MD 1899 CONCERNING SEVERE POTENTIAL…WATCH POSSIBLE FOR PARTS OF SOUTHEASTERN LOUISIANA INTO SOUTHWESTERN MISSISSIPPI

Mesoscale Discussion 1899
NWS Storm Prediction Center Norman OK
0330 PM CST Thu Dec 31 2020

Areas affected...Parts of southeastern Louisiana into southwestern
Mississippi

Concerning...Severe potential...Watch possible 

Valid 312130Z - 312330Z

Probability of Watch Issuance...40 percent

SUMMARY...Increasing discrete thunderstorm development is underway
across the region.  This could eventually pose increasing potential
to produce a tornado and/or damaging wind gusts, but it is not yet
clear that a severe weather watch will be needed, at least prior to
around 6-7 PM CST.

DISCUSSION...To the south of a strengthening warm frontal zone,
surface dew points have increased into mid 60s to near 70F across
much of southeastern Louisiana, and currently as far north as the
McComb MS vicinity.  This has contributed to modest boundary-layer
destabilization, with CAPE now generally on the order of 500+ J/kg. 

Supported by large-scale ascent associated with lower/mid
tropospheric warm advection, beneath increasingly difluent mid/upper
flow, this has contributed to increasing discrete storm development.
Some of the stronger cells have exhibited rotation, aided by strong
deep-layer shear beneath 50+ kt southwesterly 500 mb flow.  However,
southerly low-level flow, though gradually strengthening, remains
rather modest in strength.  More substantive intensification,
including to 40+ kt around the 850 mb level, is not forecast to
develop/shift across the lower Mississippi Valley until closer to
the 01-03Z time frame.  

Once this commences, residual boundary-layer instability, coupled
with enlarging clockwise-curved low-level hodographs, could pose at
least some increase in potential for supercell structures capable of
producing tornadoes.  However, until then, though perhaps not
completely out of the question, this potential seems likely to
remain relatively low.

..Kerr/Grams.. 12/31/2020

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...MOB...JAN...LIX...LCH...

LAT...LON   30989156 31659075 32149013 30278908 29328951 29419139
            30989156 

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