SPC AC 270633

   Day 1 Convective Outlook CORR 1
   NWS Storm Prediction Center Norman OK
   0133 AM CDT Thu Aug 27 2020

   Valid 271200Z - 281200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE NORTHEAST AND NORTHERN PLAINS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE LOWER MISSISSIPPI VALLEY...

   CORRECTED FOR HEADLINE VERBIAGE

   ...SUMMARY...
   Severe storms appear likely Thursday across parts of the Northeast,
   Great Lakes, and northern Plains. Damaging winds and large hail
   should be the main threats, though a few tornadoes are also
   possible. Other severe storms with a threat for tornadoes will
   continue over portions of the lower Mississippi Valley region in
   association with Hurricane Laura.

   ...Northeast...
   A west-northwest mid-level flow pattern will be in place today
   across the Northeast as a 60 to 75 kt mid-level jet moves across the
   region. At the surface, a low will move east-southeastward across
   New York as a cold front advances southward into the central
   Appalachians, to the west of the surface low. In the path of the
   surface low, surface temperatures will warm considerably allowing
   for the development of moderate instability by late morning. In
   response to surface heating and increasing low-level convergence,
   thunderstorm development should take place from western and central
   New York into southern New England by midday. Lift associated with
   the right entrance region of the mid-level jet will support
   convective development. In response, thunderstorm coverage will
   increase through the early afternoon as an east-to-west convective
   line organizes and moves south-southeastward across the region.

   NAM and RAP forecast soundings in southern New York in the early
   afternoon show veered winds at low levels with unidirectional
   west-northwesterly winds above 850 mb. The thermodynamic profile has
   very steep low-level lapse rates from 0-3 km with MLCAPE forecast to
   be in the 2000 to 3000 J/kg range. This combined with 40 to 50 kt of
   0-6 km shear should be favorable for severe storms and possibly
   supercells. The more dominant supercells that form in proximity to
   strong instability may be able to produce hailstones 2 inches in
   diameter. In addition, bowing line segments may form in parts of the
   convective line. These faster moving bowing structures will likely
   produce wind damage along the leading edge of the line. A few gusts
   of greater than 65 kt will be possible. The severe threat is
   expected to move south-southeastward into northeast Pennsylvania and
   northern New Jersey during the late afternoon and early evening.

   ..Northern Plains/Upper Mississippi Valley...
   A shortwave trough and an associated 50 to 65 kt mid-level jet is
   forecast to move eastward into the northern Plains today. At the
   surface, a low will deepen in southwestern South Dakota as a cold
   front advances across the northern Plains to the north and northeast
   of the surface low. A moist airmass will be present to the east of
   the surface low across much of South Dakota. This airmass is
   forecast to become moderately unstable by afternoon. Thunderstorms
   appear likely to develop along the front in the early afternoon
   across southwestern North Dakota and northwestern South Dakota. This
   convection is forecast to organize into a line and move
   east-southeastward across western and central South Dakota during
   the late afternoon and early evening.

   NAM forecast soundings across central South Dakota early this
   evening have very steep low to mid-level lapse rates. MLCAPE is
   forecast to be in the 2000 to 2500 J/kg range. Wind profiles have
   substantial directional shear in the low levels with speed shear in
   the mid levels. This environment will be favorable for supercells
   with large hail during the late afternoon. The more dominant
   supercells could produce hailstones greater than 2 inches in
   diameter. As a line organizes and moves eastward across
   south-central South Dakota in the early evening, damaging winds will
   be likely along the leading edge of the line. Wind gusts of greater
   than 65 kt will be possible. A severe threat should continue as the
   line segment moves across southeast South Dakota into southern
   Minnesota and northern Iowa during the overnight period.

   ...Lower to Mid Mississippi Valley...
   Hurricane Laura is forecast to be located in southwestern Louisiana
   at the start of the period. The hurricane will become a tropical
   storm during the day is it moves northward into northwestern
   Louisiana. Convective bands of heavy rainfall will be located to the
   north and east of the center across much of Louisiana extending into
   southwest Mississippi. Strong low-level shear (0-1 km shear of 30 to
   40 kt) associated with Laura will create conditions favorable for
   tornadoes. Tornadoes will be possible with the more intense cells
   embedded within the rainbands. The tornado threat may increase as
   destabilization occurs during the day. The tornado threat should
   eventually affect parts of Arkansas and northwest Mississippi as
   Tropical Storm Laura moves northward during the late afternoon and
   evening.

   ..Broyles.. 08/27/2020

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