- Day Three
acus02 kwns 191739
Storm Prediction Center ac 191738
Day 2 convective outlook
National Weather Service Storm Prediction Center Norman OK
1238 PM CDT sun Aug 19 2018
Valid 201200z - 211200z
..there is a slight risk of severe thunderstorms across portions of
the middle Mississippi...lower Ohio...and Tennessee valleys...
Isolated severe thunderstorms will be possible on Monday across the
Lower/Middle Mississippi, lower Ohio, and Tennessee River valleys,
as well as the northern rockies.
As a weak upper trough continues to depart the eastern U.S., Two
more substantial troughs will continue to traverse the Continental U.S.. one
of these troughs is progged to linger in a positively-tilted manner
over the Pacific northwest and northern intermountain region through
the period. Meanwhile, the second of these will shift out of the
plains and across the Mississippi Valley, eventually reaching the
lower Ohio/Tennessee valleys overnight.
At the surface, the primary system will be one associated with the
aforementioned central U.S. Trough. A low initially forecast over
the northwest Missouri/southwest Iowa vicinity is forecast to move
slowly east-northeast with time, reaching a position in the vicinity
of southwest lower Michigan by 21/12z. In conjunction with the
advance of this low, a cold front is forecast to cross the middle
and lower Mississippi Valley through early evening, while a warm
front shifts northeast across the Midwest -- eventually reaching the
lower Great Lakes.
..middle and lower Mississippi, lower Ohio, and Tennessee valleys
widespread showers and thunderstorms -- with only limited wind/hail
risk ongoing with a few of the strongest storms -- are expected to
be crossing the central U.S. Early in the period, including large
portions of the severe risk area. While the ongoing
clouds/precipitation continue to cast some uncertainty with respect
to potential afternoon destabilization -- and thus extent and
intensity of convective redevelopment, it appears at this time that
much of the convection should will have crossed the Mississippi
River by mid morning, decreasing in coverage and intensity with
time. This should permit some destabilization to occur in the wake
of the convection/ahead of the advancing cold front, to permit new
storm initiation near and west of the Mississippi River.
Aided by moderate low-to-mid-level westerly flow aloft, potential
for storms to organize into bands and/or rotating cells is evident,
and with resulting potential for locally damaging winds and possibly
hail a bit more certain at this juncture, an upgrade to slight risk is
being included at this time. Expect storms to cross the Mississippi
Valley through the afternoon, shifting into the lower Ohio and
Tennessee valleys with time. Risk for mainly damaging winds will
likely continue into the evening, before storms begin to diurnally
Farther north, nearer the surface low and warm front, potential for
destabilization remains much more uncertain due to likelihood for
more persistent clouds/precipitation. If ample destabilization can
occur however, backed low-level winds and proximity to the upper
system could result in risk for a few supercells -- and even some
tornado risk in addition to potential for wind/hail. At this time,
will maintain only marginal/5% risk into the eastern Iowa/northern
Illinois vicinity, but an upgrade may be needed in later outlooks
should evolution of early convection suggest greater potential for
afternoon destabilization in this area.
..parts of the northern intermountain region...
Presence of a slow-moving upper system will result in cool mid-level
temperatures and moderately strong westerly mid-level flow across
portions of the intermountain west Monday. With afternoon heating
beneath the trough resulting in a pocket of potentially sufficient
cape, storms are expected to develop over parts of the
Idaho/southwest Montana/western Wyoming area. A few stronger cells
may develop by late afternoon -- aided by the enhanced westerlies
aloft, with potential for a few/locally severe wind gusts before
storms weaken during the evening.
..maximum risk by hazard...
Tornado: 5% - slight
wind: 15% - slight
hail: 15% - slight