SPC AC 081959

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0159 PM CST Wed Feb 08 2023

   Valid 082000Z - 091200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS EVENING
   ACROSS PORTIONS OF NORTHEAST LOUISIANA...SOUTHEAST ARKANSAS...AND
   MUCH OF WESTERN AND CENTRAL MISSISSIPPI...

   ...SUMMARY...
   Severe thunderstorms are expected this afternoon into tonight,
   mainly across parts of the lower into middle Mississippi Valley. 
   Some of these will be accompanied by a risk for tornadoes, a couple
   of which could become strong this evening across parts of eastern
   Louisiana and southeastern Arkansas into western and central
   Mississippi.

   A significant uptick in convection has been observed in the last
   hour along the cold front in eastern Texas, within the moisture
   plume moving off the Gulf of Mexico, and within the uncapped
   warm/moist airmass across Louisiana and southern Mississippi.
   However, these storms are relatively unorganized thus far, owing to
   the weak shear (~20-25 knots per area VWPs). However, as the trough
   approaches and shear strengthens this evening, expect more storm
   organization, including supercells capable of tornadoes. A few
   tornadoes could be strong as the low-level jet strengthens to around
   55 knots, elongating the low-level hodograph and doubling the 0-500m
   shear (per RAP forecast soundings). 

   Farther north, the forecast remains on track. Low 60s dewpoints are
   now into western Tennessee, which will advect northward rapidly this
   evening and overnight as low-level flow strengthens.

   ..Bentley.. 02/08/2023

   .PREV DISCUSSION... /ISSUED 1027 AM CST Wed Feb 08 2023/

   ...Synopsis...
   The more prominent belt of split mid-latitude westerlies is
   undergoing considerable amplification across the eastern Pacific
   into western North America.  This will continue through this period,
   with mid-level ridging building inland of the Canadian and U.S.
   Pacific coast by late tonight.  As this occurs, a vigorous
   downstream short wave trough is forecast to dig sharply along the
   Rockies, with mid-level flow ahead of it trending southwesterly and
   strengthening across the southern Great Plains through the Ohio
   Valley.

   A significant mid-level trough/elongating low is already beginning
   to pivot east of the southern Rockies as it begins to interact with
   a belt of westerlies emanating from the subtropical eastern Pacific.
   The trough axis is likely to gradually take on an increasingly
   negative tilt across Oklahoma and Texas during the day today, before
   accelerating north-northeastward through the middle
   Mississippi/lower Ohio Valley vicinity by 12Z Thursday.  Models
   continue to indicate that associated forcing for ascent will
   contribute to significant surface cyclogenesis from near/northwest
   of the Ark-La-Tex by late this afternoon into areas north-northeast
   of the St. Louis area by the end of the period.

   In the wake of a couple of significant recent cold intrusions,
   boundary modification over the northwestern Gulf of Mexico is still
   underway.  Mid/upper 60s surface dew points have advected inland
   within at least a shallow pre-cold frontal plume across Texas
   coastal areas into portions of the Piney Woods.  A somewhat more
   substantive influx of Gulf moisture is forecast to overspread the
   Louisiana coast through much of the lower Mississippi Valley later
   today through tonight.  Near the northwestern periphery of mid-level
   subtropical ridging, centered and becoming a bit more prominent
   across the Bahamas, mid/upper-levels are relatively warm, and lapse
   rates might not become particularly steep.  However, the development
   of weak to moderate boundary-layer CAPE still appears probable
   within the evolving warm sector, in the presence of strengthening
   deep-layer shear which is expected to become sufficient for
   organized convection, including supercells.

   ...Southeastern Great Plains into Mississippi Valley...
   The primary mid-level trough may be preceded to its south and
   southeast by one or two subtropical perturbations, and the influence
   of associated forcing for ascent on thunderstorm development within
   the plume of returning moisture, while the boundary-layer attempts
   to destabilize inland of coastal areas, remains unclear.

   At the present time, the low-level moisture return associated with
   the stronger low-level flow (still around 40-50 kt around 850 mb) is
   tending to be undercut by the eastward advancing cold front, to the
   south of the developing surface low over northeast Texas. 
   Meanwhile, convection allowing models generally indicate that a
   considerable amount of convection may commence in the  destabilizing
   warm sector, well ahead of the cold front, across portions of
   Louisiana into Mississippi this afternoon.  This would likely be
   prior to substantive strengthening of the low-level wind fields and
   associated hodographs, though the environment could be supportive of
   at least weak updraft rotation.

   Depending on the impact of earlier convective development, forcing
   along and just ahead of the eastward advancing cold front likely
   will become the focus for increasing vigorous thunderstorm
   development late this afternoon and evening.  This probably will
   including one or two evolving lines, with supercells embedded within
   and perhaps preceding it.  Coupled with strengthening southerly 850
   mb flow along a corridor near/east of the Mississippi River, as
   surface cyclogenesis proceeds, a window of opportunity may develop
   this evening for one or two sustained, long tracked supercells
   capable of producing strong tornadoes.

   The possible lingering influence of at least a shallow residual
   stable surface-based layer results in more uncertainty concerning
   severe weather potential closer to the track of the surface cyclone.
    However, mid/upper forcing for ascent and deep-layer mean wind
   fields/shear will be stronger across this region, and could
   contribute to potential for damaging surface gusts and perhaps
   tornadoes.

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