- Day Three
acus01 kwns 221251
Storm Prediction Center ac 221250
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0750 am CDT sun Oct 22 2017
Valid 221300z - 231200z
..there is a marginal risk of severe thunderstorms from
southeastern Louisiana to parts of the Florida Panhandle...
..there is a marginal risk of severe thunderstorms over parts of
the northern High Plains...
A few severe storms will be possible this afternoon through tonight
near the central Gulf Coast.
The main upper-air feature for this forecast will be a
full-latitude, synoptic-scale trough -- currently arching from
southern mb across western Minnesota, eastern KS, central OK, and southwest
Texas. Approximately the northern half of this trough will break
northeastward across the upper Great Lakes and on through the
period, with a cyclone forming on its northern extremity over
northern Hudson Bay. Meanwhile, the southern portion will shift
eastward from the Southern Plains across the lower/mid Mississippi
Valley. An embedded/500-mb low should develop this evening over the
west-Central Ozarks and move generally eastward near the MO/Arkansas
border toward the Mississippi River. By the end of the period, the
trailing shortwave trough should become slightly negatively tilted,
extending south-southeastward from the low across MS to near bix and
over the adjoining northern Gulf.
Northern-stream shortwave perturbations -- now evident in
moisture-channel imagery near the coastal Pacific northwest and
coastal British Columbia -- will cross the northern rockies today. These features
phase into a strengthening, high-amplitude shortwave trough over mb,
the Dakotas, and NE by 12z, contributing to some acceleration of the
southern trough/low near the end of the period and into day 2.
At the surface, the 11z analysis showed a cold front from western
Upper Michigan across western IL, then southwestward in more diffuse form,
temporarily hung up in one convectively related wake low over
southwestern/south-central MO and another near Dal. The front then
became better-defined again southwestward across the Southern Hill
country to near drt. A warm/marine frontal zone was evident from
southeastern Arkansas across southwestern and coastal MS,
east-southeastward over shelf waters to near aaf, then in diffuse
form over the north-central Florida Peninsula. The warm front should
move slowly east-northeastward across the north-central Gulf coastal
plain through the period. By 00z, the cold front should reach
western portions of lower mi, in, KY, and MS, southwestward over the
northwestern Gulf. By 12z, the cold front should become more
meridional from eastern lower Michigan through an area of low pressure
over eastern portions of Kentucky/TN, southward across the western Florida
..central Gulf Coast...
An extensive area of scattered convection is ongoing across portions
of southeastern la and southern MS, over and southwest of the
effective warm/marine front. While deep shear is not strong over
this region, expect backed flow and well-curved hodographs near the
warm front, along with rich low-level moisture and related low local.
This will Foster a conditional risk for a brief/small tornado or two
and isolated damaging gusts from the most heavily water-loaded
downdrafts, until the main/pre-cold-frontal convective band
overtakes it from northwest-southeast late morning through early
Areas of diurnal destabilization are likely ahead of that convection
today, augmented by low-level Theta-E advection; however, the amount
and pace of that destabilization is in question given the cloud
cover associated with the ongoing convection. Regardless, surface-
based effective-inflow parcels should be available across most or
all of the risk area as the marine air shifts slowly inland,
displacing a recycled Continental/polar air mass from the frontal
passage a few days ago. Meanwhile, with the approach of the basal
trough aloft, overlying mid/upper winds should contribute to an
increase in deep shear over the warm sector and the potential for
sporadic organized convection, with damaging gusts the main concern;
a tornado cannot be ruled out.
Overnight, as the mid/upper trough and associated vorticity Banner
become more negatively tilted, the resulting adjustment of kinematic
geometry will result in an eastward acceleration of the southern
part of the corridor of most favorable lift near the front. That,
in turn, should reorient the main convective plume more north-south
with some acceleration possible the last 3-6 hours of the period.
Spreading of large-scale ascent (from both DCVA and warm advection)
over more of the warm sector also is possible, supporting
development of surface-based or nearly surface-based convection over
the Gulf, moving inland with some merger into the main line
possible. All of these factors support an eastward expansion of the
existing outlook area at least to near the Apalachicola/
Chattahoochee River system for tonight. Some subset of the "mrgl"
area may need an upgrade in succeeding outlooks, once uncertainties
are better-resolved regarding the impact of the large area of
ongoing convection on favorable destabilization to its east.
..northern High Plains...
As the northwestern shortwave trough consolidates/intensifies and
approaches, both large-scale lift and deep-layer flow will intensify
markedly. Accordingly, high-wind warnings are in effect for much of
this region. Isolated thunderstorms may supplement the already
strong-severe gradient flow at the surface late this afternoon,
resulting in damaging/severe gusts in convective downdrafts.
Forecast soundings, modified for high temps in the lower 60s f,
yield steep low/middle-level lapse rates and well-mixed boundary
layers conducive to convective downward Transfer of momentum through
the short vertical distance from a field of 50-60-kt flow in the
850-700-mb layer. Essentially uninhibited MLCAPE of 100-200 j/kg is
possible with Low Cape density, but also, buoyancy extending into
the icing layer suitable for at least isolated lightning production.
The threat should diminish markedly after dark with surface
diabatic stabilization and resultant loss of both cape and dcape.
acus11 kwns 221041
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 221041
Mesoscale discussion 1734
National Weather Service Storm Prediction Center Norman OK
0541 am CDT sun Oct 22 2017
Areas affected...southeast la...southern MS
Concerning...severe potential...watch unlikely
Valid 221041z - 221245z
Probability of watch issuance...5 percent
Summary...persistent thunderstorm clusters will pose a threat for
locally damaging wind and/or a brief tornado this morning.
Discussion...at 1030z, a very persistent thunderstorm cluster
continues to impact portions of southeast la and adjacent southwest
MS. Broad rotation has occasionally been noted with individual cells
within this cluster, and some eastward propagation has recently been
noted as cells interact and continually feed into the cluster from
the south. This activity is being sustained within a region of rich
moisture and broadly confluent low-level flow, possibly being aided
by divergent upper-level flow downstream of an amplified upper
trough over the central Continental U.S..
While midlevel flow is modest at best across the region, low-level
easterly/southeasterly flow veering to southwesterly aloft is
resulting in sufficient effective shear/helicity for organized storm
structures. Locally damaging winds and a brief tornado will be
possible with the aforementioned long-lived thunderstorm cluster,
and also potentially with ongoing storms further south across
southeast la, where weak rotation has occasionally been noted with
the strongest cells.
Watch issuance is not currently anticipated, given the marginal and
relatively localized nature of the threat.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 29609211 30289185 31109132 31399069 31428929 30828865
30428852 30038926 29468932 29168903 29019016 29289151