SPC 1730Z Day 2 Outlook

Day 2 Outlook Image

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1222 PM CDT Mon May 03 2021

Valid 041200Z - 051200Z


Widespread strong to severe thunderstorm development is possible
Tuesday from the lower Mississippi and Ohio Valleys into the
Allegheny and Cumberland Plateau regions.  This will include a risk
for large hail, damaging wind gusts and a few tornadoes.


The severe weather forecast for Tuesday remains complex with
lingering uncertainty. Most of the forecast changes for this cycle
are peripheral/modest for the Slight and Enhanced risk areas. The
Marginal risk area has been expanded quite a bit to the east, from
the South Carolina to Delmarva coast. 

On a broader scale, an upper trough extending from the Upper Midwest
to southwest TX/northeast Mexico will migrate eastward, becoming
oriented from the upper Great Lakes to GA by Wednesday morning.
Several smaller shortwave perturbations are forecast to eject
northeastward across parts of the lower MS Valley to the lower Great
Lakes vicinity as well as portions of the central
Appalachians/Mid-Atlantic vicinity. At the surface, a seasonally
moist airmass will be in place, with mid 60s to low 70s dewpoints as
far north as the TN Valley and into the Carolinas/Chesapeake Bay
vicinity. Low to mid 60s dewpoints will spread further north into
the lower Great Lakes eastward toward the I-95 corridor from
Philadelphia into southern NJ. Meanwhile, an EML characterized by
steep 700-500 mb lapse rates around 7-8 C/km will advect eastward
across the lower MS Valley into the TN Valley, aiding in strong
destabilization with MLCAPE values forecast around 2000-3000 J/kg.
Weaker, but still sufficient lapse rates will exist across parts of
the Ohio Valley eastward to the mid-Atlantic coast. A belt of
increasing southwesterly midlevel flow will overspread much of the
lower MS Valley into the Mid-Atlantic ahead of the main upper
trough, providing further support for organized convection. As a
result, a broad area of strong to severe thunderstorms are expected
across a large part of the southeastern US into the Mid-Atlantic.

...Lower MS Valley/Southeast...

The greatest/most concentrated severe threat on Tuesday appears to
extend parts of AR/LA eastward into TN/AL/GA. Some uncertainty
continues across this region as ongoing convection is expected over
parts of the area during the morning hours as an MCS spreads
eastward across the Ohio Valley and perhaps parts of the TN Valley.
How this system from the Day 1/Monday period evolves will have some
impact on how further convection develops later in the
morning/afternoon as the airmass recovers ahead of the main surface
cold front spreading southeast through the afternoon. Little capping
is expected across the area and pockets of stronger heating will
likely result in early development of thunderstorm clusters across
parts of MS/AL/GA. While effective shear will initially be modest,
this should rapidly increase through the morning. This initial
activity will pose a threat for all severe hazards, including hail,
damaging gusts and a couple of tornadoes.

By late morning/early afternoon, convection is expected to develop
along the surface cold front from parts of AR into far east TX. Some
of this activity will likely remain cellular initially amid
supercell wind profiles. With midlevel lapse rates around 7.5-8 C/km
this activity could produce significant hail, in addition to
damaging gusts and a couple of tornadoes. With time, stronger
forcing and boundary-parallel deep layer flow should result in
upscale growth into bowing/linear segments as convection shifts
eastward across MS/LA and into AL during the afternoon/evening. This
will increase the potential for more widespread damaging gusts, and
favorable low level speed shear should continue to support
mesovortex tornado potential along the line. 

...Ohio Valley to the Mid-Atlantic...

A moist boundary layer (low to mid 60s F dewpoints) beneath modest
midlevel lapse rates will be in place on Tuesday amid 35-50 kt
850-700 mb west/southeasterly flow. MLCAPE values around 1500-2500
J/kg are forecast as pockets of strong heating result in steepening
low level lapse rates. Low level flow will remain weak, but marginal
supercell wind profiles should support isolated to scattered
organized thunderstorm development during the afternoon. A
well-mixed boundary layer, with inverted-v low level thermodynamic
profiles indicate potential for locally damaging gusts. Some of the
stronger cells also could produce marginally sever hail. 

Some guidance suggests that the morning MCS over parts of OH/TN
Valley may continue eastward and spread into parts of VA/NC/SC. If
an organized convective system can be maintained, this could
increase damaging wind potential during the afternoon/evening and an
upgrade to Slight risk could be needed in later outlooks.

..Leitman.. 05/03/2021

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