are forecast to form today in an
arc near the cold front -- initially from portions of western MO to
the Arklatex region, then backbuilding/shifting over the Mid-South
and Delta regions into this evening. Damaging winds, several
tornadoes, and sporadic severe hail all will be possible in a broad
swath from IA to parts of LA/MS, with the greatest severe potential
in and near the "Enhanced" area.

The earliest development is expected from extreme eastern portions
of KS/OK into near the MKC-FSM/northern I-49 corridor around midday.
This should occur as the cold front encounters the northern fringe
of diurnal boundary-layer destabilization and continuing warm-sector
moisture advection. This will be overlain by increasing large-scale
ascent related to a vorticity lobe pivoting around the southeast
side of the mid/upper cyclone, preferentially eroding already weaker
EML-related MLCINH compared to father south. As the front
encounters even stronger boundary-layer destabilization (needed to
overcome greater capping) southward into the Arklatex region,
additional convection should develop into early/mid afternoon,
perhaps extending as far south as east TX and western/northern LA by
late afternoon.

The resulting, essentially continuous arc of thunderstorms will
encounter a favorable inflow region, with surface heating and dew
points in the 60s F contributing to a prefrontal corridor of
1000-1500 J/kg peak MLCAPE (locally/briefly higher). Buoyancy will
be supported somewhat more by cold air aloft in the north and higher
boundary-layer theta-e in the south. While the flow aloft remains
difluent, height gradients will tighten over the near-frontal warm
sector with the eastward shift of the cyclone aloft, along with some
backing with time of midlevel flow. The net effect should be for
strengthening deep shear (e.g., effective-shear magnitudes reaching
35-50 kt), but also, a more front-parallel component to winds aloft,
encouraging quasi-linear storm mode with embedded supercells and
bow/LEWP features.

The resulting MCS will encounter veering flow with height in low
levels and favorable effective SRH for occasionally tornadic
mesocirculations. The warm sector will be narrower with northward
extent, leading to convection's outrunning favorable inflow sooner
this evening in the mid Mississippi Valley region, and lasting well
into the overnight hours across portions of MS, TN, LA and AL.

..Edwards/Kerr.. 04/07/2021

Friday’s severe outlook appears to have become worse particularly central Mississippi. I don’t think it was like that earlier??

It's a pretty damn good assumption that anytime there's the threat of severe weather anywhere in the south, central Mississippi and central Alabama will be the bullseye.

It's tiring.

JAN explained the reason for the upgrade...hail.


Thermo profiles values for Friday into Saturday morning are rare for

our area this time of year. Lapse rates are steep along with

vertical totals being high in the 30s. CAPE profiles also show some

deepening in the mid levels during Friday. CAM guidance shows there

could be up to 4,000J/kg MU CAPE Friday afternoon balanced with

40kts of deep layer shear. With all these ingredients in place, a

significant severe weather event is likely. The main concern will

be large hail and damaging winds/wind gusts. There is some

potential for tornadoes and though they can not be ruled out but I

want to reiterate the fact that hail will be large.