Day 1 Severe Weather Outlook

Valid 021200Z – 031200Z


Severe thunderstorms are expected over parts of the northern Plains
and upper Mississippi Valley this afternoon through early Tuesday
morning. Damaging winds and large hail are likely. Isolated severe
storms are possible across parts of southern New England and
extending southward into northern Virginia.

…Northern Plains through upper Mississippi Valley…

A shortwave trough will amplify as it moves east southeast through
southern Alberta and Saskatchewan, reaching the northern Plains by
early this evening and the upper MS Valley late tonight. Downstream
from this feature, a warm front will advance northeast, and will be
situated from a weak surface low in northwest ND through northeast
SD and central IA by early evening. A cold front will extend
southward from the low through the western Dakotas and advance east
during the evening.

A moist warm sector will reside south of the warm front. Surface
dewpoints will range from the mid 60s to upper 70s F beneath a plume
of steep mid-level lapse rates associated with the eastward
expansion of an elevated mixed layer (EML). Warm air at the base of
the EML will result in a substantial cap in warm sector during the
day. Theta-e advection along a southerly low-level jet will
contribute to northeastward destabilization through the Dakotas and
into the upper MS Valley this afternoon and evening with MUCAPE
increasing from 2000-3000 J/kg north of the warm front. Isolated
storms with a marginal hail threat may be ongoing from SD into NE
within zone of warm advection. However, primary severe threat is
expected during the afternoon into the evening when forcing for
ascent associated with approaching shortwave trough and attendant
strengthening low-level jet will result in development of storms
north of the warm front over northern ND. These storms will be
elevated, but effective bulk shear from 40-50 kt and a favorable
thermodynamic environment will support a few supercells with large
hail the initial primary threat. Storms may eventually evolve into
an MCS with primary threat transitioning to damaging wind during the
evening and overnight as activity continues southeast through MN
into WI along instability gradient. Given favorable low-level
hodographs, a conditional threat will exist for a couple of
tornadoes with any surface-based storms that can develop on the warm
front. However, current indications are that the warm sector will
probably remain capped to parcels originating in the boundary layer.

…Southern New England through northern Virginia…

Band of convection associated with a progressive shortwave trough
currently from western NY through southeast OH will continue east
through the early morning. Destabilization of the boundary layer may
occur downstream from this activity, resulting in modest instability
and storm intensification. Similar to Sunday, wind profiles with
30-35 kt effective bulk shear will support multicell and some
marginal supercell structures with isolated damaging wind and
perhaps a brief tornado possible through early evening.

…Southeast Florida…

Hurricane Dorian is forecast by the National Hurricane center to
gradually turn to the northwest, but should remain far enough
offshore to limit the tornado threat this period. However, storms
developing in the outer rainbands will reach the eastern FL coastal
areas, and low-level hodographs might become at least marginally
supportive of a brief tornado by late this afternoon into the

..Dial/Squitieri.. 09/02/2019


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