SPC AC 280657

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1257 AM CST Tue Dec 28 2021

   Valid 291200Z - 301200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
   THE MID-SOUTH AND TENNESSEE VALLEY...

   ...SUMMARY...
   Severe thunderstorms are possible for portions of the Deep South
   into the Tennessee Valley. Large hail, damaging winds, and a few
   tornadoes will be possible.

   ...Synopsis...
   A mid-level shortwave trough can be seen off the Oregon coast early
   Tuesday morning. This shortwave will round the base of the
   large-scale trough to near the Southern High Plains by 12Z Wednesday
   and into the northern Great Lakes by Thursday night. This will
   overspread weak height falls across the warm sector during the day
   Wednesday. Meanwhile, a lee cyclone is expected to develop near the
   Oklahoma Panhandle Wednesday morning and move quickly eastward along
   the surface front through the day. Guidance has come into better
   agreement showing a closed surface low developing along the front,
   but there still remains some uncertainty in the exact location and
   strength of this surface low.

   ...Portions of the Mid-South and Tennessee Valley...
   A long-fetch Caribbean moisture feed will be in place across the
   Southeast on Wednesday morning with mid-60s dewpoints into southern
   Tennessee at 12Z. Low-level flow will strengthen in response to the
   developing surface low along the front Wednesday afternoon. This
   will advect higher theta-e air northward with mid 60s dewpoints into
   north central Tennessee and upper 60s dewpoints into portions of
   northern Mississippi and Alabama. Some breaks in the clouds are
   anticipated in this region as low-level cloud streaks advect
   northward across the warm sector, which may allow temperatures to
   warm into the mid to upper 70s. As a result, ample instability is
   anticipated across the warm sector, with MLCAPE in excess of 1500
   J/kg possible as far north as Columbus, Mississippi.

   Thunderstorms will likely be ongoing at the beginning of the period
   near the Red River in southeast Oklahoma/northeast Texas in response
   to increasing isentropic ascent as the low-level jet strengthens.
   This activity is expected to continue northeastward through the day
   and may eventually overspread the warm sector, and become surface
   based. The marginal and slight risk have been expanded northward to
   account for this possibility given the favorable severe parameter
   space across much of the warm sector Wednesday afternoon.

   The primary severe weather threat will be focused on thunderstorms
   east of this activity in a region of broad, weak isentropic ascent
   where a confluence band could set up off the Gulf of Mexico. Expect
   storms to develop out of cloud streaks which will be streaming north
   through the morning. 00Z CAM guidance appears to be subdued in
   reflectivity and updraft helicity depiction. Forecast soundings in
   proximity of this convection would suggest more intense storm
   development than is currently shown. CAM guidance has had similar
   issues in past cool-season Southeast severe events, and therefore it
   could be a false depiction of storm intensity based on the
   environment. However, there are times when lackluster CAM reflection
   of storm intensity is hinting at a greater problem with forcing.
   This is at least somewhat of a concern given the mostly neutral
   height tendency across the region for much of the event.

   Any storms which develop in this region will encounter favorable
   vertical shear for supercells with effective shear in excess of 50
   knots. In addition, the weakly forced nature of the convection will
   likely favor a mostly discrete storm mode. These storms should have
   a tornado threat, given the strengthening low-level jet, with a
   broad region of 40 knots of southwesterly flow at 850mb depicted by
   most guidance. This will yield effective SRH around 250 m2/s2 which
   will be more than sufficient for a few tornadoes.

   An upgrade to enhanced (10%/sig) was considered across northern
   Mississippi and northwest Alabama, but ultimately settled on an
   expansion of the slight risk for this outlook. If guidance continues
   to trend toward a stronger surface low, and if CAM guidance starts
   to show more intense storm development, an upgrade to enhanced may
   be necessary.

   ..Bentley.. 12/28/2021

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