Wednesday weather thread: MS, AL, LA, TN, Enhanced for MS, AL, TN

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Does this mean winter is finally coming???

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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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