Valid 051200Z – 061200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS BY MIDDAY
ACROSS PARTS OF THE UPPER MIDWEST…SOUTHWESTWARD INTO PORTIONS OF
THE MIDDLE MISSOURI VALLEY BY THIS EVENING…
…SUMMARY…
One or two organized clusters of thunderstorms may develop across
parts of the Upper Midwest, southwestward into the middle Missouri
Valley, posing a risk for swaths of potentially damaging wind gusts
and hail today through tonight.
…Synopsis…
Models indicate little general change to the large-scale pattern
through this period.
Subtropical ridging, centered over the Four Corners states, appears
likely to remain prominent across much of the Intermountain West,
Rockies and central/southern Plains. In the wake of perturbations
migrating around the western through northern periphery of this
circulation, there may be further drying across parts of the Great
Basin into the Rockies, but remnant monsoonal moisture is still
expected to support at least scattered thunderstorm activity across
the Mogollon Rim vicinity, the higher terrain of southeastern
Arizona, the Wasatch, and much of the Rockies.
Downstream, a gradual progression of troughing east of the mid/lower
Mississippi Valley, toward the mid/south Atlantic Coast, is expected
to continue. Somewhat cooler air aloft associated with this feature
probably will again support considerable diurnal thunderstorm
activity, as it overspreads lingering seasonably high moisture
content across much of the Southeast.
At the same time, in higher latitudes, blocking is forecast to
remain prominent within the westerlies, across much of the eastern
Pacific into Gulf of Alaska, with a long fetch of northwesterly
mid/upper flow downstream, from Alaska and the Yukon Territory
through the central Canadian/U.S. border vicinity. A significant
leading short wave trough within this regime is forecast to turn
eastward across northern Ontario and the Upper Great Lakes region
today through tonight, while another digs southeast of the Canadian
Northwest Territories.
A somewhat more subtle perturbation trailing the lead wave is
currently contributing substantive (30 meter) 500 mb height falls
near the North Dakota/Minnesota international border area. This may
develop southeastward through much of North Dakota into the Upper
Midwest by daybreak. Coupled with another impulse digging around
the northeastern periphery of the subtropical ridge, mid-level
ridging may become suppressed as far south/west as the mid to lower
Missouri Valley by late tonight. Associated forcing for ascent may
contribute to considerable thunderstorm development, ahead of a cold
front advancing southeast of the international border through much
of the Great Lakes region, middle Mississippi Valley and central
Plains by 12Z Tuesday. This activity appears likely to be
accompanied by the primary risk for severe weather.
…North central Plains/Mid Missouri Valley/Upper Midwest…
Mid/upper forcing for ascent is contributing to increasing
thunderstorm activity across North Dakota, which may continue to
grow upscale and organize while developing southeastward into
stronger instability across southern/eastern North Dakota through
daybreak. Due to sizable spread in model output, the extent of
continuing severe weather potential near and beyond 12Z this morning
remains unclear, as activity progresses toward the Upper Midwest,
beneath sheared northwesterly mid/upper flow.
Regardless of whether activity maintains intensity or weakens,
models generally indicate that a moistening boundary layer ahead of
the cold pool will become characterized by moderate to large CAPE
(2000-3000 J/kg) by early this afternoon across parts of the Upper
Midwest into the middle Missouri Valley. Aided by mid/upper
support, moderate to strong shear and inflow of increasingly
unstable air, substantive intensification of convection appears
possible across parts of southeastern Minnesota into Wisconsin by
midday. Thereafter, a southwestward propagation or redevelopment of
vigorous and upscale growing convection into the mid Missouri Valley
is expected through this afternoon and evening. Although
northwesterly deep-layer ambient mean wind fields may be modest to
weak in strength (around or less than 20 kt), thermodynamic profiles
appear favorable for the development of strong cold pools, which
will pose a risk for potentially damaging wind gusts while tending
to surge southeastward and southward.
..Kerr/Karstens.. 08/05/2019
$$