Conditions are ripe for severe weather development across the Chicago area this afternoon. The latest mesoscale discussion from the Storm Prediction Center in Norman, Oklahoma indicates that it is almost a certainty that a tornado watch will be issued.
Mesoscale Discussion 0831 NWS Storm Prediction Center Norman OK 1250 PM CDT Mon May 27 2019 Areas affected...portions of eastern IA...northern and central IL and northern IN Concerning...Severe potential...Tornado Watch likely Valid 271750Z - 271915Z Probability of Watch Issuance...95 percent SUMMARY...Severe thunderstorms with a threat of tornadoes (some potentially strong), large hail and damaging winds will increase over the next couple of hours. Watch issuance is likely. DISCUSSION...A warm front is rapidly lifting northward and draped across northern IL into southern/central IN as of 17z. Dewpoints will continue to climb into the mid and upper 60s this afternoon and MLCAPE values will steadily increase to around 1500-2000 J/kg over the next 2-4 hours. A shortwave trough currently migrating east/northeast across IA/MO will provide large-scale forcing, while a couple of MCVs from morning convection will result in more subtle forcing across the region. Areas downstream of these MCVs as well as along the warm front will be a focus for thunderstorm development and organization in the next 1-2 hours from eastern IA into northern and central IL. Additional storms are expected to develop along the northward advancing warm front across central into northern IN. The timing of convective initiation further east across IN is a bit more uncertain, but severe storms are expected by 21-22z. 18z RAOB from DVN showed capping just below 850mb. With the incoming shortwave, temperatures aloft are expected to cool and with continued heating and moist advection, this cap is expected to quickly erode by mid afternoon. In fact, 18z RAOB from ILX is uncapped with steep midlevel lapse rates noted in addition to steep low level lapse rates and a mean mixing ratio approaching 14 g/kg. This thermodynamic environment combined with favorable vertical shear will support supercells. The very moist boundary layer combined with ideal low level hodographs (noted in DVN RAOB) should lead to tornado potential supportive of at least a couple of strong tornadoes. A tornado watch will likely be needed by 19z across parts of far eastern IA into northern/central IL. Uncertainty in timing further east into IN may result in later tornado watch issuance, but expect additional watch issuance by 21z.