http://www.spc.noaa.gov/products/outlook/day1otlk.html

Probabilistic to Categorical Outlook Conversion Table
Public Severe Weather Outlook
The SPC is forecasting ...severe thunderstorms expected over parts of the lower mississippi...lower ohio...and tennessee valleys late this afternoon through early monday....   Please read the latest public statement about this event. Probability of a tornado within 25 miles of a point. Hatched Area: 10% or greater probability of EF2 - EF5 tornadoes within 25 miles of a point. (More Info)  Forecast Discussion SPC AC 221311 DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 0711 AM CST SUN JAN 22 2012 VALID 221300Z - 231200Z ...THERE IS A MDT RISK OF SVR TSTMS ...THERE IS A MDT RISK OF SVR TSTMS FROM PARTS OF AR...MS...AND AL NWD THROUGH SE MO...WRN TN...AND WRN KY INTO SRN IL AND SRN IND...... ...THERE IS A SLGT RISK OF SVR TSTMS ...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE MDT RISK AREA FROM THE CNTRL GULF CST NEWD INTO PARTS OF OH...... ...SYNOPSIS... ...A REGIONAL SEVERE WEATHER EVENT LIKELY TO OCCUR FROM THE LWR MS VLY NEWD INTO PARTS OF THE MID MS...TN...AND LWR OH VLYS THIS EVE THROUGH EARLY MON... POTENT UPR TROUGH NOW OVER THE CNTRL/SRN RCKYS WILL MOVE E INTO ERN KS/OK THIS EVE...BEFORE ASSUMING A NEGATIVE TILT AND ACCELERATING ENE ACROSS THE MID MS...LWR OH...AND TN VLYS TNGT AND EARLY MON. ASSOCIATED SFC LOW WILL TRACK E ACROSS KS TODAY...BEFORE TURNING NE ACROSS NRN MO AND IA TNGT...AND INTO WI EARLY MON. AHEAD OF THE LOW AND TROUGH...SATELLITE AND VWP DATA ATTM SHOW BROAD 50+ KT SSWLY LLJ EXPANDING NWD ACROSS ERN PARTS OF THE PLNS. THIS JET WILL EXPAND FARTHER E ACROSS THE MID MS AND TN VLYS TODAY...WITH STRONG MOISTURE ADVECTION OCCURRING ATOP SHALLOW SURFACE-BASED LAYER OF COLD AIR OVER THE REGION. STRONG TSTMS SHOULD FORM LATE THIS AFTN OR EARLY THIS EVE FROM IL SSW INTO PARTS OF MO/AR AND LA...ALONG LEAD COLD FRONT TRAILING SWD FROM THE LOW. GIVEN VERY STRONG WIND FIELD ASSOCIATED WITH THE UPR TROUGH AND INCREASINGLY WARM/MOIST LOW LVL ENVIRONMENT ALONG/AHEAD OF FRONT...THE STORMS WILL HAVE THE POTENTIAL TO PRODUCE STRONG TORNADOES IN ADDITION TO DMGG WIND AND HAIL AS THEY MOVE/DEVELOP E AND NE ACROSS THE TN AND LWR OH VLYS THROUGH 12Z MON. ...LWR MS VLY NWD AND EWD INTO THE TN AND LWR OH VLYS LATE TODAY THROUGH EARLY MON... SFC DEWPOINTS IN THE LOW TO MID 60S F SHOULD EXTEND NWD TO THE MO BOOTHEEL BY LATE TODAY GIVEN STRONG/PERSISTENT LOW LVL WAA/MOISTURE TRANSPORT. COUPLED WITH MODEST SFC HEATING...THIS SHOULD RESULT IN AN AXIS OF MODERATE INSTABILITY THAT WILL REACH SSWWD ACROSS ERN/SRN AR INTO MS/LA. TSTMS SHOULD FORM ALONG THE NW EDGE OF THIS INSTABILITY AXIS LATE THIS AFTN...AND SHOULD QUICKLY EXPAND IN COVERAGE NWD AND SWD AS LEAD SFC COLD FRONT OVERTAKES MOIST AXIS AND CONTINUES EWD. WITH TIME...AN EXTENSIVE SQLN SHOULD EVOLVE FROM THE LWR OH VLY SSW INTO AR/NW MS/LA. 70 TO 90 KT MID LVL JET OVERSPREADING REGION SHOULD SUPPORT AT LEAST A FEW LONGER-LIVED SUPERCELLS. LOW LVL SRH SHOULD MAXIMIZE OVER PARTS OF AR...SE MO...WRN TN AND NRN MS...ON SRN FRINGE OF RETREATING SHALLOW COOL AIR LAYER. GIVEN COMPARATIVELY GREATER SFC-BASED INSTABILITY IN THIS REGION...POTENTIAL WILL EXIST FOR STRONG TORNADOES....ESPECIALLY THROUGH EARLY EVENING. TORNADOES WILL ALSO BE POSSIBLE THROUGH EARLY MON E/SE INTO PARTS OF ERN MS AND AL AS EXISTING STORMS AND STRENGTHENING WIND FIELD ENCOUNTER MORE SLOWLY-RETREATING PART OF WEDGE AIR MASS IN THAT REGION. FARTHER N...THE MORE SIGNIFICANT SVR THREAT SHOULD BE DMGG WINDS AS THE DEVELOPING SQLN AND TURBULENT MIXING ENCOURAGE DOWNWARD TRANSPORT OF STRONG LWR TROPOSPHERIC FLOW TO THE SFC...EVEN IN REGIONS OF NEAR-ZERO SFC-BASED CAPE. THIS THREAT MAY CONTINUE...AT LEAST ON A SPORADIC BASIS...NWD AND EWD INTO PARTS OF OH AND TO THE WRN SLOPES OF THE CNTRL/SRN APPALACHIANS EARLY MON. IN THE MEAN TIME...A CONDITIONAL RISK WILL EXIST FOR A FEW STRONG STORMS OVER PARTS OF AL AND WRN GA TODAY...WHERE SFC HEATING AND ASCENT WITH WEAK LEAD UPR IMPULSE MAY SUPPORT SCTD CONVECTIVE DEVELOPMENT. ENVIRONMENT MAY PROVE FAVORABLY MOIST...UNSTABLE...AND SUFFICIENTLY SHEARED TO POSE A THREAT FOR AN ISOLD TORNADO.