Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0248 PM CDT Sat Jun 17 2017 Valid 172000Z - 181200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF EASTERN KANSAS...MISSOURI...ILLINOIS AND WESTERN INDIANA... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE ENHANCED RISK AREA FROM THE SOUTHERN AND CENTRAL PLAINS NORTHEASTWARD INTO THE SOUTHERN GREAT LAKES... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE SLIGHT RISK AREA FROM THE GREAT PLAINS NORTHEASTWARD TO THE GREAT LAKES... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY... ...SUMMARY... Severe thunderstorms are expected late this afternoon into tonight, mainly from the central Plains across the middle Mississippi Valley into southern Lower Michigan, with damaging gusts and hail likely. Isolated severe thunderstorms also are possible this afternoon over parts of the Dakotas and Minnesota. Two changes has been made to the outlook for this issuance. The first change includes adding parts of northern Illinois (including the Chicago vicinity) and northwestern Indiana into the enhanced risk area with a 30 percent probability for wind damage. A 30 percent probability for large hail has also been added into parts of northern Illinois. A corridor of moderate to strong instability is analyzed by the RAP across northern Illinois with a zone of enhanced low-level convergence evident on visible satellite imagery. Convection that initiates along this zone will have a severe threat with a chance for more widespread severe possible...see MCD 1071. The second change is to remove the slight risk from parts of eastern Oklahoma which was an area to account for this morning's convection. ..Broyles.. 06/17/2017 .PREV DISCUSSION... /ISSUED 1137 AM CDT Sat Jun 17 2017/ ...Central Plains across the middle Mississippi Valley into southern Lower Michigan... Prior convection from yesterday/last night has modified the environment across this region with slightly lower surface dew points evident this morning. However, thinning clouds across much of this corridor from Kansas into Illinois will promote stronger diabatic heating to occur this afternoon, and some recovery of low-level moisture with increasing surface dew points from Missouri eastward is likely this afternoon. 12Z soundings over the Plains maintain steep lapse rates that will spread eastward toward the Mississippi Valley this afternoon, contributing to moderate/strong instability with MLCAPE ranging from 3500-4000 J/kg over eastern Kansas to 1500-2000 J/kg over in the Indiana/Michigan region. The presence of the EML is also likely to delay convective initiation from Kansas into Missouri until the late afternoon/evening. As a short wave trough over the northern high plains moves southeastward in response to stronger west-northwest winds in the mid/upper levels, flow over the central plains and mid-Mississippi Valley will strengthen enhancing deep layer shear over the region. Model guidance is reasonably consistent indicating that storms are likely to develop near an advancing cold front, initially over southeast Iowa/northern Missouri, then gradually developing westward along/near the front into parts of central/eastern Kansas late this afternoon and evening. The strong instability coupled with 35-45 kt deep layer shear will support development of multiple clusters/lines of severe storms capable of producing large hail with significant hail possible, especially during the early stages of storm maturation, and damaging winds as activity moves eastward and southeastward through the evening hours. There are indications that some bowing line segment will develop which may promote significant wind gusts mainly during the evening hours. Farther west over the central High Plains, model consensus indicates convective coverage will be more isolated with a few storms developing from extreme northeast Colorado area moving southeastward into parts of western Kansas. A deep mixed layer/steep lapse rates will be favorable for strong/severe wind gusts and possible hail late this afternoon and evening. ...Eastern Oklahoma into Eastern Texas and Southwest Louisiana... A small complex of strong/severe storms continues to move southward into southeast Oklahoma. The air mass in advance of the activity is continuing to warm/destabilize, although a capping inversion on the 12Z SHV sounding associated within EML provides some uncertainty regarding the persistence of the system into the afternoon. A low probability of severe storms has been added into eastern Texas to account for this scenario, with potential for additional strong storms to develop as far south as southwest Louisiana. ...Eastern Dakotas, Minnesota... Behind the surface cold front, widely scattered to scattered thunderstorms are expected to develop this afternoon with the risk for isolated severe hail and gusts. This potential will be supported by the strengthening large-scale ascent and deep shear associated with the approaching northern-stream trough in mid/upper levels, overlying diabatically minimized afternoon MLCINH and weak convergence near a surface trough. Though conditions will be seasonally cool at the surface, with temps in the 60s and dew points 50s F, this still will support MLCAPE 500-1000 J/kg, with some areas of effective-shear magnitudes reaching 30-35 kt. This threat should abate quickly after dark as the boundary layer stabilizes, due to a combination of cold advection and diabatic cooling.Read more
from SPC Forecast Products http://ift.tt/u32qoI
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