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Source: US National Oceanic and Atmospheric Administration

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Aug 11, 2019 1730 UTC Day 2 Convective Outlook
Updated: Sun Aug 11 17:19:17 UTC 2019 (Print Version |   |  )
Probabilistic to Categorical Outlook Conversion Table

 Forecast Discussion

SPC AC 111719

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1219 PM CDT Sun Aug 11 2019

Valid 121200Z – 131200Z

…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR CENTRAL
ILLINOIS…

…SUMMARY…
Severe thunderstorms are likely on Monday over the middle
Mississippi Valley to the Ohio Valley. Damaging gusts are the
primary risk but a few tornadoes are possible.

…Synopsis…
Northern-stream upper trough will deamplify as it moves across the
Dakotas into MN. In the low levels, a cold front will extend from
central ND southward through western NE by Monday evening. A
low-amplitude, convectively enhanced shortwave trough will move from
the NE/IA area into the southern Great Lakes and OH Valley during
the evening into the overnight. A weak surface low will develop
eastward from the central Great Plains into IL by late afternoon. A
quasi-stationary front will extend eastward from the low
through southern IA then southeast as a warm front toward the TN
Valley.

…Middle Mississippi Valley into the Ohio Valley…
Considerable uncertainty for this forecast is largely related to
thunderstorm development expected late tonight into Monday morning
across the central Great Plains into the lower MO Valley. This
forecast thinking is similar to the previous outlook. Storms will
likely be ongoing Monday morning from eastern NE into IA in
association with the progressive, low-amplitude shortwave trough and
will continue east within zone of isentropic ascent north of the
front and on nose of an eastward-migrating low-level jet. A cluster
of storms is possible farther southeast along the warm front over
northeast MO into IL.

By the afternoon, the warm front activity will have weakened and
moved farther east. Despite modest 700-500mb lapse rates, strong
heating amidst high-quality moisture (16-19 g/kg lowest 100mb mean
mixing ratio) will lead to a moderately to very unstable airmass
developing across IL south of an effective boundary. A strong belt
of westerly 700mb flow (30-40kt) is forecast to move across IA and
into the northern half of IL by the mid-late afternoon. It is a
possibility that convection is maintained from IA into IL during the
day ahead of the mid-level wave moving from eastern NE to Lake MI by
early evening. Models are in good agreement in the depiction and
evolution of a south-southwesterly LLJ over lower MO Valley being
maintained and perhaps intensifying to 40kt as it shifts east into
north-central IL by early evening. However, the variability shown
by different model solutions in the magnitude of destabilization is
very large and is due mostly to convective possibilities modulating
the position of the buoyancy gradient across the mid MS Valley.
With those uncertainty concerns, it seems prudent at this time to
introduce a corridor of higher probabilities given the overlap of
shear (40+ kt effective shear) and a moisture-rich/buoyant boundary
layer. Supercells are possible, especially early during the
convective lifecycle before upscale growth into bows favors
potential for severe gusts and a corridor of wind damage.

…Northern Plains region…
The atmosphere may become at least marginally unstable east of a
cold front across the eastern Dakotas and into eastern NE where a
belt of stronger winds within base of the northern-stream shortwave
trough will reside. A few storms may redevelop in this region during
the afternoon, posing some risk for hail or locally strong wind
gusts.

…Maine…
A few storms may develop along pre-frontal trough during the
afternoon where modest low-level moisture and mid-level lapse rates
will support marginal instability. This region will reside within
belt of stronger winds aloft within base of an amplifying upper
trough supporting 35-40 kt effective shear. A few storms may produce
locally strong wind gusts and small to marginally severe hail, but
overall threat should remain limited by the expected weak
thermodynamic environment.

…MAXIMUM RISK BY HAZARD…
Tornado: 5% – Slight
Wind: 30% – Enhanced
Hail: 15% – Slight

..Smith.. 08/11/2019

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