- Day Three
acus02 kwns 191731
Storm Prediction Center ac 191730
Day 2 convective outlook
National Weather Service Storm Prediction Center Norman OK
1230 PM CDT Mon Aug 19 2019
Valid 201200z - 211200z
..there is an enhanced risk of severe thunderstorms Tuesday across
parts of southeastern Iowa and northwest Missouri...much of central
Illinois and central/southern Indiana...
An organized cluster of thunderstorms may develop and overspread
portions of the middle Mississippi and lower Ohio valleys Tuesday,
accompanied by the risk for severe wind and hail.
The stronger westerlies will remain confined to areas near/north of
the Canadian/U.S. Border, but may undergo amplification during this
period, as a significant short wave trough pivots inland across the
British Columbia coast. Downstream, positively tilted mid/upper
ridging is forecast to build across the Canadian prairies into
Nunavut and western Hudson Bay, while another significant short wave
trough digs southeastward across Manitoba and northwest Ontario,
toward the upper Great Lakes region.
To the south, a broad plume of very warm elevated mixed-layer air is
already beginning to emerge from the intermountain west/Mexican
plateau/rockies region. The northeastern periphery of this air mass
appears to likely to continue advecting across the middle Missouri
Valley region, toward the upper Midwest, through daybreak Tuesday.
As this overspreads seasonably moist boundary layer air, it is
expected to contribute to the maintenance of moderate to strong
potential instability across a broad area of the interior United
States. By early Tuesday, it appears that this will be focused
ahead of a stalling initial front extending across southern portions
of the Great Lakes region into middle Missouri Valley, and ahead of
another surface cold front advancing into the northern plains and
This instability is expected to provide support for scattered
vigorous thunderstorm activity, as far east as the upper Ohio Valley
and northern mid Atlantic region, Tuesday into Tuesday night. Some
of these storms will pose a risk for severe wind and hail,
particularly across portions of the middle Mississippi into lower
..middle Mississippi/Ohio valleys...
A considerable signal remains evident within the latest model output
suggesting the potential for the evolution of a large, organized
convective system during this period. It appears that forcing for
ascent associated with a low amplitude impulse migrating around the
northern periphery of the subtropical ridging, near/south of the
stronger westerlies, will provide support for the initiation of this
activity, perhaps as early as late tonight across southern Iowa.
Aided by inflow of moderate to large cape, particularly as the
boundary layer begins to warm with insolation, there appears
potential for considerable intensification Tuesday morning into
afternoon, across southeast Iowa and adjacent northwestern Missouri
through central Illinois. Aided by deep-layer shear associated with
a 30-50 kt mid-level jet streak, and steep mid-level lapse rates,
the environment may become supportive of large to very large hail
and damaging surface gusts, associated with both downbursts and a
strengthening convectively generated surface cold pool.
Some uncertainty exists concerning the subsequent track of the
convective system, and highest severe probabilities, but this seems
most likely to become focused near the northeastern periphery of
mid-level ridging, centered (around 700 mb) across southern
Missouri. Based on this, activity may gradually turn
south-southeastward into/through the lower Ohio Valley by Tuesday
evening, although eastward development toward portions of the upper
Ohio Valley may not be out of the question.
..Central Plains into mid Missouri Valley...
More uncertainty exits concerning coverage and location of
convective development, particularly with regard to mid/upper
support for activity. However, a zone of stronger differential
surface heating may provide the general focus for strong/severe
storm development across central/southern Nebraska and northeast
Kansas late Tuesday afternoon and evening.
Models suggest that an area of mid/upper forcing for ascent,
developing ahead of the primary digging short wave trough, may
provide support for thunderstorm development during the peak heating
across parts of east central Minnesota into northern Wisconsin and
adjacent portions of Upper Michigan, where the environment may
become at least marginally conducive to isolated supercells posing a
risk for severe hail and wind.
..maximum risk by hazard...
Tornado: 2% - marginal
wind: 30% - enhanced
hail: 30% sig - enhanced