SPC 1730Z Day 2 Outlook
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1224 PM CDT Wed May 17 2023
Valid 181200Z - 191200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THURSDAY
AFTERNOON AND EVENING ACROSS PARTS OF THE SOUTHERN HIGH PLAINS...AND
PORTIONS OF THE UPPER MIDWEST...
...SUMMARY...
Strong thunderstorms may impact parts of the southern Great Plains
and the Upper Midwest on Thursday. Some of these may pose at least
some risk for severe weather.
...Synopsis...
Within the prevailing split flow across the Pacific, a broad and
deep mid-level low is forecast to evolve in association with strong
cyclogenesis to the southeast of the Aleutians during this period.
Downstream of this feature, weak mid-level ridging is forecast to
build inland of the U.S. and Canadian Pacific coast, while low
amplitude troughing lingers in a separate stream near/west of Baja
California.
Farther east, weak, broadly confluent mid-level flow will generally
prevail across much of the U.S. However, an initially vigorous
mid-level low may continue digging south of the central
Canadian/U.S. border, gradually elongating across the Upper Midwest
into middle Missouri Valley by late Thursday night. Downstream of
this feature low amplitude ridging is forecast to shift across and
east of the lower Great Lakes, in the wake of a vigorous impulse
accelerating east of the Canadian Maritimes.
At the same time, downstream of the Baja mid-level troughing, short
wave ridging may tend to build across the northern Mexican Plateau
and southern Great Plains into the lower Mississippi Valley.
However, this may be slowed by a smaller-scale perturbation of
subtropical eastern Pacific origins, which is forecast to gradually
progress around its northern periphery. Farther east, broad, weak
mid-level troughing is forecast to continue slowly migrating
eastward across the Southeast.
In the wake of the stronger lead northern branch perturbation, and
the weaker lead southern branch perturbation, boundary-layer
moisture across the southern Great Plains into surface troughing to
the lee of the southern Rockies will initially be seasonably modest.
Substantive moisture advection appears unlikely, but
evapotranspiration may contribute to moistening across this region
during the day, as well as ahead of a significant cold front
accompanying the northern branch low digging south of the
international border. A significant preceding cold front appears
likely to stall and weaken across parts of the southern Atlantic
coast into Tennessee Valley vicinity.
...Southern High Plains...
While models suggest that mid-level lapse rates will be initially
modest across much of the region, some gradual steepening with
lower/mid-tropospheric warming is forecast through the period.
During the day, it appears that the boundary-layer, becoming deeply
mixed with daytime heating, may become sufficiently unstable to
support scattered thunderstorm posing a risk for marginally severe
hail and wind.
Deep-layer wind fields and shear appear likely to remain generally
weak, but it is possible that a 30-40 kt west-southwesterly jet
streak in the 700-500 mb layer (accompanying the subtropical
perturbation) could enhance convection as it spreads across the
southern Texas Panhandle/South Plains into Red River vicinity
Thursday evening. An outflow boundary from prior convection, or at
least a zone of stronger differential surface heating across the
Texas Panhandle into southwestern Oklahoma/western North Texas, may
become a focus for stronger lower/mid tropospheric warm advection by
Thursday evening. This could support an isolated supercell
structure or two, and eventually an upscale growing cluster which
could be accompanied by strong surface gusts for a period late
Thursday evening, before forcing shifts to the cool side of the
boundary and convective intensities wane overnight.
...Upper Midwest...
The cold core of the approaching mid-level low/trough is forecast to
remain displaced to the northwest of the surface warm sector.
Coupled with the modest warm sector boundary-layer moisture, maximum
instability is forecast to remain weak and largely confined to a
narrow pre-frontal corridor. However, it appears that this may be
sufficient to support a developing line of storms by late Thursday
afternoon across parts of north central through southwest Wisconsin
and adjacent portions of Minnesota/Iowa, aided by favorable
large-scale forcing for ascent. This may coincide with sufficient
deep-layer shear beneath 40-50 kt southwesterly mid-level flow, to
support a few supercell structures posing at least some risk for
severe hail and wind gusts, before convection weakens after dark
while spreading southeastward.
..Kerr.. 05/17/2023
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