- Day Three
acus01 kwns 211300
Storm Prediction Center ac 211258
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0758 am CDT Mon Oct 21 2019
Valid 211300z - 221200z
..there is a slight risk of severe thunderstorms from the lower
Mississippi Valley to parts of Alabama and the Florida Panhandle...
The severe-weather threat will shift eastward today through this
evening from the lower Mississippi Valley to parts of Alabama and
the Florida Panhandle. Damaging wind gusts and a few tornadoes are
In mid/upper levels, a large area of cyclonic flow covers the
western/central Continental U.S. To the Ohio/Tennessee valleys, anchored by a
strong, stacked, occluded cyclone centered near Yukon. The associated
500-mb low will pivot northeastward across the upper Midwest through
the period. A speed Max and related vorticity Banner now behind the
height trough -- over the central rockies -- will pivot through the
trough and reach the mid-south region by 12z. This will lessen the
synoptic trough's positive tilt and spread height falls across most
of the eastern conus, aside from ridging crossing New England
At the surface, a warm front was drawn across central Florida and the
northeastern Gulf to near mob and Jan, northward to where it was
being overtaken by a squall line in western Tennessee. The convection will
continue to overtake the warm front across western Tennessee and northern
MS through the afternoon. Meanwhile, a cold front -- initially over
northern Arkansas to near txk and Sat -- will move southeastward across
most of the rest of Texas and the lower Mississippi Valley by 00z.
Weak frontal-wave cyclogenesis is possible late in the period over
portions of eastern Alabama or GA, with a resultant low over eastern Georgia
or SC by the end of the period. By then, the cold front should
extend from that secondary low southwestward across southwestern GA,
the western Florida Panhandle, and the north-central/west-central Gulf.
Through the remainder of the morning, a band of thunderstorms should
continue to outpace the slow eastward shift of suitable warm-sector
air, from northwest-southeast across the mid-south region, and into
parts of the Tennessee Valley. Isolated severe gusts are possible
in the meantime. For near-term details, refer to remaining portions
of Storm Prediction Center watches 681 and 682, and related mesoscale discussions.
Additional scattered thunderstorms should pose a risk of damaging
gusts today and tonight, along with a few tornadoes closer to the
Gulf Coast from portions of southern la to the Florida Panhandle. Some
spatial and timing uncertainty exists on onset of greatest severe
potential over the southern la area, because of an expansive shield
of clouds/precip related to an mesoscale convective system offshore from the Texas/western la
coast. Ultimately, the near-frontal convective band should overtake
the remnants of the mesoscale convective system. The merged area of convection may
intensify as it proceeds eastward near the coast, the inland portion
encountering a destabilizing and favorably moist air mass.
Additional storms, perhaps including a few supercells, may develop
in a weakly capped, moisture-rich warm sector near the coast.
Damaging winds and a few tornadoes are possible, given the very
moist environment with strengthening vertical shear.
Forecast soundings suggest that the preconvective environment will
destabilize mainly from Theta-E advection, with some diurnal
warming. This will offset modest mid/upper-level lapse rates enough
to yield warm-sector/preconvective MLCAPE 1000-2000 j/kg near the
coast, decreasing inland to around 300-800 j/kg over central
sections of MS/al. Low-level mass response to the synoptic
troughing will lead to enlarged hodographs with effective srh
commonly in the 150-400 j/kg range within about 100 nm of the
central Gulf Coast from late afternoon into tonight, where buoyancy
should be greatest. Any relatively discrete cells sustained in this
environment will pose a tornado threat, in addition to localized
damaging wind, with better-organized wind threat from any Gulf
mesoscale convective system/antecedent qlcs combination.
acus11 kwns 211400
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 211359
Mesoscale discussion 2102
National Weather Service Storm Prediction Center Norman OK
0859 am CDT Mon Oct 21 2019
Areas affected...northeast Louisiana through western and central
Concerning...Severe Thunderstorm Watch 682...
Valid 211359z - 211530z
The severe weather threat for Severe Thunderstorm Watch 682
Summary...threat for mainly isolated damaging wind is expected to
persist another couple hours from northeast la through western and
north central MS, followed by a diminishing trend. Storms are
approaching the eastern bound of ww 682. While, the spatial and
temporal extent of the threat appears too limited for a downstream
watch, ww 682 can be extended in area as needed.
Discussion...this morning an expansive squall line extends from
northern MS southwest into northeast la moving east at 30 kt. Storms
are moving through a narrow corridor of marginal instability with
500-1000 j/kg MLCAPE along with 40-50 kt effective bulk shear. This
environment should support maintenance of the squall line next
couple hours. However, the downstream thermodynamic environment is
very marginal due to a stable boundary layer and dry air between
900-800 mb as evidenced on the 12z Jan radiosonde observation. Modest destabilization
associated with Theta-E advection along the southwesterly low-level
jet may occur east of ongoing storms. However, widespread
multi-layer clouds will significantly limit surface-based
instability, suggesting the squall line should begin a gradual
weakening trend with eastern extent into MS.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 31519195 32529080 33888944 33618894 32478949 31449057
30529221 30589290 31519195