Severe weather threat increasing- Northern Illinois and portions of the Chicago area targeted for possible tornado watch later this afternoon
The Storm Prediction Center in Norman, Oklahoma has just issued a mesoscale discussion highlighting the potential for a tornado watch issuance across northern Illinois and portions of the Chicago area later this afternoon.
Despite the fact, that it’s still February, atmospheric dynamics are strong and temperatures are quite springlike with Illinois temperature’s ranging from the 50s north to the 70s south. Thunderstorms are expected to develop rapidly later this afternoon and could quickly become severe, producing large hail, strong damaging winds and even tornadoes. Heavy rainfall is also possible with these storms.
Mesoscale Discussion 0219 NWS Storm Prediction Center Norman OK 0203 PM CST Tue Feb 28 2017 Areas affected...Portions of northern IL...northern IN...southeast IA...extreme northeast MO Concerning...Severe potential...Tornado Watch likely Valid 282003Z - 282230Z Probability of Watch Issuance...80 percent SUMMARY...The area is being monitored for increasing severe-thunderstorm potential, including the risk for tornadoes, severe hail (possibly significant), and damaging wind gusts. The issuance of a Tornado Watch will likely be required during the next few hours. DISCUSSION...A northwest-southeast-oriented warm front extending from surface low pressure in northeastern IA to southern IN will continue to develop northeastward, as the leading edge of a high-level speed maximum also spreads northeastward. Recent water vapor imagery indicates a notable surge of midlevel drying advancing through central IA, likely associated with an impulse embedded within the flow aloft. As forcing for ascent associated with these features continues to overspread the northeastward-developing warm sector (where modest pressure falls are observed), convection should increase in coverage/intensity during the next several hours Already, isolated elevated convection has been intensifying northeast of the warm front in east-central IL, with additional convection increasing in the open warm sector across south-central IA. Despite the return of only modest boundary-layer moisture (surface dewpoints in the middle 50s to around 60F), steep midlevel lapse rates associated with a well-established elevated mixed layer overlying the area will support sufficient buoyancy for intense updrafts. With 40-60 kt of effective shear, rotating updrafts will likely be capable of producing severe hail -- possibly significant. Damaging wind gusts and tornado potential will exist, especially near and south/southwest of the warm front, where effective inflow layers will extend to the surface. Significant tornadoes could even occur owing to ample streamwise vorticity in the boundary layer (effective SRH around 200-400 m2/s2), and especially in proximity to the warm front given pre-existing vertical vorticity. There is some uncertainty regarding the timing of the more robust increase of surface-based convective risk. Regardless, the risk for all severe hazards will be increasing, likely warranting Tornado Watch issuance during the next few hours.