...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF   NORTH-CENTRAL AND NORTHWEST ILLINOIS...       ..SUMMARY     A SIGNIFICANT SEVERE WEATHER OUTBREAK IS POSSIBLE FOR PORTIONS OF   THE MIDDLE MISSISSIPPI VALLEY ON SATURDAY. THE THREATS WITH THE   STRONGER STORMS INCLUDE TORNADOES, A FEW WHICH MAY BE SIGNIFICANT,   LARGE TO VERY LARGE HAIL, AND SEVERE GUSTS.      

A RAPIDLY EVOLVING ENVIRONMENT WILL CHARACTERIZE THE ENH AND MDT RISK AREAS -- I.E. THE MIDDLE MISSISSIPPI VALLEY AREA -- DURING THE FIRST HALF OF THE PERIOD, AS THE UPPER SYSTEM CLOSES OFF AND THE SURFACE LOW DEEPENS, AND EVENTUALLY OCCLUDES. EARLY-PERIOD CONVECTION ACROSS THE RISK AREA IS FORECAST TO SHIFT NORTHEASTWARD AND DISSIPATE, ALLOWING HEATING -- IN CONJUNCTION WITH RAPID STEEPENING OF MID-LEVEL LAPSE RATES -- TO YIELD MODERATE DESTABILIZATION. NEW STORM DEVELOPMENT IS FORECAST TO OCCUR DURING THE MID AFTERNOON TIME FRAME IN THE FAR EASTERN IOWA/FAR NORTHEASTERN MISSOURI/NORTHWESTERN ILLINOIS AREA, AND CELLULAR MODE IS ANTICIPATED. PRONOUNCED LOW-LEVEL VEERING IN THE WIND FIELD -- PARTICULARLY IN THE VICINITY OF THE WARM FRONT OVER NORTHERN ILLINOIS -- IS EXPECTED, WITH FLOW INCREASING TO SOUTHWESTERLY AT NEAR 100KT AT MID LEVELS, AS THE UPPER SYSTEM APPROACHES. THE RESULTING SHEAR WILL BE STRONGLY SUPPORTIVE OF STRONG UPDRAFT ROTATION -- AND THUS ATTENDANT RISK FOR TORNADOES, A FEW OF WHICH COULD BE SIGNIFICANT. VERY LARGE HAIL WILL ALSO BE POSSIBLE GIVEN UPDRAFT INTENSITY -- AIDED BY THE VERY STEEP LAPSE RATES ALOFT AND STRENGTH OF THE SHEAR PROFILE. LOCALLY DAMAGING DOWNBURST WINDS WILL ALSO BE POSSIBLE LOCALLY, WITH THE STRONGER STORMS. PEAK TORNADO RISK THROUGH LATE AFTERNOON SHOULD BEGIN TO GRADUALLY DIMINISH INTO THE EVENING, AS DIURNAL COOLING BEGINS. STILL, SEVERE RISK -- INCLUDING POTENTIAL FOR LARGE HAIL WITH ELEVATED STORMS NORTH OF THE WARM FRONT -- WILL LIKELY CONTINUE THROUGH THE EVENING BEFORE DECREASING OVERNIGHT.

This parameter space along with low LCLs will support potential for supercells capable of producing long-track, strong tornadoes and very large hail. Assuming sufficient destabilization occurs, other supercells might develop in an arcing band within zone of ascent accompaying the primary vorticity maximum and just east of the surface low across central IA. These storms will also pose a risk for large hail and tornadoes. An upgrade to high risk will be considered in later day 1 updates for a portion of this region.

...Mid Mississippi Valley and vicinity... Scattered thunderstorms are expected to form in episodic arcs from midday through this evening across parts of central/eastern IA, eastern MO, IL, and IN, moving northward to northeastward generally at 45-60 kt. This will include supercells with tornadoes (some long-tracked and significant at EF2+ damage levels), large and sometimes very large hail, and occasionally severe non-tornadic gusts. Given the fast storm motions, any tornadoes that do form may persist for nearly as many miles as minutes of time -- at least, until supercells cross more than a short distance into what should be a very sharp warm-frontal zone.

A POSSIBLE UPGRADE REMAINS POSSIBLE FOR THE 20Z OUTLOOK UPDATE, GIVEN THE LARGER-SCALE PATTERN/ENVIRONMENT WHICH CONTINUES TO APPEAR FAVORABLE FOR SEVERAL SIGNIFICANT TORNADOES.