SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0747 AM CDT Tue Jul 05 2022
Valid 051300Z - 061200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
OF CENTRAL/SOUTHERN SOUTH DAKOTA...NORTHEASTERN
NEBRASKA...NORTHWESTERN IOWA...AND SOUTHWESTERN MINNESOTA...
...SUMMARY...
Clusters of severe thunderstorms are forecast this afternoon and
evening in a broad area from parts of the northern/central Plains
into the Midwest, Ohio Valley, and Mid-Atlantic. Severe gusts and
large hail are the main threats. The most intense gusts and very
large hail are expected across parts of the northern Plains.
...Synopsis...
The mid/upper-level pattern will continue to be dominated by a Rex
block over northwestern Canada, with the cyclonic component over the
Pacific west of BC, WA and OR. As a shortwave trough now over
extreme southern BC ejects northeastward, ridging will build through
the northern Rockies, connecting the longstanding anticyclones over
the Mid-South and northwestern Canada. Downstream, westerly to
west-northwest flow will prevail in a belt from the northern Plains
to the Mid-Atlantic. In that flow belt, several embedded shortwaves
and vorticity maxima are expected, including those related to
convection now over the northern Plains and OH. The former will
shift southeastward across the Dakotas to eastern IA through the
period. The OH activity, and associated MCV, are in the southern
part of a longer shortwave trough that extends over Lake Huron and
adjoining parts of southern/central ON. That perturbation should
move east-southeastward across the Mid-Atlantic today.
The surface analysis at 11Z showed a wavy stationary to warm front
near Cape Cod, northern NJ, central PA, to a low over northeastern
Lower MI. A slow-moving cold to stationary front was drawn from
there across central lake Michigan, southern WI, northern IA,
southeastern SD, and north-central NE, to another low between
AKO-GLD. Outflow boundaries shunted the effective baroclinic zone
farther southward over parts of the middle/upper Ohio Valley region
and Upper Great Lakes. The front should move slowly southeastward
through Lower MI, the Lower Great Lakes, and much of NY through the
period, while moving little and being diffused further by more
strongly baroclinic convective processes over the Upper Midwest and
central Plains regions.
...Northern Plains to IA...
Widely scattered to scattered thunderstorms -- some severe -- are
moving roughly eastward over parts of eastern MT and the western
Dakotas. See SPC severe thunderstorm watch 435 and related
mesoscale discussions for near-term coverage of the ongoing
activity.
As the associated perturbation and area of large-scale ascent pivots
southeastward today near the low-level baroclinic zone -- additional
development is expected mainly over southern SD. This activity will
arise from strong diurnal destabilization, weakening MLCINH,
favorable low-level moisture, steep low/middle-level lapse rates,
and both orographic (in the Black Hills vicinity) and deep-layer
lift. Specifics of the convective evolution remain somewhat
uncertain, though a strong and reasonable guidance consensus remains
for relatively maximized concentration of convection in and near the
"enhanced" area. The immediately preconvective environment this
afternoon will contain a parameter space suitable for both organized
multicells and a few supercells prior to upscale evolution.
Forecast soundings along and northeast of the front show strong
veering of winds with height, offsetting modest midlevel flow enough
to contribute to effective-shear magnitudes of 40-50 kt. Near the
low-level moist axis, surface dew points in the upper 60s to low 70s
F will contribute to MLCAPE in the 3000-4000 J/kg range.
Farther northwest, additional convective development is likely
across parts of southern/central and eastern MT, as warm/moist
advection and diurnal heating contribute to recovery behind the
current eastern MT complex and near its residual outflow boundary.
The environment will become favorable again for supercells and
bowing convective clusters, with residual moisture near the theta-e
axis, diabatic heating, and steep low/middle-level lapse rates
contributing to a corridor of 1000-2000 J/kg MLCAPE, superimposed by
45-55-kt effective-shear magnitudes. The width of this corridor,
and coverage of related severe convection, each is in doubt, with
surface dew points likely to be less than the previous day, in a
narrower zone of relatively maximized but convectively processed
moisture. As a result, while at least a few severe thunderstorms
are still expected, perhaps even a supercell or two with significant
hail/gusts possible, a 30% unconditional wind coverage appears less
justified than in previous outlooks.
...Mid-Atlantic to Ohio Valley...
A currently non-severe, quasi-linear MCS is moving east-
southeastward to southeastward across portions of OH and
northwestern PA. The associated clouds/precip and UVV field should
impinge on a diurnally destabilizing boundary layer in the
Mid-Atlantic from later this morning into afternoon. As that
occurs, strengthening and/or additional development is expected,
with potential for damaging/isolated severe gusts and isolated large
hail to result. Forecast soundings in the preconvective environment
indicate weak low-level flow/hodographs, but enough deep shear
(effective-shear magnitudes generally 35-40 kt) for a mix of
organized storm modes. Diabatic heating and surface dew points in
the 60s F at lower elevations will offset modest midlevel lapse
rates enough to yield:
1. A well-mixed boundary layer supporting damaging to locally
severe gusts, as well as maintenance of any supercell-generated
severe hail to the surface, and
2. MLCAPE reaching 500-1000 J/kg in a corridor east of, and
parallel to, the Blue Ridge into southeastern PA.
Already modest buoyancy should decrease considerably farther
northeast, while shear and overall organization diminish southward.
However, strong heating and steeper lapse rates aloft will spread
across parts of the Ohio Valley region and into the adjoining
Appalachians this afternoon, persisting into early evening. This
should support additional development near the front and residual
outflow boundaries, some of which may become severe, with damaging
gusts likely and isolated large hail possible.
..Edwards/Goss.. 07/05/2022
Read more
