A Mesodiscussion is issued by the National Weather Service's Storm Prediction Center when there is a chance of a significant weather event impacting an area. These are routinely issued before weather watches or as updated to weather watches.

SPC MD 568

MD 0568 Image

Mesoscale Discussion 0568
NWS Storm Prediction Center Norman OK
0736 PM CDT Wed Apr 26 2017

Areas affected...western Tennessee...northern Mississippi

Concerning...Severe Thunderstorm Watch 166...

Valid 270036Z - 270200Z

The severe weather threat for Severe Thunderstorm Watch 166

SUMMARY...A risk for primarily potentially damaging wind gusts
continues at least another couple of hours with the squall line
advancing across northern Mississippi and portions of western
Tennessee.  This may include areas in close proximity to, but east
of watch 166, before activity weakens, but it is not yet certain
that a new watch will be needed.

DISCUSSION...As the significant short wave trough approaching the
lower Mississippi Valley takes on an increasing negative tilt,
stronger low-level convergence and mid-level forcing for ascent
likely will become focused across northern Mississippi northward
into the lower Ohio Valley during the next few hours.  This is
forecast to coincide with further strengthening of southerly 850 mb
flow to 50+ kt ahead of the ongoing pre-frontal squall line which is
now advancing east of the Mississippi River.  However, the northward
return of richer boundary layer moisture is becoming increasingly
cut-off from this region, and the maintenance of vigorous boundary
based storm development beyond the next couple of hours is
increasingly unclear.  The segment of the squall line north of
Greenville MS into the vicinity of the MISSISSIPPI/Tennessee state
border seems to provide the greatest risk for potentially damaging
wind gusts through the 01-03z time frame, with at least some
northeastward acceleration possible.

..Kerr.. 04/27/2017

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


LAT...LON   36158881 36488844 36418745 34788792 33908807 33508891
            33628998 34278996 34808962 36158881 

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SPC MD 566

MD 0566 Image

Mesoscale Discussion 0566
NWS Storm Prediction Center Norman OK
0702 PM CDT Wed Apr 26 2017

Areas affected...Parts of central/northeastern
Louisiana...western/central Mississippi

Concerning...Tornado Watch 164...

Valid 270002Z - 270130Z

The severe weather threat for Tornado Watch 164 continues.

SUMMARY...The risk for a couple of tornadoes and potentially
damaging wind gusts with storms overspreading the region will
continue at least into the 9-10 pm CDT time frame.  It is not
certain that an additional watch will be needed to the east of watch
164, but trends will continue to be monitored for this possibility.

DISCUSSION...Sustained vigorous thunderstorms within a pre-frontal
squall line continue to advance eastward into/through the lower
Mississippi Valley, as a supporting short wave trough pivots into an
increasing negative tilt orientation as it approaches the region. 
New convective development is ongoing ahead of the line, but is
generally confined to a narrow corridor ahead of it, before merging
into or being overtaken by the line, as the leading edge of
mid-level cooling contributes to a gradual west to east erosion of
the plume of capping elevated mixed layer air.  

The leading edge of  an inland returning plume of seasonably moist
boundary layer air (characterized by surface dew points in the upper
60s/lower 70s) remains stalled over southern/eastern Louisiana and
southwestern Mississippi, and the squall line appears likely to
increasingly cut-off inland return of higher boundary layer moisture
from the Gulf of Mexico.  As it does, easterly inflow of less
unstable air into the storms may tend to contribute to weakening of
updrafts as early as 02-03Z, although it is possible that this may
be counter balanced somewhat by at least some strengthening of
southerly 850 mb flow (and low-level shear).  Until convection
begins to wane, a period of increasing risk for strong surface gusts
remains possible into early evening, along with continued potential
for a couple of tornadoes.

..Kerr.. 04/27/2017

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


LAT...LON   33469078 33969011 33888938 32978950 31949013 30499167
            30179265 30449347 32549152 33469078 

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