Severe Storm Outlook Narrative (AC)
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000
ACUS01 KWNS 170558
SWODY1
SPC AC 170556

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1256 AM CDT Thu Aug 17 2017

Valid 171200Z - 181200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
LOWER MICHIGAN...EASTERN INDIANA...AND NORTHWESTERN OHIO...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM EXTREME
SOUTHEAST COLORADO INTO THE OKLAHOMA AND TEXAS PANHANDLES...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE OHIO
VALLEY INTO THE CENTRAL GREAT LAKES...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM WESTERN
NEBRASKA SOUTH INTO THE SOUTHERN HIGH PLAINS...

...SUMMARY...
Thunderstorms, some severe, are expected across the Ohio Valley into
Lower Michigan.  Scattered thunderstorms are also expected across
the central and southern High Plains, where some severe storms are
also possible.

...Central Great Lakes and Ohio Valley...
Early morning water-vapor imagery shows a vigorous shortwave trough
over the Upper Midwest and this feature will move through the Upper
Great Lakes by early evening and into Ontario by daybreak Friday.  A
surface low will likewise develop northeast from MN/WI to the Upper
Peninsula of MI and then to the northeastern shore of Lake Superior.
 A warm frontal zone over the southwestern Great Lakes will advance
north into central Lower MI by midday.  A moisture-rich airmass
--near 16 g/kg lowest 100mb mean mixing ratio sampled by 00Z DVN and
ILX RAOBS-- will move into the region.  Scattered showers/isolated
thunderstorms are forecast during the morning within the warm
conveyer from northern IN/OH into Lower MI.  Despite relatively
modest mid-level lapse rates, the rich moisture coupled with heating
will destabilize the boundary layer with 500-1250 J/kg MLCAPE
expected by midday.  Renewed thunderstorm development is expected by
early-mid afternoon as weak 500-mb height falls (30 m per 12 hr) and
a belt of 40-50 kt southwesterly mid-level flow overspreads the warm
sector.  It seems plausible a supercell risk will develop in Lower
MI during the afternoon with a risk for damaging winds.  Model
forecast soundings vary considerably in hodograph size in whether
low level flow veers, which would reduce the risk for a tornado, or
if surface-500 m flow can remain relatively backed augmenting the
tornado risk.  Regardless, organized storms are forecast to develop
during the afternoon and move east with this activity weakening
during the evening.

To the south of I-70, weaker flow fields will tend to favor a
multicellular mode and an isolated threat for strong to locally
severe gusts may arise.

...Central and Southern High Plains...
An effective front over the southern Plains will likely become
stationary by early-mid morning as convective outflow over the
southern Plains becomes increasingly distant both temporally and
spatially.  A lee trough is forecast to sharpen over the central
High Plains as shortwave trough over the Northern Rockies this
morning quickly moves southeast through the Dakotas by late tonight.
 Steep 700-500 mb lapse rates will overspread the central-southern
High Plains during the day amidst strong heating.  Thunderstorms are
likely to develop near the orographically favored Raton Mesa and
move southeastward into the moist axis.  Hail/wind are the primary
threats with the stronger storms.  A gradual waning in storm
coverage/intensity is expected during the evening.

..Smith/Dean.. 08/17/2017

$$


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