SPC AC 280054
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0754 PM CDT Tue Jun 27 2017
Valid 280100Z - 281200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
CENTRAL AND EASTERN NEBRASKA INTO FAR SOUTHERN SOUTH DAKOTA...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE
ENHANCED RISK ACROSS PARTS OF THE CENTRAL AND NORTHERN PLAINS...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE
SLIGHT RISK AREA ACROSS PARTS OF THE NORTH-CENTRAL STATES...
Severe thunderstorms capable of large hail and damaging winds are
expected over much of Nebraska and South Dakota this evening, with
the potential for hailstones greater than 2 inches in diameter and
damaging wind gusts in excess of 65 knots.
...Central and Northern Plains/Mid Missouri Valley...
The latest water vapor imagery shows a shortwave ridge over eastern
parts of the Dakotas extending southward into the mid Missouri
Valley with a shortwave trough in the northern Rockies. At the
surface a cold front is moving eastward across the western Dakotas
and far western Nebraska. Ahead of the front, a corridor of
maximized low-level moisture is located across the central and
northern Plains. The RAP is analyzing moderate to strong instability
along this corridor with MLCAPE in the 2000 to 4000 J/kg range.
Thunderstorms are ongoing along the western edge of the stronger
instability in southwestern South Dakota and west-central Nebraska.
This activity should continue to grow upscale, moving eastward into
higher quality moisture and stronger instability. As the shortwave
trough moves into the region from the west and low-level flow
strengthens across the Plains, MCS development will be possible.
In addition to moderate instability, the WSR-88D VWP at North
Platte, NE shows a supercell wind profile with strong directional
shear in the lowest 2 km AGL and 30 to 35 kt of westerly flow in the
mid-levels. The 00Z sounding from North Platte also shows a very
steep mid-level lapse rate exceeding 8.0 C/km. This environment
should support supercells with large hail and the more dominant
storms may be capable of producing hailstones greater than 2 inches
in diameter. Supercells will be most likely over the next 2 to 3
hours but a transition to linear mode is expected to take place
during the mid to late evening. This should result in an increasing
wind damage threat especially if a linear MCS can organize across
the region. Due to the strong instabililty and steep low-level lapse
rates, any persistent bowing line of storms may be able to produce
damaging wind gusts exceeding 65 knots. The severe threat should
eventually reach the mid Missouri Valley by during the late evening
and overnight period. See MCD 1165 for more information on the South
Dakota part of the Enhanced risk area.
Isolated severe storms will be possible further to the south across
western Kansas and the Oklahoma Panhandle where a narrow corridor of
moderate instability is analyzed by the RAP. Farther west and north
across parts of the northern High Plains and North Dakota,
convective coverage should remain more isolated but a marginal
severe threat will be possible due to an adequate combination of
instability and deep-layer shear.
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