Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0752 AM CDT Mon Jul 15 2024
Valid 151300Z - 161200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
IOWA TO SOUTHWESTERN LOWER MICHIGAN AND NORTHERN INDIANA...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENHANCED RISK...ALSO SEPARATELY OVER PARTS OF NEW YORK/PENNSYLVANIA
AND THE CENTRAL HIGH PLAINS...
...SUMMARY...
Severe thunderstorms are possible this afternoon and evening across
portions of the Midwest, central High Plains, lower Great Lakes, and
Arizona. The greatest potential for severe gusts is over eastern
Iowa into Illinois and Indiana.
...Synopsis...
Mid/upper-level pattern amplification in the northern stream will
contribute to weakening (but not dissipation) of the persistent
anticyclone over the Four Corners region. A 500-mb low over
northern MB will dig south-southeastward to northwesternmost parts
of ON through the period, combining with a series of small shortwave
perturbations in the cyclone's southern semicircle to increase both
cyclonic flow speeds and height curvature over upper parts of the
Midwest, Mississippi Valley and Great Lakes.
As this occurs, a shortwave trough -- evident in moisture-channel
imagery over the Black Hills and vicinity -- will accelerate
eastward to southern MN and northern/western IA by 00Z, then reach
Lake Huron, northwestern OH and northern IN by the end of the
period. That feature should be preceded by a smaller shortwave
trough and MCV -- now apparent over western MN -- and forecast to
move across northern WI and Upper MI through this evening. To its
southeast and well downstream from the Black Hills perturbation,
another MCV was quite evident in composite radar imagery over
southern Lower MI, with shortwave trough southwestward to parts of
central/eastern IL. This perturbation should move westward to
western parts of NY/PA by 00Z, then near-coastal areas of central/
southern New England and the northern Mid-Atlantic by 12Z tomorrow.
At the surface, 11Z analysis showed a low over northeastern SD with
cold front southwestward across western NE and northern CO. By 00Z,
the low should move/redevelop to a position near DBQ, with cold
front southwestward across northeastern to northwestern KS and
east-central/southeastern CO. By 12Z, the front should reach Lower
MI, northern/western IL (perhaps obscured by convective effects),
southern KS, and northeastern NM. Relevant prefrontal boundaries
are discussed below.
...IA to southern Lower Great Lakes...
The decaying remnants of the prior/overnight MCS responsible for the
southern Lower MI MCV also have left an outflow boundary over
southern portions of IN/IL, arching northwestward and becoming
quasistationary over western IL into southeastern/central IA.
Clouds/precip from the prior MCS, and shortwave subsidence/DNVA
immediately following its associated shortwave trough, should be
long gone by mid/late afternoon, when the airmass along the outflow
boundary will have had several hours to modify favorably from west
to east. The boundary itself also may drift northward amidst weak
but steady ambient warm advection, with the boundary layer favorably
destabilizing on both sides.
A combination of lift near the boundary, increasing large-scale
ascent ahead of the upstream mid/upper-level shortwave trough,
steepening low-level lapse rates from diurnal heating, and rich
moisture (surface dewpoints commonly upper 60s to low 70s F) are
expected to prepare the corridor in and near the "enhanced risk"
area for severe potential through the afternoon. Isolated to
scattered thunderstorms should develop near and north of the western
limb of the outflow boundary (mainly IA, but also possibly western
IL) from mid/late afternoon into early evening. Some of this
activity may be supercellular in the first few hours, with large
hail possible.
However, given the presence of a convergent, moisture-rich, broad
gradient of instability north of the boundary aligned strongly
parallel to favorably strong mid/upper winds, large low/middle-level
lapse rates, and a related reservoir of high buoyancy (MLCAPE of
3000-5000 J/kg) this activity should evolve quickly upscale to an
MCS with severe gusts (some over 75 mph) becoming the greatest
hazard. With a vorticity-laden boundary, and potential for backed
surface winds and vertical-shear/hodograph enhancement to its north,
a corridor of relatively maximized tornado potential (from
supercells and/or QLCS-embedded circulations) also exists -- likely
narrower than depicted here due to current boundary-position
uncertainty. Uncertainty exists also on how far the MCS will travel
(and whether far enough to qualify as a derecho) before weakening
tonight. This will be related to depth/breadth of the cold pool and
mesobeta-scale rear-inflow jet dynamics, and their ability to exert
forced ascent on a moist but gradually stabilizing inflow layer with
eastward extent.
...Northeast...
Scattered thunderstorms are expected to move eastward across this
area this afternoon into early evening -- some of which may initiate
late morning into midday back in ON closer to the MCV. Activity
should aggregate into clusters and/or a short QLCS, yielding
damaging to severe gusts. Isolated discrete or line-embedded
supercells also are possible, with some attendant severe-hail
potential.
As the MCV crosses southernmost parts of ON and also Lake Erie
today, the foregoing boundary layer over parts of NY/PA will
destabilize diurnally, while remaining favorably moist (with lower-
elevation surface dewpoints commonly in the mid 60s to near 70 F).
This will support a buildup of preconvective MLCAPE into the 1500-
2000 J/kg range (locally higher). Note that models with RAP physics
(RAP, HRRR) may be over-mixing higher elevations and
underforecasting buoyancy as a result (a common bias). Increase and
slight veering of winds with height will contribute to around 30-40
kt effective-shear magnitudes, which may be locally greater in any
channels of gradient-flow enhancement from the MCV that extend to or
past the zone of convective forcing. Meanwhile, a well-mixed
subcloud layer will supports damaging-downdraft potential.
...Central High Plains...
Isolate to scattered, high-based thunderstorms are expected to form
this afternoon over a broad area from central WY to the Front Range
and foothills of CO, with the most concentrated convection still
progged for portions of southeastern WY, western NE and northeastern
CO. Steep low/middle-level lapse rates (8.5-9 deg C/km) are
expected by midafternoon over this region, with enough easterly
component of flow behind the front (and relate moist advection) to
offset mixing and maintain mid 40s to low 50s surface dewpoints in
the "slight risk" area. These should contribute to a field of
peak/preconvective MLCAPE ranging from around 300 J/kg just east of
the mountains to around 1500 J/kg over parts of west-central/
southwestern NE. The post-frontal boundary-layer flow also will
enhance both storm-relative low-level winds and deep shear
(effective-shear magnitudes in the 35-45-kt range). This, along
with potential for cold-pool clustering, favors some organization
and maintenance of convection across the outlook this evening before
diminishing in a more strongly capped, nocturnal boundary layer over
lower Plains elevations.
...AZ...
Widely scattered thunderstorms are expected to form again this
afternoon over portions of the Mogollon Rim and smaller terrain
maxima to its south, offering locally strong-severe downbursts.
Although the synoptic anticyclone will be weakening through the
period, associated flow still will support motion toward the west or
southwest, atop deep/well-mixed boundary layers of the desert floor.
Airmass recovery from the prior day's convection by afternoon will
be aided by warm/moist advection and strong diurnal heating. Large
dewpoint depressions and deep subcloud layers in lower elevations --
beneath about 1000-1500 J/kg MLCAPE -- will support the gust
potential, especially wherever activity can become clustered and
maximize regenerative potential through cold-pool-aided lift.
..Edwards/Broyles.. 07/15/2024
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