Day 1 Severe Weather Outlook

Valid 041200Z – 051200Z


Scattered severe thunderstorms may impact the northern High Plains,
much of the Dakotas and northern Minnesota this afternoon and
evening, posing a risk for potentially damaging wind gusts and large

Downstream of a blocking mid/upper high centered near/south of the
Gulf of Alaska, models indicate that the westerlies will trend
northwesterly during this period, across the Yukon Territory into
the central Canadian prairie provinces. Within this regime, a
significant short wave trough, currently progressing through the
Canadian prairies, appears likely to continue eastward and
southeastward, across Manitoba and adjacent portions of the
Canadian/U.S. border area into northwestern Ontario and portions of
the upper Great Lakes region by 12Z Monday. Stronger mid-level
height falls may remain north of the international border through
much of the day today, but at least some suppression of mid-level
ridging is forecast into areas south of the border, across parts of
the Upper Midwest and northern U.S. Plains this evening into the
overnight hours.

Otherwise, subtropical ridging appears likely to remain prominent
inland of the Pacific coast, centered over the Four Corners states,
but encompassing much of the Intermountain West, Rockies and
central/southern Plains. Farther east, weak mid-level troughing
will prevail within the subtropical westerlies, east of the
mid/lower Mississippi Valley into the mid and southern Atlantic
Coast region. A gradual eastward progression of the trough is
likely, but it is forecast to lag to the south and west of troughing
within the mid-latitude westerlies progressing across and east of
the Canadian Maritimes and New England.

Low-level cooling and drying associated with the lead wave in the
mid-latitude westerlies is expected to overspread much of the
Northeast today. A few thunderstorms are possible this afternoon
along or ahead of the cold front, across parts of the lower Great
Lakes region into northern Mid Atlantic and southern New England.
However, seasonably high moisture content probably will remain
confined to the Gulf and south Atlantic Coast states, with richer
tropical moisture also in a narrow plume across southern portions of
the Florida peninsula into the western Atlantic, focused on the
western periphery of ridging over the subtropical Atlantic. This
moisture is expected to contribute to more substantive potential for
thunderstorm activity, which will be mostly diurnal in nature.

Around the western periphery of the Western subtropical high, models
suggest that there may some eastward progression of drying across
the Great Basin, but remnant monsoonal moisture may still support
fairly widespread thunderstorm activity this afternoon and evening
across the higher terrain of the eastern Great Basin into Rockies.

Lee surface troughing across the northern Plains, and a cold front
near the central Canadian/U.S. border vicinity, are expected to
provide focus for additional thunderstorms today into tonight. This
activity is expected to pose the most substantive risk for severe
wind and hail.

…Northern Plains…
There is expected to be sufficient boundary layer moisture to
support CAPE in excess of 2000 J/kg, beneath fairly steep lapse
rates associated with elevated mixed-layer air. Models suggest that
this will become focused within a corridor of stronger daytime
heating, ahead of the stalling or slow moving surface front near the
Canadian/U.S. border, and perhaps in the vicinity of a thermal low
developing near the Black Hills region. The Black Hills region
remains a little more unclear, with model output indicating
potential for early convective development, possibly aided by
forcing for ascent associated with a perturbation migrating around
the northern periphery of the subtropical ridge.

Otherwise, mid/upper forcing for ascent south of the international
border prior to this evening remains unclear, but strengthening of
westerly mid/upper flow (including 30-40 kt near or above 500 mb)
probably will contribute to sufficient deep-layer shear for
organized convection, including isolated supercells, capable of
producing severe hail and strong surface gusts. Mid-level
inhibition may weaken sufficiently near/after peak daytime heating
to support the initiation of at least scattered thunderstorm
activity, with potential for at least some upscale growth through
this evening, before boundary-layer instability wanes with the loss
of daytime heating.

..Kerr/Nausler.. 08/04/2019


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