SPC AC 191730
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1230 PM CDT Wed Jul 19 2017
Valid 201200Z - 211200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE LOWER
GREAT LAKES INTO PARTS OF PENNSYLVANIA AND NEW YORK...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE NORTHERN
AND CENTRAL HIGH PLAINS INTO THE MIDDLE MISSOURI VALLEY...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE HIGH
PLAINS ACROSS THE MIDDLE MISSISSIPPI VALLEY INTO PARTS OF THE
There is potential for severe storms to spread across the lower
Great Lakes region and the Northeast on Thursday. Other severe
storms are expected to develop in the afternoon over the northern
High Plains area and spread toward the middle Missouri Valley during
The stronger westerly flow aloft is forecast to persist across the
northern states as an upper ridge over the south-central US
gradually weakens. Several short wave troughs within the westerlies
are expected to impact the primary convective potential tomorrow.
One trough will be enhanced by Day 1 MCS development over the
northern Plains and upper Mississippi Valley region, and is expected
to progress east-southeastward across the lower Great Lakes and
northeast states during the period. Farther west, a series of weak
perturbations are forecast to rotate around the northwest periphery
of the aforementioned upper ridge and move across the northern
At the surface, a cold front will move southeastward across the
northeastern states and lower Great Lakes region. This boundary
will extend westward over parts of Iowa into the central High Plains
and into eastern Montana.
...Lower Great Lakes into parts of Pennsylvania and New York...
Strong-to-severe storms may be ongoing at the start of the period
over parts of Lower Michigan and/or northern Indiana in association
with a long-lived nocturnal MCS. There is potential for this system
to progress eastward/east-southeastward across parts of the lower
Great Lakes during the day, as a very moist and moderately unstable
air mass will be in place as far east as Ohio. Over the eastern
part of the risk in New York and Pennsylvania, the threat area has
been adjusted southward to align better with projected MCS movement
and the axis of modest instability.
The details of the convective evolution are somewhat unclear at this
time, as new development may occur in advance of the initial MCS.
Further, there may be potential for the initial storms to generate
an outflow boundary along its southern periphery with subsequent
later development along the boundary and/or cold front during the
afternoon. Despite these uncertainties, the overall pattern is
favorable with a southwesterly low-level jet forecast to shift
eastward across the lower Great Lakes in tandem with a zone of
mid-level height falls and coincident 45-55 kt northwest winds,
resulting in strong low-level convergence and deep-layer shear to
promote severe weather potential. The primary threat is expected to
be damaging wind gusts.
...Northern and Central High Plains into the middle Missouri
A southerly component to the low-level winds over the central and
northern High Plains will transport moisture northward, with surface
dew points in the 55-60 F range extending into eastern Montana.
Steep lapse rates and strong diabatic heating during the afternoon
will contribute to destabilization with MLCAPE increasing to
1000-1500 J/kg. Widely scattered storms are expected to develop by
mid-late afternoon over the High Plains as heating and weak ascent
in advance of mid-level perturbations weaken the cap sufficiently
for convective initiation. Veering winds with height and resultant
30-40 kt deep layer shear will enhance storm organization and
intensity with isolated supercells possible. Large hail and
damaging wind gusts are the most likely hazards. There is potential
for activity to grow upscale during the evening with one or more
MCSs developing in response to a strengthening southerly low-level
jet over the Plains. These systems are expected to spread eastward
during the night with a continuing threat for strong wind gusts and
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