LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0755 AM CDT Fri Jul 28 2023
Valid 281300Z - 291200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
SOUTHEASTERN SOUTH DAKOTA TO THE SOUTHERN LAKE MICHIGAN VICINITY...
...SUMMARY...
Severe storms are most probable from southeast South Dakota across
Iowa, northern Illinois and southern Wisconsin this afternoon and
evening, with corridors of damaging wind and large hail. Other
scattered strong to severe storms are expected from Ohio into
Virginia, and across the northern High Plains.
...Synopsis...
In mid/upper levels, a persistent, strong anticyclone will remain
centered over the southern Rockies, with ridging eastward across the
Mid-South and southwestward over northern Baja. The northern-stream
belt will start the period mostly zonal, but start to buckle some
through the period as a series of mostly low-amplitude shortwaves
traverses a re-amplifying eastern CONUS mean trough. Ridging also
will amplify from the 500-mb high north-northwestward over the
northern Rockies. These processes will yield a belt of difluent,
northwesterly to west-northwesterly flow over the northern Plains,
Upper Midwest, Upper Great Lakes, and lower Ohio Valley.
At the surface, 11Z analysis showed a cold front across northeastern
ON to Lower MI, where it was broken up by an area of convective
outflow. A warm front became apparent farther southwest over parts
of northern IA, and southern MN, to a low in the SUX area, with the
cold front extending westward over northern NE to northern WY. The
low should migrate eastward across the WI/IL border region to near
TOL by the end of the period, along the aggregate baroclinic zone
from synoptic and convective influences.
...North-central Plains to central Appalachians...
Scattered to locally numerous thunderstorms are possible through the
period in this corridor. The main concern remains upscale growth of
one or more organized thunderstorm clusters into MCSs with organized
rear-inflow jet(s), cold pool(s) and attendant severe wind. Early-
stage, relatively discrete thunderstorms also should produce large
hail -- perhaps significant/2+ inch diameter in western parts of the
outlook area, where deep-layer lapse rates and low-level hodographs
will best favor hail-producing supercells.
In the area from central/southeastern SD to northern IL, strong
diurnal heating will steepen low-level lapse rates amid 60s and low
60s F surface dewpoints, combining with steep midlevel lapse rates
to yield MLCAPE in the 3000-4500 J/kg range. This will support
rapid development and upscale growth of convection once the cap is
broken, likely preferentially near the baroclinic zone and surface
low where low-level convergence should be maximized. Deep shear
will be favorable for early-evolution supercells as well as bowing
structures during upscale aggregation, with effective-shear
magnitudes in the 45-55-kt range.
Current unconditional probabilities don't depict a substantially
denser, greater-magnitude wind threat that *could* develop somewhere
within the broad swath of the "slight" and "enhanced" lines. The
"enhanced" area simply represents what still appears to be the best
apparent environment for such an event, but still may shift around
in position and shape today, as mesoscale/convective trends and
shorter-fused progs warrant. This uncertainty is partly because of
mesoscale questions regarding the influence (and possible downshear
intensification of) ongoing activity and its boundaries, as well as
substantial differences among both CAM and synoptic-scale guidance
on convective coverage/organization later today, in that more-ideal
environment from the Plains to the Corn Belt.
...Northern High Plains...
Widely scattered to scattered thunderstorms should form this
afternoon over the higher terrain of southwestern MT and northern
WY, moving eastward the east-southeastward. Isolated severe gusts
and marginal hail will be possible. This projected path conforms
both to:
1. The gradual anticyclonic curvature of ambient/mean flow near the
mid/upper ridge, and also
2. The orientation of a supportive, boundary-layer moist plume
whose axis should remain across southern MT into western SD.
Near that moist axis, which should feature 50s F surface dewpoints,
diurnally steepened low/middle-level lapse rates will contribute to
500-1000 J/kg MLCAPE, a well-mixed boundary layer, and minimal
MLCINH ahead of the orographic initiation regime by mid/late
afternoon. Surface flow should veer with time from northerly to
easterly and southeasterly, maximizing storm-relative winds in the
inflow layer, though absolute speeds may be weak. Still, strong
veering with height will contribute to 45-55-kt effective-shear
magnitudes. Some of this activity may persist well into tonight
and portions of the Dakotas with a sporadic wind threat, as it
encounters increasing moisture and continued favorable storm-
relative winds above a nocturnally stabilizing near-surface profile.
...Northern ME...
Widely scattered to scattered thunderstorms are expected to develop
along/ahead of a cold front over southern QC, upshear from this
region, during the afternoon. Some of this activity may cross
northern ME before weakening, with the potential for isolated,
marginally severe gusts and/or hail. Forecast soundings suggest a
combination of diurnal heating and sufficient moisture (e.g.,
mid-60s F surface dewpoints) will offset modest mid/upper-level
lapse rates and yield around 1000 J/kg MLCAPE. Nearly
unidirectional deep-layer wind profiles will limit low-level shear,
but with strong flow in mid/upper levels, effective-shear magnitudes
around 35-45 kt reasonably appear in modeled wind profiles, prior to
convective influences. As such, organized multicells and isolated
supercells may occur.
...AZ...
Isolated severe gusts are possible again this afternoon from
thunderstorms propagating westward to northwestward over portions of
southern AZ -- mainly developing over orographically favored areas
early, then perhaps on a couple of subsequent outflow boundaries.
IR satellite imagery indicates extensive cloud cover from earlier
convection over MX is breaking up, and substantial diurnal heating
once again is expected across the region. MLCAPE up to about 500
J/kg should develop as enough moisture is maintained in a deep, hot,
well-mixed boundary layer that supports locally intense downdrafts.
..Edwards/Broyles.. 07/28/2023
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