U.S. Severe Weather Forecast (Convective Outlook)

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 
and vicinity... 
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 

.Goss.. 08/19/2018