Significant Severe Weather Event Likely Wednesday

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.