Valid 181200Z – 191200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER LOWER
MI…NORTHEAST IN…AND FAR NORTHWEST OH…
…SUMMARY…
Scattered thunderstorms capable of severe gusts and large hail are
possible over Lower Michigan, northeast Indiana, and far northwest
Ohio this afternoon into the early evening.
…Middle MS Valley through OH Valley and Upper Great Lakes…
Well-organized MCS currently moving through southern MN and northern
IA will likely be in the vicinity of the IA/IL border at the
beginning of the period. The resulting convectively induced
vorticity maximum and attendant isolated showers and thunderstorms
are expected to gradually move eastward this morning. A very moist
and diurnally destabilized air mass is expected downstream across
Lower MI and the OH Valley. As a result, thunderstorms associated
with this vorticity maximum are expected to reintensify as they move
into Lower MI and the middle OH Valley this afternoon.
The anticipated trajectory of vorticity maximum currently takes it
over Lower MI during the afternoon. Enhanced mid-level flow will
accompany this vorticity maximum, resulting in strong enough
vertical shear to support more organized storm structures capable of
damaging wind gusts and isolated hail. Highest concentration of
severe storms is currently expected to be across southern portions
of Lower MI and adjacent northeast IN and northwest OH.
Another round of thunderstorms is possible over the region as the
cold front moves through this evening, but development of these
storms is largely dependent on the strength of the antecedent storms
and air mass recovery in their wake.
…Lower Great Lakes into the Northeast…
Low-amplitude shortwave trough currently moving through the middle
OH Valley will continue eastward/northeastward, reaching the Lower
Great Lakes/Northeast region by the early afternoon. Despite
relatively modest lapse rates, surface dewpoints in the upper 60s
(isolated low 70s possible) and surface temperatures in the mid 80s
will still support moderate buoyancy capable of occasionally strong
updrafts. Vertical shear will be modest and a predominantly linear
multicell mode is anticipated. Water-loaded downbursts are possible
with strongest storms.
…Ozark Plateau into OK…
A very warm and moist low-level air mass will remain in place across
the region with afternoon dewpoints in the 70s and temperatures in
the upper 90s/low 100s. Steep mid-level lapse rates atop this warm
and moist air mass will result in very strong buoyancy (i.e. MLCAPE
greater than 3000 J/kg) but also modest convective inhibition.
Impetus for overcoming this convective inhibition and initiating
deep updrafts is expected to be the cold front moving south through
the region. However, location of the cold front this afternoon is
uncertain, primarily because of potential modification of its
position as a result of outflow from the ongoing MCS across KS.
There is also some chance for the MCS to remain over northeast OK
and the Ozark Plateau long enough to prevent air mass recovery.
These uncertainties merit keeping Marginal severe probabilities for
this forecast, but an upgrade may be needed in a subsequent outlook
as these mesoscale details become more certain.
..Mosier/Nauslar.. 08/18/2019
$$