Valid 172000Z – 181200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR
SOUTHEASTERN SOUTH DAKOTA INTO NORTHWEST IOWA AND SOUTHWEST
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR CENTRAL NEW
YORK INTO NORTHERN PENNSYLVANIA…
Scattered severe thunderstorms with damaging winds and large hail as
the primary threats are expected across a large portion of the
northern to central Great Plains into the Midwest, generally during
the late afternoon into tonight. Damaging winds are also possible
into early evening in parts of the Northeast States.
Only a few changes were made to the previous outlook for this
1) Added a 65-kt or greater risk area for wind for adjacent parts of
southern NE into northern KS. Recent HRRR time-lagged members
indicate the possibility for a bowing system moving eastward within
a plume of steep 700-500mb lapse rates (in excess of 8 degrees C/km)
and a wind profile supportive of organized storms. Uncertainty
remains regarding this scenario and therefore will keep at a Slight
Risk for this outlook update.
2) Reduced severe probabilities west of the squall line over NY
where convective overturning will limit the magnitude of
destabilization and the potential for severe later this afternoon
into the early evening.
3) With passage of an MCV and the stabilizing influence of a
thunderstorm line that moved through IL into eastern IN, have
reduced hail/wind probabilities into a Marginal-Risk highlight.
4) Nudged higher severe probabilities westward into east-central CO
to account for ongoing and expected storm development forecast to
move east off the Palmer Divide towards the northwest KS vicinity.
.PREV DISCUSSION… /ISSUED 1130 AM CDT Sat Aug 17 2019/
…Northern Great Plains/Upper Midwest…
A broad positive-tilt shortwave trough will advance east across the
southern Prairie Provinces and MT to ND through tonight. A surface
front well in advance of this wave extends from the Red River Valley
southwest towards the Black Hills and should remain quasi-stationary
before accelerating south/east tonight. The bulk of stronger
mid-level west-southwesterlies accompanying the wave will remain
confined to the cool side of the front, while a separate belt of
enhanced westerlies is confined south across NE. In addition, the
northern periphery of steeper mid-level lapse rates from an elevated
mixed-layer will probably only overlap the SD portion of the front.
Low to mid 60s F surface dew points are expected along the front and
should support moderate buoyancy with MLCAPE around 1500-2500 J/kg.
Owing to a lack of stronger capping, at least scattered
thunderstorms will develop in the late afternoon. A few transient
supercells should develop initially, but hodographs do not appear
particularly elongated or enlarged given modest low-level winds and
weakness aloft in the split flow regime. Given the frontal forcing
and predominance of outflow consolidation, broader clusters fusing
into one or more lines are expected with eastern extent tonight.
Some risk for damaging winds will probably continue across southern
MN and northern IA along the northern extent of the larger buoyancy
…Central Great Plains…
High-based convection that develops off the Front Range this
afternoon should impinge on the recovering moist sector near the
CO/KS/NE border towards early evening. On the southern periphery of
a belt of enhanced mid-level westerlies centered from southern WY
across NE amid a very steep lapse rate environment, a swath of
severe wind gusts will be possible into north-central KS tonight.
…Mid-MS Valley to IN vicinity…
An MCV is centered near the MO/IA/IL border area and should drift
east. Several CAMs indicate redevelopment of scattered storms ahead
of this feature. This will be highly dependent on more robust
destabilization occurring in the wake of a leading MCS in IN. At
least a conditional threat for damaging winds and severe hail
exists, mainly during the evening. Warm-advection storms overnight
may also pose an isolated severe risk.
A low-amplitude shortwave impulse over southern Ontario should
slowly move across the Saint Lawrence Valley. Scattered convection
is ongoing in association with this wave and will likely persist
through the afternoon as further surface-based destabilization
occurs downstream. Weak low-level winds and modest mid-level lapse
rates will be limiting factors. Most of the stronger speed shear
should be confined to the upper portion of the buoyancy profile.
Setup should foster a risk of isolated marginally severe hail and
scattered damaging winds.