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.

A summary of everything is available in a slideshow created by Andrew of the Team

https://onedrive.live.com/embed?cid=434931F6880ADF4C&resid=434931F6880ADF4C%2118078&authkey=ALbDZLTMO3MUzOs&em=2&wdAr=1.7777777777777777

It’s no secret that there has been grave concern regarding severe weather Wednesday. Ingredients and parameters are coming together for this to be a potentially dangerous weather day. We could see some dangerous thunderstorms that could produce significant wind gusts, large hail, and a few tornadoes.

SUMMARY/TIMING: For today, March 16th 2021, a few strong to severe thunderstorms will be possible for central Alabama south of Cullman, Gadsden and Anniston. Looking at a Marginal Risk (Level 1/5) for these areas. Brief damaging wind gusts and a brief tornado are possible in this areas.

On Wednesday, thunderstorms will increase in coverage statewide late in the morning. The morning commute overall looks to be in good shape. There will be breaks in between the morning storms, which will cause instability to rise and put ingredients in place for severe thunderstorms. Expecting to see an increase in overall coverage and intensity during these late morning/afternoon thunderstorms.

Summary of expected hazards:

• Damaging winds of 60-80 MPH
• Tornadoes, some of which could be strong and long-tracked
• Large hail, up to golf ball size
• Flooding, especially in low-lying areas and areas that see repeated rainfall

TECHNICAL DISCUSSION/IMAGES

This is what the radar will look like from now until 1:00 PM Thursday

This is supercell composite radar, which is where supercell development is most likely given the parameters (Parameters being CAPE, high dew points, shear etc.)

This is the STP (Significant Tornado Parameters) index, indicating where tornado development is likely given the parameters in place. Also the Enhanced Helicity Index is showing where tornadoes could form and “tap in” to be long-tracked.

SUMMARY:

There is a threat for some strong thunderstorms on Tuesday, with a greater threat on Wednesday. On Tuesday, the threat will be mainly south of Cullman, and only a Marginal Threat (Level 1/5) for a severe thunderstorms. Gusty winds of 40-50 MPH and heavy rainfall will be the primary threat during this time.

On Wednesday, a more widespread and more intense chance for severe thunderstorms exists as a warm front moves north. Conditions will be favorable for large hail, damaging straight-line winds (70+ MPH), and tornadoes – one or two which could be strong.

CONCLUSION

• Round 1/2- Marginal Risk for severe storms mainly south of Cullman to Alexander City. Primary threats: small hail, gusty winds, heavy rain
• Round 2/2- Enhanced Risk for severe storms Coverage area-wide. All modes of severe weather possible, including damaging straight-line winds of 60-70 MPH, large hail, and tornadoes – one or two which could be strong. Timing: Beginning Wednesday afternoon lasting through the evening
• Flash flooding threat: minimal due to

DISCUSSION

…SUMMARY…
    A broad area of severe weather potential -- including risk for large
    hail, damaging winds, and several tornadoes -- is anticipated
    Wednesday from  the Arkansas/Louisiana vicinity eastward across the
    central Gulf Coast states/southern Appalachians.
 …Synopsis…
    A vigorous upper low moving across central portions of the U.S. will
    once again be the primary upper feature of interest, as it tracks
    from the Texas/Oklahoma Panhandle vicinity early, to southern
    Missouri through the end of the period.  Ahead of this system,
    moderately strong/accompanying flow will spread across the lower
    Mississippi and Tennessee Valleys and central Gulf Coast states with
    time.
 At the surface, a low is forecast to cross Oklahoma during the day,
    and then should cross the Ozarks through the evening eventually
    reaching the western Illinois vicinity by 18/12Z.  Widespread
    thunderstorms, and substantial/accompanying severe risk, can be
    expected in advance of this system.
 …The AR/LA vicinity eastward to portions of TN/GA and the FL
    Panhandle…
    Showers and thunderstorms are forecast to be ongoing from Kansas
    southward to Texas ahead of the advancing upper system and
    associated cold front, and eastward across the central Gulf Coast
    states in a zone along a west-to-east warm front.  Some ongoing
    severe risk will likely exist at the start of the period.
 With time, as moisture streams northward from the Gulf of Mexico
    beneath steepening lapse rates, modest heating will push
    surface-based CAPE into the 1000 to 2000 J/kg range with a broad
    warm sector stretching from the AR/LA vicinity eastward to the
    southern Appalachians.  Convection is forecast to increase in
    response to the destabilization, as persistent UVV occurs not only
    in the vicinity of the cold and warm fronts, but also more broadly
    within the general warm-advection regime.
 Southerly low-level flow, veering and increasing to around 50 kt
    from the west/southwest at mid levels will provide shear favorable
    for supercells.  Additionally, ample low-level shear is expected to
    evolve through the day -- particularly near the aforementioned warm
    front which should drift northward across Arkansas and align
    west-northwest to east-southeast from far southern MO to central GA
    by early evening.  As such, tornado risk may be maximized near this
    boundary -- across the AR area during the day, and then later
    increasing eastward across AL and perhaps into GA as well, as
    low-level flow increases into the evening/overnight.  
 In addition to tornado potential -- including the risk for a couple
    of significant tornadoes across a broad area represented by the
    ENH/30% risk area, large hail and damaging winds will also occur in
    some areas.  Risk will continue through the overnight hours,
    tapering from west to east across the lower Mississippi Valley but
    continuing across the central Gulf Coast states and into the
    southern Appalachians through 18/12z.