Valid 051300Z – 061200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER UPPER
PARTS OF THE MIDWEST…MISSISSIPPI VALLEY AND GREAT LAKES…
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
CENTRAL HIGH PLAINS…
…SUMMARY…
Organized severe-storm clusters will pose a threat mainly of severe
winds today, across upper parts of the Midwest, Mississippi Valley
and Great Lakes.
…Synopsis…
In mid/upper levels, the longstanding longwave pattern will continue
to feature eastern troughing and a western anticyclone anchored over
the southern Rockies, with ridging northward to the northern High
Plains. East of the mean ridge, a positively tilted shortwave
trough is evident in moisture-channel imagery from extreme southern
MB across northern/western ND. This perturbation is forecast to
move east-southeastward across the remainder of ND, northern MN and
Lake Superior through the period, crossing most of the Upper Great
Lakes region by 12Z.
At the surface, the 11Z analysis showed a slow-moving warm front
from central MN south-southeastward across IA to central/southern MO
and the eastern Ozarks. This boundary should shift diffusely
eastward/northeastward through the period into the upper Great Lakes
region. A cold front was drawn from southeastern MB across western
SD, beneath or slightly ahead of the 500-mb vorticity axis, and
parallel to it. The cold front was (and should remain mostly)
preceded by convection and convective outflows, located initially
across MN, ND and northeastern SD. The cold front should cross most
of Lake Superior, WI and northern/western IA overnight, preceded by
outflow, while trailing back across the central High Plains.
…Upper Midwest/upper Mississippi Valley/upper Great Lakes…
Scattered thunderstorms are ongoing over parts of northern MN, near
the low-level front. Large-scale lift (warm advection and DCVA) and
low/middle-level mass responses/shear associated with the
aforementioned mid/upper perturbation have been contributing to the
sporadic severe pulsing within this convection across ND overnight,
and it recently has become better-defined in radar and satellite
presentations. See severe thunderstorm watch 552 and related
mesoscale discussions for near-term guidance on this scenario.
Whether evolving from morning convection, forming anew on the front
and outflow boundaries, or some combination of both, one or more
quasi-linear complexes of convection should persist/develop through
the day and move southeastward through the outlook area. The
primary threat area has shifted northeastward from the previous
outlook across more of MN/WI, and a relative min in risk may occur
across portions of southern SD and central/eastern NE where
deep-layer forcing will be less than farther east, even with the
unstable prefrontal boundary layer. Additional convection may
develop still farther east across Upper MI along southern-shore
lake-breeze boundaries, perhaps merging with activity moving from
northern MN/WI into the area. Throughout the region, damaging
and/or severe wind will the main concern. Isolated large hail and a
brief tornado cannot be ruled out.
The main process at work should involve a combination of cold-pool
amalgamation/reinforcement by convection, with related forward
propagation and further lift along the leading edge. Lateral
expansion of an MCS, through backbuilding along the outflow/front
and subsequent cold-pool development, also is possible. The
preconvective warm sector is expected to destabilize through the
afternoon from a combination of direct/diabatic heating at the
surface, and low-level theta-e advection. Steep midlevel lapse
rates and favorable low-level moisture, with surface dew points
mid/upper 60s F, should contribute to a corridor of 1500-2500 J/kg
MLCAPE ahead of th front/outflows, from the southern MN/northern IA
region toward western Upper MI. Convection may outpace the
strongest deep shear, though 30-35 kt effective-shear magnitudes are
possible, despite weak near-surface winds.
…Central Plains…
Isolated to widely scattered thunderstorms cannot be ruled out over
central/eastern NE near the front where mid/upper-level support is
reduced. However, severe thunderstorms now appear most probable
late afternoon into early tonight from northeastern WY across parts
of southwestern SD into northwestern NE. Activity is forecast form
over terrain of northern WY late this afternoon in a regime of
upslope flow and diabatically minimized MLCINH. Severe hail and
gusts will be the main concern, though considerable uncertainty
remains regarding storm longevity and potential support from
self-sustaining cold-pool processes. Convection should follow a
moisture/instability axis southeastward at least into parts of the
Black Hills/Sandhills area this evening, with forced ascent along
the cold pool.
..Edwards/Marsh.. 08/05/2019
$$