SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0750 AM CDT Wed Jul 13 2022
Valid 131300Z - 141200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS WESTERN AND
CENTRAL MONTANA...AS WELL AS PARTS OF MISSISSIPPI AND ALABAMA...
...SUMMARY...
Thunderstorms capable of producing severe gusts and isolated hail
will be most likely across western and central Montana, as well as
parts of Mississippi and Alabama.
...Synopsis...
A rather stagnant large-scale pattern will persist over the CONUS,
characterized by mean troughing in the east, and ridging extending
north-northeastward from an anchoring anticyclone over the Four
Corners States. North of the high, some height falls are likely
across the northern Rockies and High Plains, as weak perturbations
eject northeastward through/from southwest flow ahead of a Pacific
trough. downstream, two closely spaced shortwave troughs are
apparent in moisture-channel imagery over:
1. Parts of Lower MI, and forecast to pivot across southern ON and
OH by 18Z, then over the northern Appalachians this evening;
2. ON, between Lake Superior and James Bay, and forecast to move
across eastern Lower MI, Lake Huron and adjoining areas of
southwestern ON by 00Z, then pivot eastward to eastern ON and Lake
Ontario overnight.
The 11Z surface chart showed a slow-moving cold to quasistationary
front near a line from ACK-PHL-LYH-CSV-BNA-MEM-DAL-MAF-TCS. This
boundary should drift southeastward over the northern Tidewater
region today, while remaining quasistationary elsewhere. A weak
cold front -- related to the mid/upper troughing over the Upper
Great Lakes region -- was drawn from southwestern QC across Lake
Huron, central Lower MI, to near the IL/WI border. This boundary
should move slowly southeastward through the period, reaching
western portions of NY/PA, and southern parts of OH/IN/IL, by 12Z
tomorrow.
...MT...
Scattered thunderstorms are expected to develop this afternoon over
higher terrain of western/southwestern MT, where MLCINH will be
removed quickest by surface diabatic heating. This convection
should move northeastward across the outlook area the remainder of
the afternoon into early evening, offering occasional severe gusts
(with isolated gusts near 75 mph possible).
Morning dewpoints in the upper 40s to mid 50s over lower elevations
will lessen through the afternoon as heating/mixing occurs. This
should result in a very deep boundary layer characterized by nearly
dry-adiabatic lapse rates, and dew points in the 30s to mid 40s --
still sufficient to support thunderstorms. In this profile,
activity will be high-based, with substantial DCAPE (over 1000 J/kg)
present over a broad area. The convective environment will be
supported further by large-scale ascent preceding one of the subtle/
southwest-flow shortwave troughs, currently apparent over the
northern NV/southern ID region. While isolated severe gusts may
occur anywhere in a large swath of MT and northwestern WY, the
greatest potential for clustering and cold-pool organization appears
to be across the "slight" area.
...Southeast...
Widely scattered to scattered thunderstorms -- some in locally dense
multicellular clusters -- are possible this afternoon along/ahead of
the cold front. Damaging to severe gusts should be the main
concern. Initiating foci will include the front and prefrontal
outflow/differential-heating boundaries, including an outflow
boundary from prior/overnight convection, now evident over northern
MS and northern/central AL. With one substantial mesoscale focus
already readily apparent diagnostically, and a subtle boost to
large-scale support (below), a relatively maximized area of
convective-wind probabilities is evident over part of MS/AL that has
improved in predictability since the previous outlook.
The warm sector will remain characterized by rich moisture, with
surface dew points in the upper 60s to mid 70s F, associated with
areas of PW in the 2-2.25-inch range. A strip of sustained diurnal
heating is likely -- between areas of ambient cloud cover, parallel
to and ahead of the front -- which will overcome modest MLCINH and
boost MLCAPE into the 1500-2500 J/kg range. Hail probabilities also
have been added in and near the "slight" wind-threat area for today.
Despite the weak low/level and deep shear, forecast soundings
across portions of the area indicate a deep growth zone containing
with abundant moisture and buoyancy, along with several 1-1.75-inch
historic hail analogs in multiple locations' worth of sounding
samples across the area. A little extra large-scale support for
destabilization aloft also is evident on the southern rim of the
right-entrance region of a cyclonically curved, 250-mb jet. An
approximately southward-moving convective band is possible, yielding
the most-concentrated severe potential.
...Northern OH/IN/southeastern MI...
Isolated to scattered thunderstorms are expected to develop by late
this afternoon, as the southern part of a mesoscale area of strong
DCVA/ascent aloft moves over the region, ahead of the northern
shortwave trough. This will steepen midlevel lapse rates that will
have been poor for much of the afternoon, following the passage of
the previous perturbation. As that occurs, marginal low-level
moisture and associated buoyancy are expected, with MLCAPE generally
in the 300-800 J/kg range (locally/briefly higher). A few
downdrafts penetrating a well-mixed subcloud layer may produce
damaging winds. The potential should be confined to just a few
hours' time window around late-afternoon/early evening, before too
much nocturnal near-surface stabilization occurs. The modest
moisture and late timing of the strongest forcing for ascent
preclude a better-organized severe threat.
..Edwards/Leitman.. 07/13/2022
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