Day 1 Severe Weather Outlook

Valid 091200Z – 101200Z


Thunderstorms capable of producing large hail, and strong/gusty
winds, and possibly a tornado will be possible across parts of the
northern Plains and upper Mississippi Valley Monday afternoon and
evening. A few strong/severe storms may also occur over the
southern Virginia/northeast North Carolina area.

A short-wave trough is forecast to shift northeastward across the
central and northern Plains, toward the upper Mississippi Valley
through the day, ahead of a much larger trough gradually evolving
southeastward across the West. Meanwhile, a weak short-wave trough
will move quickly east-southeastward from the upper Ohio Valley
across the Mid Atlantic region through the first half of the period.
Otherwise, short-wave troughing will prevail across the southeastern
quarter of the country.

At the surface, a baroclinic zone extending from Montana
southeastward across the mid Missouri Valley is forecast to shift
gradually northeastward across the northern Plains/upper Mississippi
Valley/upper Great Lakes region through the period. Meanwhile, a
weak/remnant front is forecast to linger from the northeast North
Carolina area west-southwestward across the Southeast states, south
of Northeast U.S. high pressure.

…Parts of the central and northern Plains into the Upper
Scattered to isolated thunderstorms are forecast to be ongoing
across the risk area at the start of the period, in conjunction with
the short-wave trough advancing northeastward across the
central/northern Plains. The effects of this convection complicate
the forecast — particularly as it pertains to areas where storm
reintensification and/or redevelopment may be most concentrated
during the afternoon.

Overall, a moist boundary layer will remain in place near and south
of the warm front, such that where heating can occur, moderate
instability will evolve. As such, storm development is expected,
and at this time it appears will be most concentrated over the
Iowa/southern Minnesota vicinity near where the warm front is
expected to reside during the afternoon.

Other, more isolated storms may develop southwestward across
Nebraska and into western Kansas, though with this area in the wake
of the short-wave troughing, subsidence should limit storm coverage
across this region. Still, with ample instability and modest shear,
a stronger storm or two may produce large hail or gusty winds during
the afternoon.

Still, greater risk will remain farther north, nearer the
northeastward-advancing warm front and ahead of the mid-level
feature. With stronger flow aloft across this region, organized
storms are expected — though mode remains uncertain. Any isolated
cells which can develop — particularly near the warm front — would
be capable of producing large hail and locally damaging winds, and
possibly a tornado. However, a more clustered/loosely linear storm
mode appears likely to evolve, with attendant risks for mainly
damaging winds and hail.

As a south-southwesterly low-level jet strengthens into the evening,
elevated convection should increase in coverage into the Great Lakes
area, where some risk for hail may linger well after dark.

…Southern Virginia/northeast North Carolina…
As a short-wave trough moves quickly across the central Appalachians
and into the Mid Atlantic region through the afternoon, a few
thunderstorms are forecast to develop — possibly over the higher
terrain, as well as in the vicinity of the surface boundary progged
to be lingering over the northeast North Carolina area. An amply
moist/unstable environment will support the storms, and with
moderate mid-level west-northwesterly flow shifting across the
region, a few stronger storms may evolve. As such, local risk for
damaging winds — and possibly marginal hail — remains apparent,
prior to the onset of diurnal cooling.

..Goss/Elliott.. 09/09/2019


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