SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0755 AM CDT Fri Jul 22 2022
Valid 221300Z - 231200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
THE CENTRAL AND NORTHERN PLAINS INTO THE UPPER MIDWEST...SEPARATELY
OVER PARTS OF NORTHERN UTAH...AND THE UPPER OHIO
VALLEY/NORTH-CENTRAL APPALACHIANS REGION...
...SUMMARY...
Isolated strong to severe thunderstorms may occur from parts of the
central and northern Plains into the Upper Midwest, separately over
parts of northern Utah, and the upper Ohio Valley/north-central
Appalachians region.
...Synopsis...
In mid/upper levels, the persistent anticyclone will remain anchored
over the Four Corners region, but with height falls and more zonal
flow to its north, as a series of shortwaves traverse the
westerlies. Downstream, the eastern mean trough also will deamplify
gradually, but with broadly cyclonic flow lingering from the upper
Mississippi Valley across the Great Lakes, Ohio Valley and
Northeast. The most prominent northern-stream shortwave trough is
evident in moisture-channel imagery over portions of WA/OR. This
perturbation should move eastward to the northern Rockies by 00Z,
with slight deamplification. By 12Z, it should extend from northern
ND southwestward over western MT.
At the surface, 11Z analysis shows a weak cold front across northern
parts of ME and NY, becoming warm to a triple point over southern
Lower MI, with an occluded front northward to northern ON. A cold
front was drawn from there across northern/western IN, becoming warm
to quasistationary over west-central IL, northern MO, eastern NE,
and eastern SD, to a low near PIR. The front then curved back
southwestward across the NE Sandhills to eastern WY. The SD low
should migrate eastward to IA by 00Z, while the trailing frontal
segment becomes nearly stationary over eastern to southwestern NE
and weakens somewhat. An area of surface cyclogenesis and
frontogenesis is expected over northern ND by 00Z, with the
resulting front extending east-southeastward and southwestward from
a southeastward-shifting low.
...Northern Plains...
At least isolated thunderstorms are possible this afternoon and
evening near the surface low and the frontal segment to its
east/southeast, as strong diurnal heating minimizes MLCINH.
Additional development may occur overnight farther east and
southeast as a LLJ intensifies to 35-45 kt, with accompanying
theta-e advection above the surface. Any sustained convection will
offer strong-severe gusts and hail. The environment across the
region will be characterized by a well-mixed subcloud layer with
enough moisture to support about 500-1500 J/kg MLCAPE, highest near
the warm front (which will correspond closely with a low-level moist
axis). Forecast soundings suggest 45-55-kt effective-shear
magnitudes, strengthening northward under stronger mid/upper winds
and more-backed frontal-zone flow in the boundary layer.
A conditional threat for organized severe exists near the
international border north of the front, with favorable moisture and
enlargement of both hodographs and low-level shear vectors. Any
convection that can develop near the front -- perhaps across parts
of southeastern SK or extreme southern MB -- accordingly may become
supercellular and potentially grow upscale into a small, eastward to
southeastward-moving MCS on either side of the international border.
However, progs that do overcome MLCINH and develop such convection
(some still do not) remain inconsistent on timing by a few hours, as
well as initiation/track location. Any such activity that does form
will have access to a corridor of around 1000-1500 J/kg MLCAPE above
a well-mixed subcloud layer, even with or without a stable nocturnal
or frontal layer near the ground. If progs initialized from 12Z
onward, and/or mesoscale trends, impart more confidence in such
development/maintenance on the U.S. side, a corridor of greater
hail/wind probabilities would be needed.
...Central Plains, Corn Belt...
Isolated to scattered thunderstorms are possible this afternoon and
evening in a broad, poorly focused convective regime, with any
sustained activity posting a risk of strong-severe gusts and
isolated large hail. Intense surface heating along the northwestern
rim of favorable warm-sector moisture, and near the weak frontal
segment, should reduce CINH enough by mid/late afternoon to support
an area of high-based convection over parts of southwestern/NE and
perhaps northwestern KS. Activity then should move southeastward
over a deep/well-mixed subcloud layer suitable for strong/damaging
gusts. Buoyancy and MLCINH each should increase with eastward
extent. Initial thunderstorms may access around 1000-2000 J/kg
MLCAPE, with not much less DCAPE below. Light but backed
near-surface winds will underlie around 25-35-kt 500-mb flow,
contributing to effective-shear magnitudes around 30-40 kt.
Outside the more-confident but weaker-buoyancy scenario over the
southwestern parts of the outlook area, confidence is low (and
guidance highly variable in any specific initiation/propagation
scenario in or near the frontal zone. That said, activity may
develop near the front across eastern NE and/or IA this evening, and
as late as 04Z-07Z tonight over eastern parts, west of Lake Michigan
and somewhat northeast of the surface warm front, with marginal
severe hail/wind potential.
...Upper Ohio Valley, north-central Appalachians...
Widely scattered to scattered thunderstorms are expected to develop
from midday through the afternoon over this area and move
predominantly eastward, offering sporadic damaging gusts. Isolated,
marginally severe convective gusts may occur in the most intense
cells.
Satellite and radar imagery indicates two low-amplitude but still
potentially influential perturbations:
1. Over northwestern PA and extreme western NY, with an MCV evident
south of BUF, forecast to move eastward astride the PA/NY line and
over southern New England today.
2. A convectively enhanced shortwave trough over the upper
Mississippi Valley, poorly depicted by most synoptic models and
CAMs, and forecast to reach southern ON, Lake Erie and OH by 00Z.
Combined, these features should help to maintain a sufficiently
tight height gradient to maintain strong-magnitude and somewhat
cyclonic mid/upper flow over the area. Development should be aided
by diurnal heating, orographic lift, and the residual outflow/
differential-heating boundary from clouds/convection related to the
leading perturbation/MCV. Moisture advection/transport from the
Ohio Valley region should combine with that heating to yield
1000-1500 J/kg peak/preconvective MLCAPE. Effective-shear
magnitudes around 30-35 kt will support organized multicells.
...UT...
Widely scattered high-based thunderstorms are possible this
afternoon across much of northern through south-central UT, with
activity over northern deserts into the Wasatch Range (and perhaps
slightly farther east) posing a threat for isolated strong-severe
gusts. Although lower-elevation surface dew points are commonly in
the 40s to low 50s at this time, those values should decrease to the
30s to low 40s amidst intense diurnal heating and mixing.
Nonetheless, that still supports around 300-800 J/kg of MLCAPE,
starting in the 500-600-mb layer, atop a dry-adiabatic/inverted-V
thermodynamic profile with large DCAPE. Though low-level winds and
shear will be weak, some increase in mid/upper flow is expected this
afternoon as the shortwave trough to the northwest and north
tightens the height gradient, leading to around 35-40-kt cloud-layer
shear.
..Edwards/Gleason.. 07/22/2022
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