SPC AC 182000
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0300 PM CDT Wed Jul 18 2018
Valid 182000Z - 191200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF SOUTHERN SOUTH DAKOTA AND CENTRAL/EASTERN NEBRASKA...
Severe thunderstorms are likely this afternoon into tonight across
parts of the northern and central Plains including Nebraska and
southern South Dakota.
...SD and NE...
No major changes have been made to the primary risk area across
portions of South Dakota and Nebraska. The 19Z Rapid City sounding
shows strong midlevel northwesterly flow that will begin impinging
on a moderately unstable environment further east. The potential
remains for a few supercells later this afternoon capable of large
hail and a tornado or two, followed by upscale growth this evening
that may be accompanied by an increased severe wind risk.
5% wind and hail probabilities have been added across portions of
northwest ND, where thunderstorms have recently intensified. Despite
generally weak shear across this region, moderate instability will
support some localized hail and wind threat with the strongest
...Ozarks into the Lower MS River Valley...
The Marginal Risk has been adjusted across portions of AR into the
lower MS River Valley. Parts of western/central AR have been removed
due to persistent cloudiness limiting destabilization across this
area. Parts of northeast AR into western TN have been added, where
convection has recently intensified. Portions of northeast LA into
western MS have also been added to the Marginal area, ahead of a
growing thunderstorm cluster that will move into an axis of
moderate-to-strong instability, posing a threat of isolated damaging
.PREV DISCUSSION... /ISSUED 1149 AM CDT Wed Jul 18 2018/
...North-central Plains/Middle MO Valley...
Severe thunderstorms are likely this afternoon into evening and
possibly the overnight across the region, although some uncertainty
exists related to the most likely sub-regional corridor of severe
storms. This is attributable to some lingering influences of prior
outflow and multi-layer cloud cover at midday. Convection-allowing
guidance spatial variability also exists between this morning's HRRR
runs and other 12Z-based CAMs such as the NSSL-ARW.
Regardless, the most probable area for surface-based convective
initiation appears to be across west-central/south-central SD into
far northern NEB this afternoon. With strong high-level winds
already in place, an eastward-developing belt of increasingly strong
mid-level westerlies in conjunction with south-southeasterly
boundary layer winds will contribute to 35-45 kt of effective shear.
This will be favorable for supercells capable of large hail, with
moderate buoyancy and lengthy hodographs supportive of some larger
hail magnitudes in spite of sub-optimal lapse rates. Some tornado
risk may exist as well with enhanced low-level SRH in proximity to
the surface boundary.
Otherwise, relatively quick upscale growth should occur by evening
with one or more southeastward-moving clusters likely to evolve.
This would pose an increased damaging wind risk this evening across
parts of NEB, which could continue into the overnight potentially as
far south as portions of KS.
...Lower MO Valley to Mid-South/Lower MS River Valley...
Weak outflow/zones of differential heating will spatially influence
sub-regional corridors of diurnally intensifying storms today within
a very moist air mass. Some of the stronger updrafts/downdrafts will
be capable of localized wind damage.
...Northern AZ/Lower CO River Valley...
Widely scattered thunderstorms are expected later today, growing in
coverage and becoming more clustered as they shift westward through
afternoon into early evening. Isolated severe-caliber wind gusts are
expected. A moist air mass, with PW of 1-1.5 inches, and steep
low-level lapse rates in an increasingly well-mixed boundary layer
this afternoon, will contribute to MLCAPE commonly reaching
1000-1500 J/kg, and locally exceeding 2000 J/kg. This will
contribute to potential for wet microbursts in cores and hybrid
wet/dry processes in deeper/lower-ground-elevation boundary layers.
Forecast soundings reasonably suggest (given the pattern) that a
deep layer of easterlies in the midlevels will be the dominant
contributor to the weak steering for early-stage convection, until
enough upscale growth occurs with westward extent that aggregated
cold pools force forward propagation.
...Coastal Carolinas vicinity...
A warm and very moist air mass to the south of a southward-sagging
front in conjunction with sea breeze influences could yield a couple
of strong to locally severe thunderstorms capable of isolated
damaging wind gusts this afternoon.
CLICK TO GET WUUS01 PTSDY1 PRODUCT
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z