Day 1 Severe Weather Outlook

Valid 061200Z – 071200Z


Scattered severe thunderstorms are possible across the northern
Plains late this afternoon and evening, particularly across parts of
the central Dakotas, where a few storms may become capable of
producing very large hail and strong wind gusts.

Models indicate that much of the Intermountain West and Rockies,
into the southern Plains, will remain under the influence of
mid/upper subtropical ridging, centered over the Four Corners
states. While lingering moisture beneath the ridge may once again
support scattered diurnal thunderstorm activity across the Mogollon
Rim vicinity, Wasatch, and southern Rockies, weak perturbations
progressing around the periphery of the ridge may contribute to
widely scattered to scattered thunderstorms across the northern
intermountain region and northern Rockies into portions of the
northern and central Plains.

Downstream of the ridging, the remnants of initial weak mid-level
troughing are forecast to accelerate northeast of the Atlantic
Seaboard. A preceding plume of tropical moisture will continue
advecting through the western Atlantic, although it may still trail
back into the Florida Peninsula, where scattered diurnal
thunderstorm activity appears likely. Despite low/mid-level drying
in the wake of the troughing, weak residual low-level moisture may
remain sufficient to support isolated to widely scattered
thunderstorm activity across the Gulf and mid/south Atlantic coast
region, with daytime heating, in the presence of relatively weak

Meanwhile, it appears that larger-scale troughing east of the
Mississippi Valley into the Atlantic Seaboard may be reinforced by a
digging/amplifying perturbation across parts of the middle
Mississippi into lower Ohio Valley, today through tonight. Forcing
for ascent associated with this feature may contribute to
thunderstorm development along/south of an initial southward
advancing cold front, which is expected to reach the lower Great
Lakes and middle Mississippi Valley by 12Z this morning, before
continue into southern New England/northern Mid Atlantic coastal
areas and the Ohio Valley by daybreak Wednesday.

While blocking remains prominent within the mid/upper flow over the
mid-latitude eastern Pacific into the Gulf of Alaska vicinity, a
long fetch of northwesterly flow is expected to persist across
Alaska and the Canadian Rockies, into the central Canadian/U.S.
border vicinity. Within this regime, it does appear that another
significant short wave trough will dig across the central Canadian
provinces, with a deepening embedded closed low. Stronger forcing
for ascent likely will remain north of the Canadian/U.S. border
through much of this period, but an associated surface cold front is
forecast to advance south of the international border and may
provide a focus for increasing thunderstorm development tonight.

…Northern Plains…
Moderate to strong northwesterly mid/upper flow (including 30-50 kt
at 500 mb) likely will contribute to strong deep layer shear ahead
of the cold front advancing across and south of the international
border, and within pre-frontal troughing across the central Dakotas
by late afternoon. Primary uncertainties with regard to severe
weather potential concern forcing for convective development, and to
a lesser extent boundary-layer destabilization in the wake of a
preceding surface front.

It does appear that there will be a narrow corridor of modest
moisture return on southerly low-level flow, east of the surface
troughing across the Dakotas during the day, and northeastward ahead
of the cold front into northern Minnesota by tonight. This is
expected to contribute to CAPE at least on the order of 1000-2000
J/kg, with daytime heating, with some guidance indicating even
greater destabilization is possible. Even so, most guidance
indicates that mid/upper support for the initiation of late
afternoon/evening thunderstorm activity will likely be associated
with a subtle mid-level impulse rounding the periphery of the
subtropical ridge. The extent to which forcing associated with such
a feature can support convective development remains unclear.

Based on a consensus of model output, convective initiation seems
most probable within pre-frontal troughing, perhaps near a
developing thermal low, over parts of southwestern into south
central North Dakota by late this afternoon, before tending to
propagate southeastward and southward through portions of
central/eastern South Dakota this evening. Given wind profiles with
generally small low-level hodographs, but strong deep-layer shear, a
few supercells are possible, which may pose a risk for large to very
large hail and strong surface gusts, before convection wanes with
the loss of heating this evening.

With stronger destabilization, there may be at least some potential
for the evolution of an upscale growing, organized convective system
and southward development into Nebraska this evening/overnight,
accompanied by severe wind/hail potential. However, at this point,
probabilities for this still appear low.

…Mid Mississippi/Ohio Valley into lower Great Lakes…
Wind fields and shear are expected to be generally weak, but
moderate boundary-layer destabilization along/ahead of the lead
southward advancing front may contribute to considerable
thunderstorm development this afternoon and evening, some of which
may pose a risk for potentially damaging wind gusts.

..Kerr/Dean.. 08/06/2019


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