SPC AC 021948

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0248 PM CDT Thu Nov 02 2017

   Valid 022000Z - 031200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON AND EVENING ACROSS PARTS OF NORTHERN MISSISSIPPI AND
   ADJACENT PORTIONS OF NORTHWESTERN ALABAMA...

   ...SUMMARY...
   A couple of strong thunderstorms could impact parts of northern
   Mississippi and adjacent portions of northwest Alabama late this
   afternoon and evening, accompanied by some risk for severe weather.

   ...20Z Outlook Update...
   A substantive influx of moisture (characterized by upper 60s to near
   70f surface dew points) is ongoing on south/southwesterly low-level
   flow off the western Gulf of Mexico.  This is contributing to a
   sizable area of weak to moderate potential instability, from parts
   of the southeastern lower Plains and Ozark Plateau through the lower
   half of the Mississippi Valley.  CAPE is maximized from southeast
   Texas into the Ozark Plateau, where lower/mid tropospheric lapse
   rates are steepening as elevated mixed-layer air advects east of the
   southern Plains.  However, mid-level inhibition is also increasing,
   and may tend to suppress convective development.  

   In general, mid-level heights are also rising appreciably within
   both branches of the split upper flow, through most areas across and
   east of the Rockies.  However, a weak southern branch
   perturbation/cyclonic vorticity center now crossing southeastern
   Arkansas does appear to be suppressing mid-level height rises.  As
   this feature gradually shifts eastward/northeastward late this
   afternoon and evening, associated forcing for ascent could enhance
   showers now likely forming in response to low-level warm advection
   across parts of northern Mississippi.  

   Prior to the onset of boundary layer cooling this evening, some
   strengthening of mid-level wind fields and shear appear possible
   with the progression of the upper impulse, across northern
   Mississippi.  And there still appears a window of opportunity for a
   couple of storms to develop and intensify within an environment
   becoming marginally conducive to supercells.

   Farther north, across parts of southeastern Missouri and
   northeastern Arkansas into the lower Ohio Valley, guidance is
   suggestive that an increasing divergent upper flow field between the
   two branches of westerlies, coupled with the ongoing
   destabilization, could contribute to increasing thunderstorm
   development.  However, this seems most probable after dark, with
   activity tending to be rooted above the stable or stabilizing
   boundary layer.  Depending on the timing of initiation, and residual
   boundary layer instability, a window of opportunity for
   strong/severe storm development may not be entirely out of the
   question.  But, this still seems even more uncertain than the
   potential farther to the south (across northern Mississippi).  As a
   result, severe weather probabilities are being maintained at less
   than 5 percent.

   ..Kerr.. 11/02/2017

   .PREV DISCUSSION... /ISSUED 1130 AM CDT Thu Nov 02 2017/

   ...Northern portions of Mississippi and Alabama...
   A weak mid-level impulse over the Ark-La-Tex this morning will
   continue eastward and cross the region later this afternoon into
   early evening. Low clouds remain relatively prevalent across
   northern portions of MS and AL late this morning, but a gradual
   erosion of low-level clouds will occur this afternoon while the
   boundary layer otherwise continues to moisten (middle to upper 60s F
   surface dewpoints). Ascent related to the aforementioned mid-level
   impulse should contribute to isolated thunderstorm development by
   mid/late afternoon within an increasingly unstable air mass,
   particularly across northern MS.

   While mid-level winds this morning (12Z observed soundings) were
   relatively weak, a gradual strengthening will occur today with
   deep-layer shear increasing to around 35-40 kt by late afternoon.
   This would conditionally support a supercell mode with all severe
   hazards possible, potentially including a tornado risk in the
   presence of a sufficiently moist environment and approximately
   150-200 m2/s2 0-1 km SRH.

   ...Southeast Missouri to central/southern Indiana...
   Scattered convection should develop and increase this evening along
   a cold front that will accelerate southeastward across the Midwest
   towards the Ohio Valley. With both low and mid-level winds
   decreasing in time ahead of the front, deep-layer shear will likely
   be modest, especially compared to typical late autumn severe setups.
   Even though low-level moistening will be occurring ahead of the
   front, current thinking is that near-surface inhibition and the
   undercutting nature of the front should limit surface-based
   convection. Additionally, hail beyond small sizes appears rather
   unlikely given relatively weak mid-level lapse rates.
   Observational/short-term guidance will be reevaluated through the
   afternoon for any possible needed introduction of low severe
   probabilities across this region.

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