Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0751 AM CDT Wed May 26 2021
Valid 261300Z - 271200Z
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS
NORTHWEST/NORTH-CENTRAL KS AND SOUTHWEST/SOUTH-CENTRAL NE...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS NORTHEAST
STATES...
...SUMMARY...
A regional outbreak of severe thunderstorms is expected across the
central Great Plains this afternoon through tonight. The greatest
tornado threat, including the potential for strong tornadoes, is
expected between 5 to 11 PM CDT. Intense damaging winds and very
large hail are expected as well.
...Central Great Plains...
A shortwave trough over the northern Rockies should reach the
Dakotas by 12Z tomorrow. An attendant belt of enhanced mid-level
southwesterlies should overspread WY to the Dakotas this afternoon.
Meanwhile, a confluent belt of enhanced southwesterlies in the
southern stream will become centered from AZ/NM to the central High
Plains. A surface cyclone will initially deepen over southeast WY as
a warm front advances north across KS and NE. Early afternoon
thunderstorm development is expected in advance of the shortwave
trough across north-central WY and far southeast MT, with additional
development a couple hours later farther east-southeast in eastern
WY near the surface low. An initially narrow surface-based buoyancy
plume should broaden on continued low-level moist advection from the
southeast, yielding development and intensification of supercells
across the WY/SD/NE border area by late afternoon.
Farther south, the most likely scenario is for discrete supercells
developing across parts of northeast CO and at the apex of the
thermal ridge along the dryline near the southeast CO/west-central
KS border by late afternoon. This activity should impinge on a plume
of large buoyancy with MLCAPE of 2500-3500 J/kg. In addition, a
strengthening low-level jet between 00-03Z renders substantial
concern for potential long-track supercells capable of significant
tornadoes centered on the northwest KS vicinity this evening. With
time, numerous supercell structures will likely grow upscale as they
mature along/south of the warm frontal zone, aided by the strong
low-level jet. An intense MCS may evolve by late evening with bowing
structures, yielded an increasing threat for significant severe wind
gusts across portions of KS and southern NE into the overnight. This
wind threat may spread as far east-southeast as the Lower MO Valley
by 12Z tomorrow.
Variance in CAM guidance appears greater-than-normal regarding the
aforementioned potential convective evolution. The 00Z HRW-NSSL and
HRW-ARW both suggest that surface-based convection may develop by
early to mid-afternoon across parts of western to central KS,
overturning the air mass prior to substantial strengthening of the
low-level jet. This would limit the likelihood of significant
tornado/wind potential and coverage of all severe probabilities as
far north as indicated by the current forecast. This scenario may be
unlikely in the presence of mid-level ridging but appears plausible
given persistent low-level theta-e advection and weak MLCIN.
...Southern High Plains...
Isolated severe thunderstorms will develop south along the dryline
through west TX where strong diabatic heating will once again allow
surface parcels to reach their convective temperatures by late
afternoon. Modest southwesterly flow aloft will support a few
supercells, and large hail/severe wind are the greatest threats.
...Northeast...
A low-amplitude shortwave trough over the Upper Great Lakes should
quickly progress towards the Saint Lawrence Valley by tonight. While
the bulk of strong 500-mb west-southwesterlies will remain prevalent
across Ontario and Quebec, moderately enhanced 700-mb flow will
persist across much of the Northeast States through this afternoon.
Scattered thunderstorms will develop this afternoon ahead of the
warm conveyor from OH across the central to northern Appalachians as
robust boundary-layer heating occurs. Weak mid-level lapse rates and
unidirectional wind profiles will largely support scattered damaging
winds as the primary hazard, with multicell clusters spreading
toward the coastal plain.
..Grams/Mosier.. 05/26/2021
