SPC 1730Z Day 2 Outlook
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1227 PM CDT Mon Aug 01 2022
Valid 021200Z - 031200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
UPPER MIDWEST NEAR THE INTERNATIONAL BORDER AND LAKE SUPERIOR...
...SUMMARY...
Large hail and damaging to severe thunderstorm gusts are most likely
Tuesday evening and night over parts of the Upper Midwest near the
international border and Lake Superior.
...Synopsis...
The large-scale pattern will continue to be characterized by an
anticyclone over AZ/NM and vicinity, with ridging eastward over the
southern Plains and Gulf Coast States. Mean troughing will persist
over the Lower Great Lakes and central Appalachians, south of a
strong synoptic cyclone moving erratically over the northern Hudson
Bay region. However, that troughing should deamplify as heights
fall in the upstream west-northwest flow belt, across the
northwestern/north-central CONUS and adjoining Canada. Those height
falls will be related largely to two strong shortwave troughs, the
trailing one forecast to move from AK into BC.
The leading shortwave trough, and most relevant for this forecast,
is apparent in moisture-channel imagery from northwestern NC
southward off the coast of Vancouver Island. This perturbation is
forecast to move east-southeastward to the northern Rockies by the
start of the period (02/12Z), penetrate a low-amplitude mean ridge
during the day, followed by entering the confluent-flow belt south
of the Hudson Bay cyclone. By 03/12Z, the trough should reach the
southern part of the MB/ON provincial border, southward over western
MN. There it should phase with an initially separate/weaker
perturbation currently evident over parts of central CA and southern
NV, that will be entrained into the northern-stream westerlies early
day 2.
At the surface, a cold front -- initially located over Lake
Michigan, northern IL and southern IA -- should move eastward into
the Lower Great Lakes and Northeast early day-2. The western
segment of this frontal zone should become a warm front by the start
of the period, extending from central IL northwestward to a frontal-
wave low over eastern/southeastern ND, then northwestward to a
primary low over southern SK. By 03/00Z, the latter low should move
eastward to southern MB, with warm front over northwestern MN,
southeastern MN/western WI, and central IL. A cold front will
extend from the low across parts of northeastern ND, northwestern SD
and central/southern WY. By 12Z, the warm front should reach the
western U.P., central Lake Michigan, southwestern Lower MI, near the
IN/OH line, to central KY.
...Upper Midwest...
During the afternoon, a corridor of strong buoyancy will develop in
the warm sector over southern/eastern MN, with strong diurnal
heating and rich low-level moisture fostering MLCAPE in the
3000-4000 J/kg range. Still-favorable but lower values will extend
northwestward toward the main surface low, with 1500-2500 J/kg
possible over northeastern ND. However, strong MLCINH and nebulous
deep-layer forcing cast great uncertainty on diurnal development
near the warm front, over the eastern MN/WI area.
Thunderstorms may develop during the afternoon and evening over
northeastern ND and northwestern MN, as well as the Boundary Waters,
MN Arrowhead and western Lake Superior region. Activity northeast
of the warm front will be elevated, but still pose a threat for
severe hail and isolated damaging gusts. Development over the warm
sector is more uncertain in timing/coverage due to strong
EML-related capping, but the potential for at least isolated to
widely scattered surface-based thunderstorms will increase as the
mid/upper trough approaches and associated mass response strengthens
overnight. That mass response also will include strengthening of
the associated zone of low-level warm advection, moisture transport
and isentropic lift to LFC over the warm-frontal zone. The entire
convective regime is expected to shift eastward or east-
southeastward across the lake, and perhaps northern/eastern parts of
the Upper Peninsula, through the period.
Forecast soundings early in the episode suggest very steep low/
middle level lapse rates above the warm-frontal surface, supporting
MLCAPE and elevated MUCAPE increasing to the 2500-3500 J/kg range,
amidst effective-shear magnitudes of 40-50 kt (perhaps higher).
This will support supercell and large-hail potential, especially
early in the convective cycle and along the inflow (south to
southwest sides) of any resulting complex, where relatively
unimpeded access persists to the 45-55-kt nocturnal LLJ. The track
of this convective regime remains somewhat uncertain, but it should
outpace the advance of the warm front and become more elevated with
time. Unconditional probabilities are being extended eastward over
more of the U.P. to accommodate the threat from this activity.
...Northeastern Plains...
A conditional threat exists for strong-severe gusts with
thunderstorms that may form during the evening and overnight, ahead
of the cold front over eastern parts of SD and northeastern NE.
While moisture/buoyancy will be less than areas farther northeast
near the warm front, increasing large-scale ascent and cooling aloft
will occur ahead of the southern/phased branch of the shortwave
trough. A residual very warm and deeply mixed boundary layer from
prior diurnal heating will foster the potential for strong-severe
gusts with any convection that can develop; however, storm coverage
appears too uncertain for more than marginal unconditional
probabilities at this time.
...Lower Ohio Valley to Ozarks...
Scattered showers and thunderstorms may be ongoing over northern
and/or eastern parts of this region from prior/overnight MCS
activity discussed in more detail in the day-1 outlook. This
convection should leave outflow/differential-heating boundaries to
focus additional, diurnal development. Though deep-layer shear will
be weak, strong diabatic heating away from any persistent cloud
cover, combined with very rich low-level moisture (dewpoints
commonly in the 70s F) will underlie moderately steep midlevel lapse
rates of around 6.5 deg C/km, within a deep troposphere. The result
should be areas of 3000-4000 J/kg MLCAPE. MLCINH will be weak east
of the EML, supporting diurnal development and perhaps upscale
growth into one or more wind-producing clusters. Any more-focused
area of potential within the broader outlook will be very dependent
on mesoscale processes yet to evolve, and this outlook area may
shift or change shape substantially as time gets closer.
..Edwards.. 08/01/2022
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