Enhanced Risk In Place for Tuesday 5/4

Enhanced Risk in place for most of the Coverage Area on Tuesday, May 4th. Damaging winds and large hail will be the main threat, but there will be a secondary threat for tornadoes.

Summary: Thunderstorms are expected to come in 2 waves on Tuesday, the first wave will be in the morning with gusty to damaging winds being the primary threat. After this passes, the atmosphere will destabilize and make room for the next wave of thunderstorms. Thunderstorms that do develop will have the potential for produce damaging winds gust, large hail, and a few tornadoes.

NWS Graphics

NWS Huntsville 5/4

NWS Birmingham 5/4 PM

NWS Birmingham 5/4 AM

SPC Outlook

 ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   FAR SOUTHEAST
   ARKANSAS...LOUISIANA...MISSISSIPPI...TENNESSEE...ALABAMA AND
   NORTHWEST GEORGIA...

   ...SUMMARY...
   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.

   ...Synopsis...

   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.
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