SPC AC 242001
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0301 PM CDT Sun Jun 24 2018
Valid 242000Z - 251200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM SOUTHEAST
COLORADO THROUGH OKLAHOMA AND INTO PARTS OF KENTUCKY AND
Isolated to widely scattered severe thunderstorms capable of severe
gusts and large hail are expected from the south-central High Plains
eastward through Oklahoma and Arkansas and into central Kentucky and
The notable change from the previous outlook's probability forecast
is to reduce severe probabilities (hail, wind, tornado) and result
in a categorical Slight (2) Risk from southeast CO through the TX
Panhandle and into western OK. An ongoing severe MCS over central
OK is continuing to the east-southeast during the mid afternoon.
Extensive cold-air outflow from the MCS has permeated the TX
Panhandle into northeast NM and southeast CO. The 19Z AMA RAOB
sampled a boundary layer much less unstable than short-term model
guidance depicted (even the 18Z RAP model struggled substantially
with a difference in 3000+ J/kg MLCAPE noted for a 1-hr forecast).
Given the inability for both convection-allowing and deterministic
guidance to reasonably capture the pervasiveness of the reduction in
theta-e, have opted to heavily lean on observed surface and special
RAOB upper-air data for an environmental expectation for later this
afternoon/evening. Models continue to show thunderstorms developing
near the higher terrain later today and moving southeast into the TX
Panhandle. The steep 700-500 lapse rates sampled above the boundary
layer and supercell-wind profiles will support a large hail risk
along with the potential for severe gusts.
Elsewhere, left the forecast largely unchanged except to include
low-wind probabilities in Upstate SC.
.PREV DISCUSSION... /ISSUED 1131 AM CDT Sun Jun 24 2018/
A midlevel trough over the central Rockies will continue to progress
east-southeastward over CO today, and the trough will reach western
NE/KS overnight. This synoptic wave is preceded by a series of MCVs
from overnight convection across KS/OK/MO/AR, and associated
...Southern High Plains this afternoon through early tonight...
The outflow and stabilization appears likely to limit the severe
storm threat for areas north of the ongoing convection across
northwest OK, given the reduction in lapse rates aloft and low-level
moisture per surface observations and the 12z soundings from DDC and
LMN. The outflow associated with the ongoing storms continues to
surge southward across northwest OK and the TX Panhandle, and should
consolidate with the prior outflow from the weakening MCS now over
AR. This will tend to result in a southward shift of the main
severe threat area to where the outflow eventually stalls at the
triple point with the lee cyclone and dryline. An influx of 68-72 F
boundary-layer dewpoints along the residual southern outflow and
surface heating, beneath steep midlevel lapse rates noted in the 12z
AMA sounding, will drive strong buoyancy (MLCAPE in excess of 4000
The convective outflow has overspread almost all of southeast CO,
where widespread low clouds may tend to slow surface heating.
Still, sufficient destabilization is expected to support severe
storm development this afternoon as forcing for ascent overspreads
the area with the approach of the midlevel trough and a reinforcing
frontal surge from the north. The initial storms should be
supercells capable of producing large hail, though upscale growth
into another convective cluster is expected by this evening, with an
attendant threat for damaging winds. The convective cluster should
subsequently develop east-southeastward toward the northern TX
Panhandle and western OK along the lingering outflow and buoyancy
gradient from the ongoing storms. A separate area of storm
development will be possible at the outflow/dryline intersection
(probably near or just north of PVW), where strong surface heating
and deep mixing will reduce convective inhibition and drive strong
buoyancy. This area will be along the southern fringe of the
stronger mid-upper flow and will rely on backed low-level flow along
the outflow to support supercells. Very large hail and damaging
winds will be possible with any storms that form along the boundary,
and a couple of tornadoes could also occur in the zone of enhanced
low-level shear and moisture on the cool side of the boundary, once
it slows its southward movement this afternoon.
...Western TN and vicinity...
The MCS across AR should continue to weaken today, though a
resultant MCV will continue to drift eastward through the afternoon.
Some destabilization will occur along a weak outflow boundary
across western TN, and in advance of the remnants of the AR
convection. There will be some chance for redevelopment of
strong/severe storms with the MCV this afternoon, and damaging gusts
will be the main threat.
...Mid Atlantic and vicinity this afternoon...
Scattered thunderstorms will form in a moist environment and in
association with embedded speed maxima progressing eastward from the
OH Valley toward the northern Mid Atlantic and southern New England.
Though lapse rates will remain rather poor, moderately strong
midlevel flow could support a few storms capable of some downward
momentum transfer and isolated strong/damaging gusts.
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