RSS Mesoscale Discussions from Storm Prediction Center
No watches are valid as of Sat Aug 17 13:05:03 UTC 2019.
MD 1767 CONCERNING SEVERE POTENTIAL...WATCH UNLIKELY FOR CENTRAL AND NORTHERN ILLINOIS AND WESTERN INDIANA
Mesoscale Discussion 1767
NWS Storm Prediction Center Norman OK
0800 AM CDT Sat Aug 17 2019
Areas affected...central and northern Illinois and western Indiana
Concerning...Severe potential...Watch unlikely
Valid 171300Z - 171500Z
Probability of Watch Issuance...20 percent
SUMMARY...Thunderstorms will continue to move out of Missouri and
into central Illinois this morning. Additional thunderstorms will be
possible across northern Illinois as well. Hail and gusty winds may
be possible with the strongest cores. The area will be monitored for
potential watch, but current indications are that the threat should
remain sufficiently isolated to not warrant one.
DISCUSSION...Long-lived cluster of thunderstorms continues to move
east from northern Missouri into central Iowa. Most-unstable CAPE
along and ahead of this cluster is around 1000-1500 J/kg; however,
deep-layer shear is generally less than 25 knots. Thus, although a
severe hail or strong thunderstorm wind gust will be possible with
the strongest cores, the threat should remain sufficiently
isolated/episodic that a watch is not currently anticipated for this
activity.
To the north, across northern Illinois, additional thunderstorms may
develop within a belt of stronger midlevel flow, likely in response
to ageostrophic forcing from the mesoscale convective system to the
southwest. The result will be thunderstorms developing in an
objectively analyzed environment of most-unstable CAPE around
1000-1500 J/kg and deep-layer shear of 40-50 knots. Although this
would normally suggest a threat for organized severe thunderstorms,
considerable uncertainty remains regarding convective coverage and
longevity. As such, area will continue to be monitored for potential
watch, but expectations are that the threat will remain too isolated
in nature to warrant watch issuance.
..Marsh/Edwards.. 08/17/2019
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...IWX...IND...LOT...ILX...LSX...DVN...
LAT...LON 39948994 41609062 42338945 41488669 39198715 38529025
39948994
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SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0758 AM CDT Sat Aug 17 2019
Valid 171300Z - 181200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE NORTH-CENTRAL PLAINS REGION...
...SUMMARY...
The greatest threat for severe storms appears to be over parts of
the northern and central Plains into the upper Midwest late this
afternoon into tonight.
...Synopsis...
In mid/upper levels, a highly amplified and blocky pattern inhabits
North America's neighborhood in the Northern Hemisphere. A large,
complex cyclone will remain anchored over the north-central Pacific,
with sharp ridging to its north through east, from the Bering Sea to
waters west of both CA and Baja. Broadly cyclonic mean flow will
persist over much of Canada and the northern half of the CONUS, much
of it accompanied by seasonally cold mid/upper-level temperatures.
The most influential shortwave feature appears to be the trough now
moving eastward out of the northern Rockies region, and forecast to
amplify as it crosses MT and the Dakotas through the period. By
12Z, this perturbation should reach southeastern MB, eastern ND and
central/eastern SD. Meanwhile, convectively generated/enhanced
vorticity maxima from prior activity will aid convective potential
on the mesoscale across portions of the marginal-risk area from
IN-NY.
At the surface, the 11Z analysis showed a surface low over extreme
northern Lower MI with cold front southwestward across northern IL,
becoming quasistationary and somewhat diffused by convective
activity over southern IA and southern NE. Baroclinicity was
stronger on an outflow arc extending from eastern MO across the
eastern KS/OK border region to west-central KS. The latter boundary
should weaken from west to east through the day, with some
northeastward retreat possible, while the frontal zone to its north
moves little. Meanwhile, a better-defined cold front was drawn from
southern MB across east-central ND and northwestern SD, to a low
over northeastern WY. This front should move eastward/southeastward
across the remainder of the northern Plains through the period. By
12Z the cold front should reach eastern MN, western/central IA,
southeastern NE, and northeastern CO.
...North-central Plains, upper Midwest...
Scattered thunderstorms should develop over western parts of the
north-central Plains this afternoon, predominantly along the cold
front and over higher terrain in and near the Black Hills and
Bighorns, offering the potential for severe hail/gusts. Supercells
are possible relatively early in the convective cycle. Then the
potential exists for some of the western SD/frontal activity to
evolve upscale toward a more wind-dominant severe risk, perhaps
aided by additional convective development to its east.
Being fairly well-removed from the ongoing extensive clouds/precip
in the eastern KS/MO region, areas of strong heating are expected in
the prefrontal warm sector. This, combined with 60s F surface dew
points and steep low/middle-level lapse rates, should render areas
of 2000-3500 J/kg late-afternoon/preconvective MLCAPE in central/
eastern SD and much of NE, decreasing westward toward the higher
terrain as moisture diminishes (but still should be sufficient to
support convection). Vertical shear should strengthen through the
afternoon and evening in the warm sector, with 40-55-kt effective-
shear magnitudes supporting supercell potential before upscale
growth occurs. The threat accordingly should transition from hail
to wind with time into the overnight period.
...IA, eastern NE, portions of KS, northern MO...
Additional scattered to numerous thunderstorms may form farther
south tonight over parts of the eastern NE/IA/northern MO area in a
zone of strong low-level warm advection and moisture transport, with
severe/significant hail potential. This regime is a good deal more
uncertain in how it may develop and evolve, including how much
influence may persist from High Plains activity persisting into the
region vs. in situ development with the warm-advection/LLJ corridor.
The extensive area of antecedent convection over KS/MO -- and
related airmass effects on timing/extent of recovery farther north
this afternoon and evening -- yield the bulk of uncertainty in this
forecast. The severe that occurred late over KS/MO during the past
overnight period required 12-18 hours of postconvective recovery
following extensive convection to the south. Loosely similarly, a
delayed onset of convective/severe potential into the
evening/overnight hours is probable across the outlook area again
this day-1 period. Two main differences will be:
1. Somewhat weaker but still adequate low-level moisture/theta-e,
and
2. Stronger but still peripheral mid/upper-level influences
southeast of the pronounced shortwave trough, in terms of
synoptic-scale lift via warm advection and weak DCVA/cooling, and
strengthening gradient winds aloft for favorable deep shear.
At this time, uncertainty remains too large to increase or areally
refine the unconditional severe probabilities.
..Edwards/Marsh.. 08/17/2019
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