SPC AC 090658

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1258 AM CST Thu Dec 09 2021

   Valid 101200Z - 111200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE LOWER MISSISSIPPI VALLEY...MID-SOUTH...AND LOWER OHIO VALLEY...

   ...SUMMARY...
   Severe thunderstorms capable of producing several tornadoes and
   scattered to numerous damaging winds appear probable from mainly
   Friday evening into Friday night across parts of the lower/mid
   Mississippi Valley into the lower Ohio Valley and Tennessee Valley.
   Some of these nocturnal tornadoes may be strong.

   ...Synopsis...
   A highly amplified upper trough initially over the western states
   Friday morning will advance eastward across the Plains through the
   day, eventually reaching the MS Valley by late Friday night into
   early Saturday morning. A broad area of enhanced southwesterly
   mid-level flow will be present ahead of the upper trough over much
   of the lower/mid MS Valley and OH/TN Valleys. At the surface, a low
   over southeastern CO Friday morning will develop eastward across the
   southern/central Plains through the day, before turning
   northeastward while deepening across parts of the Midwest to Great
   Lakes Friday evening/night.

   Substantial low-level moisture, with surface dewpoints generally
   ranging from the mid 60s to low 70s, will likely already be
   established at the start of the period Friday morning across
   central/east TX into the lower MS Valley and Mid-South. As both the
   large-scale upper trough and related surface low move eastward, this
   moist low-level airmass is expected to spread quickly northward in
   tandem with a warm front into parts of the mid MS Valley, OH/TN
   Valleys, and Southeast by Friday evening. A cold front attendant to
   the surface low will sweep eastward from the southern/central Plains
   to the vicinity of the TX Coast and MS River by the end of the
   period early Saturday morning.

   ...Lower/Mid Mississippi Valley into the Ohio/Tennessee Valleys and
   Southeast...
   A low-level inversion should tend to limit robust thunderstorm
   development through much of the morning and afternoon on Friday.
   But, with continued low-level moistening and diurnal heating,
   between 500-1500 J/kg of MLCAPE will probably develop across the
   warm sector by early Friday evening. This instability will be more
   than sufficient to support severe, surface-based thunderstorms.
   Large-scale ascent preceding the upper trough will eventually erode
   the cap, and convective initiation appears increasingly likely
   around 00-03Z Friday evening. Even so, there is still some signal in
   guidance that surface-based thunderstorms may develop slightly
   earlier along a pre-frontal confluence band across the lower MS
   Valley and perhaps into the Mid-South as a subtle embedded shortwave
   trough overspreads this region through the day. Regardless of the
   timing of convective initiation, thunderstorms will increase in
   coverage and intensity by late Friday evening through early Saturday
   morning along/ahead of the cold front as strong forcing/mid-level
   height falls preceding the upper trough overspread the warm sector.

   Deep-layer shear will become quite strong by Friday evening owing to
   the broad area of enhanced mid-level southwesterly winds. 50-60+ kt
   of effective bulk shear will easily support updraft organization,
   including supercells. There remains uncertainty regarding dominant
   convective mode and overall evolution Friday night. Thunderstorms
   may develop ahead of the cold front along one or more pre-frontal
   confluence bands late Friday evening. If this occurs, then a broken
   band of supercells may develop, with additional thunderstorms
   forming later along the cold front farther west. Eventually, this
   convection should congeal into broken line segments and small
   clusters ahead of the front, while a squall line develops along the
   cold front itself. The southwesterly winds around 850 mb are still
   forecast to rapidly increase to 50-60+ kt Friday night across most
   of the warm sector. Even though winds may not veer much with height
   through the boundary layer, there will be ample speed shear. Strong
   0-1-km SRH will likely support low-level rotation and the potential
   for tornadoes in any supercell than can develop, and with
   circulations embedded within the QLCS. Isolated large hail may also
   occur with any initially discrete storms. In addition, scattered to
   numerous severe/damaging winds appear likely given the strength of
   the low/mid-level flow, as it will not take much for convective
   downdrafts to bring the enhanced flow aloft to the surface.

   Confidence has increased in a more favorable corridor for organized
   severe thunderstorms Friday night into early Saturday morning from
   roughly the vicinity of the MS River in eastern AR and northern MS
   northward into parts of the lower OH Valley. Across this region, the
   best overlap of sufficient instability, strong low-level and
   deep-layer shear, and a potentially favorable supercell mode may
   overlap for several hours (generally around 03-09Z). Have therefore
   increased severe wind and tornado probabilities across this region
   and have introduced and Enhanced Risk. Various forecast soundings
   from the 00Z NAM across this region show around 1000-1500 J/kg of
   MLCAPE, and effective SRH in excess of 300 m2/s2. This suggests some
   potential for nocturnal strong tornadoes. The northern extent of the
   severe risk across the Midwest and OH Valley remains uncertain. Some
   northward expansion of severe wind and tornado probabilities was
   made across this region based on latest guidance showing upper 50s
   to low 60s surface dewpoints possibly advancing a little farther
   north. The Slight Risk was also expanded eastward and southward
   across parts of the lower MS Valley into the TN Valley to account
   for a line of convection along or ahead of the cold front likely
   continuing to pose an isolated threat for damaging winds and perhaps
   a few tornadoes through the end of the period.

   ..Gleason.. 12/09/2021

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