Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
SPC AC 221259
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0759 AM CDT Thu Jun 22 2017
Valid 221300Z - 231200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
MISSISSIPPI DELTA...TENNESSEE VALLEY AND CENTRAL GULF COAST
REGIONS...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
CENTRAL AND SOUTHERN HIGH PLAINS AND COLORADO FRONT RANGE...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF IOWA
TO PARTS OF LOWER MICHIGAN...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK IN THE SOUTH...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING AND
CONNECTING THE OTHER TWO SLIGHT RISKS...
...SUMMARY...
A few tornadoes and isolated damaging gusts are possible over a
broad area east and northeast of the center of inland Tropical Storm
Cindy. Elsewhere, severe thunderstorms with damaging winds and
large hail are possible from the south-central High Plains to
portions of the Great Lakes.
...Synopsis...
The mid/upper-level pattern will remain dominated by three primary
synoptic to mesoalpha-scale features:
1. TC Cindy and its accompanying upper-air perturbation;
2. An intense ridge that will remain entrenched over the
southwestern U.S.;
3. A belt of relatively fast northern-stream flow from the Pacific
Northwest across the Northern Plains and Great Lakes, to New
England. The northern Plains and Upper Midwest segment of this
pattern will become more cyclonic with time, as a cyclone now over
MB and trailing positively-tilted trough move southeastward. By
00Z, the trough should extend from the southern section of MB/ON
border across ND to southwestern MT. By 12Z the mid/upper trough
should reach the MN Arrowhead and extend to southwestern MN then
westward over SD.
At the surface, the 11Z analysis showed a weak frontal-wave low over
north-central NE, with a cold front southwestward across
southwestern NE and south-central CO. The frontal zone extended
from the low across southern NM -- north of an area of convective
outflow with a southern boundary arching across northeastern IA.
The front then was analyzed over central lower MI and Lake Erie.
That boundary will sag southward as a cold front today across WI, IA
and eastern NE, as well as the south-central High Plains, as the
northern-stream trough moves southeastward. By 00Z the front should
extend from northern lower MI to central IA,
northwestern/west-central KS and northeastern NM. By the end of the
period, the front should extend from southern lower MI across
southwestern MO, southwestern OK and east-central NM.
...Southern States: Tropical Storm Cindy...
T.S. Cindy is forecast to progress farther inland/northeastward
through the period toward an eventual merger with the aforementioned
frontal zone day-2, experiencing a general weakening of surface
winds and filling of central pressure, as forecast by NHC. However,
as often is the case with inland-penetrating tropical and
subtropical systems, low-level flow above the surface will be slower
to weaken. This will lead to maintenance (and perhaps areas of
mesobeta-scale strengthening) of low-level shear/hodographs in the
favorable northeastern and eastern sector through the day. In
addition, this system's dramatically asymmetric structure will
continue to spread a rather large area of at least marginally
favorable shear across the South.
Last night's 00Z 500-mb analysis showed several pockets of
distinctive, relatively warm and cold (and dry and moist) air
wrapping into this system, revealing complex internal baroclinic
influences revolving around the circulation that will contribute to
moving swaths of relative clearing today. With low/middle-level
lapse rates nearly moist adiabatic, only a few deg F of surface
heating will be needed to boost the magnitude and depth of buoyancy
such that MLCAPE 200-500 J/kg should be fairly common, with patches
of 500-1000 J/kg values this afternoon, amidst effective SRH 200-600
J/kg. Any sustained discrete or semi-discrete cells moving through
such a favorable environment will pose a tornado risk, though
coverage of those may be isolated overall. Also, differential-
heating boundaries and convergence bands will be corridors to
monitor for more localized augmentation of tornado potential within
this broad-brushed outlook area. The tornado potential should peak
mid-late afternoon and generally ramp down overnight (but not go
away altogether), amidst weaker boundary-layer instability, lower
convective coverage, and more nebulous foci.
Isolated damaging/nontornadic wind gusts also are possible from the
deeper cells even well away from center, especially where areas of
diabatic heating create shallow but well-mixed subcloud layers that
augment downward momentum transfer out of the layer of faster winds
just above surface. For near-term coverage of tornado potential,
see SPC watch 363 and related mesoscale discussions. For forecast
of Cindy's path/intensity and tropical-related watches/warnings,
refer to NHC advisories.
...Central/southern High Plains, CO Front Range...
Widely scattered thunderstorms are expected to develop near the
front this afternoon, in an environment characterized by a hot and
well-mixed warm-sector boundary layer supporting potential for large
hail and damaging gusts generated aloft to reach the surface.
Concerns exist over storm coverage, and some areas of this outlook
may not experience any. The threat itself is probably more locally
clustered than the outlook area indicates; however it is too soon to
narrow that down until mesoscale destabilization/lift trends near
the frontal zone become more readily apparent. Though low-level
winds will be weak, strong directional shear will contribute to
30-40 kt effective-shear magnitudes amidst MLCAPE 1000-1500 J/kg,
supporting a few clusters of organized and probably outflow-dominant
convection.
Post-frontal flow is expected to veer to a substantially easterly
component by this afternoon, setting up an upslope regime into the
Front Range region, beneath difluent northwest flow aloft. This
will contribute to moist advection, increasing deep speed/bulk
shear, and favorable storm-relative low-level wind vectors for
supercells moving either leftward or rightward off a long, nearly
straight hodograph. Large hail and isolated damaging gusts each
will be possible, given the deep/well-mixed subcloud layers likely
with 40s F surface dew points. A conditional threat for isolated
significant/2+ inch hail may develop in the I-25/I-76/I-70 areas
near and downshear from the DEN/FCL corridor, though uncertainties
remain related to both optimal cloud-water content and maintenance
of discrete storm modes most suitable for it. Mesoscale trends will
be monitored for enough coverage of this threat to introduce an
unconditional significant-hail area in a later outlook. Convection
will offer some wind/hail threat eastward across the High Plains of
eastern CO before diminishing.
...Portions of the Corn Belt to Great Lakes...
Multiple episodes of scattered thunderstorms are possible through
the period, with the greatest cumulative concentration expected
across the 15% wind/hail corridor representing the categorical
slight risk. This includes ongoing convection that will move
eastward from northern IA across at least parts of southern WI and
perhaps northern IL, offering a threat for sporadic large hail.
Additional thunderstorms currently north of the surface low and
frontal zone across central/eastern SD, as well as in southern MN
near the front and north of the outflow boundary, will pose a
marginal risk of isolated large hail as well as damaging gusts in
the next few hours, The SD/MN activity, based on modifications to
the OAX sounding and RAP forecast soundings, resides in an
environment of steep midlevel lapse rates and elevated moistening,
atop a dry layer between 750-900 mb that may support evaporative
downdraft-parcel accelerations through the near-surface stable
layer.
Additionally, surface-based thunderstorms are expected to develop
this afternoon along the front and outflow boundary, as well as in a
low-level warm advection/convergence zone farther east across parts
of Lake Michigan and Lower MI. This activity should occur mainly
south of the southern rim of optimally favorable mid/upper level
flow. Still, sufficient deep shear will exist for organized
multicellular convection or transient/marginal supercells, with hail
and damaging gusts possible. Diabatic surface heating and dew
points 60s F should contribute to MLCAPE 1500-2500 J/kg amidst 30-40
kt effective-shear magnitudes.
..Edwards/Picca.. 06/22/2017
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