U.S. Severe Weather Forecast (Convective Outlook)

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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 
upper Midwest. 


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 
Ohio valleys. 


..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. 


..upper Midwest... 
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 


.Kerr.. 08/19/2019 


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