SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0803 AM CDT Wed Apr 14 2021

Valid 141300Z - 151200Z


Hail should be the most common threat with severe thunderstorms
today over portions of southeast Texas, Louisiana and southwestern

The upper-air pattern over the CONUS will continue to be dominated
by two cyclones and a belt of enhanced, nearly zonal flow to their
south.  The first cyclone is located over the northern WI/Upper MI/
Lake Superior region, and is forecast to shift east-southeastward
across the Upper Great Lakes to Lake Huron by 12Z tomorrow.  The
western cyclone -- apparent in moisture-channel imagery over the
Great Basin and southern parts of the interior Northwest -- will
move slowly eastward through the period, while remaining part of a
loosely organized Rex configuration with a high over BC.  By 12Z,
the 500-mb low should reach northern UT.  To its south, a weak
shortwave trough was evident over AZ and northern Sonora.  This
feature should move through the broader-scale ridge over the
southern High Plains by around 00Z, then reach the Mid-South region
by the end of the period.

Surface analysis at 11Z depicted a diffuse cold front from western
PA across southeastern KY, middle TN, and southeastern AR, becoming
somewhat better defined southwestward over the Arklatex to near DRT.
The front is forecast to shift slowly southward/southeastward,
extending from a weak low over western VA across northern GA,
southwestern AL, southern LA, the mid/upper TX Coastal Plain, and
north-central Coahuila by 00Z.  By 12Z, the low should move/
redevelop just offshore from the Delmarva Peninsula, with cold front
over central GA, southeastern AL, and southeastern LA, becoming
quasistationary over south-central TX.

...West Gulf Coastal Plain...
Scattered thunderstorms are ongoing over a broad swath of the
Arklatex, east TX, LA, and the lower Mississippi Valley region, with
the potential for at least isolated severe hail.  Additional
development is expected through this evening, especially on the
western and southern fringes of the ongoing activity, as isentropic
lift to LFC continues atop a cold pool and near the frontal zone.

The cold pool was located north and northeast of an outflow boundary
from yesterday's convection, and is being reinforced over much of
east TX, AR, and central/northern LA by ongoing activity.  The
boundary was analyzed between rig stations GRY and GHB over the
Gulf, south of the LA coastline, northwestward across the HOU area
then diffusely northward to a frontal intersection roughly between
UTS-CRS.  This boundary should move slowly northward with time over
southeast TX and southwestern-coastal south-central LA, gradually
leading to an increase in theta-e and more robustly surface-based
effective-inflow parcels over that region.  The 15% wind and
marginal tornado probabilities are more dependent on such inflow
parcels, and have been shaped accordingly.  Inland diurnal heating
also should support a sea-breeze boundary today over mid/upper TX
coastal areas, with isolated to widely scattered thunderstorms
possible thereon, also offering a wind/hail threat.

Atop the cold pool, elevated MUCAPE of 1000-2000 J/kg will be
enabled by a nearly saturated layer near 850 mb, 6.5-7.5 deg C/km
midlevel lapse rates and a tropopause near 200 mb.  Highly variable
effective-shear magnitudes are expected, mainly in the 30-45 kt
range, supporting a blend of multicell and supercell modes. 
Clustering also is likely given the weak elevated MUCINH, which
would reduce potential hail duration and size somewhat in any given
area.  However, any cluster/MCS that can build into nearly surface-
based parcels closer to the coast may offer a more-organized wind
threat on the mesoscale.

Widely scattered to scattered, high-based thunderstorms should
develop late this afternoon into this evening, initially near the
low and trailing surface/lee trough, as the leading edge of a plume
of low/middle-level ascent reaches the area.  Damaging gusts, with
isolated severe/50+ kt gusts possible, will be the main concern,
though a supercell or two may develop this evening and produce hail
as well.

Sustained diurnal heating of the boundary layer over this region is
expected, given current and forecast clouds trends. This will lead
to steep low-level lapse rates with a well-mixed boundary layer, and
in turn, support downdraft acceleration in any convection that
encounters this air mass before nocturnal/diabatic cooling becomes
too stabilizing.  The main inhibiting factor for a more-robust/
organized severe event will also be that which contributes to the
gust potential:  lack of richer low-level moisture.  Surface dew
points in the mid 40s to low 50s F are common over the region, and
may increase somewhat today via moist advection, before afternoon
mixing levels them off or even reduces dew points in some areas. 
Accordingly modified model soundings max out MLCAPE only around 500

Low-level shear vectors and hodographs are expected to enlarge with
time from midafternoon into early evening in the preconvective
environment, as prefrontal mass response strengthens southeast of
the increasingly progressive Great Lakes cyclone.  Effective SRH may
grow into the 200-300 J/kg range while parcels still are surface-
based, along with 50-60 kt effective-shear magnitudes.  This will
support supercell potential, and a risk for at least isolated,
marginally severe hail, despite the modest midlevel lapse rates and
lack of greater buoyancy.

..Edwards/Broyles.. 04/14/2021

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