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SPC Mesoscale Discussion Changes

As part of SPCs efforts to more efficiently convey critical forecast information, standardized content structure within the Mesoscale Discussion (MD) product will increase, including new information on the probability of watch issuance for severe potential MDs. Starting May 1, 2012, MDs will include the following sections:

  • For Severe Potential MDs,
       AREAS AFFECTED...(PORTION OF STATES OR GEOGRAPHICAL AREAS) 
    
       CONCERNING...(TYPE OF MD) 
    
       VALID DDHHMMZ - DDHHMMZ 
    
       PROBABILITY OF WATCH ISSUANCE...[increments of 20% from 20-80%,
       including 5% and 95%]
    
       SUMMARY...A concise one or two sentence statement regarding the 
       forecast (timing, coverage, intensity, and mode) severe threat.
    
       DISCUSSION...The description of significant mesoscale features and 
       atmospheric processes which will likely result in the expected event.
    
       Definitions:
       PROBABILITY OF WATCH ISSUANCE in percent...5 and 20 for Unlikely,
       40 and 60 for Possible, 80 for Likely, 95 for Needed Soon. 
    
  • For all other MDs,
       AREAS AFFECTED...(PORTION OF STATES OR GEOGRAPHICAL AREAS)
    
       CONCERNING...(TYPE OF MD)
    
       VALID DDHHMMZ - DDHHMMZ
    
       SUMMARY...A concise one or two sentence statement regarding the
       forecast (timing, coverage, intensity, and mode) severe threat.
    
       DISCUSSION...The description of significant mesoscale features and
       atmospheric processes which will likely result in the expected event.
    

    Example of Severe Potential Mesoscale Discussion - Watch Possible

       MESOSCALE DISCUSSION 2046
       NWS STORM PREDICTION CENTER NORMAN OK
       0247 AM CST TUE NOV 16 2010
       
       AREAS AFFECTED...THE FL PANHANDLE
       
       CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE  
    
       VALID 160847Z - 161115Z
    
       PROBABILITY OF WATCH ISSUANCE...40 PERCENT
    
       SUMMARY...SHORT-LINE SEGMENT MAY GROW UPSCALE AND BRUSH COAST WITH 
       ATTENDENT INCREASE IN SEVERE THREAT AFTER 10Z.
    
       DISCUSSION...CONVECTIVELY ENHANCED BOUNDARY STRETCHES FROM SOUTH OF 
       APALACHICOLA WWD AND INTERSECTS A NE-SW ORIENTED BAND OF THUNDERSTORMS   
       EXTENDING FROM SOUTH OF GULF SHORES SWWD TO JUST OFF SERN TIP OF LA.   
       ACTIVITY IS DEVELOPING WITHIN A PRE-FRONTAL CONFLUENCE ZONE ASSOCIATED 
       WITH A STRENGTHENING SWLY LOW-LEVEL JET. THIS FEATURE WILL DEVELOP NWD
       THROUGH THE WRN FL PANHANDLE INTO AL AS PRIMARY SURFACE LOW DEEPENS 
       AND LIFTS NWD INTO THE TN VALLEY IN RESPONSE TO FORCING ASSOCIATED 
       WITH NEWD EJECTING VORT MAX. BUOY OBSERVATIONS SHOW A RESERVOIR OF 70F   
       DEWPOINTS OVER THE NRN GULF SOUTH OF E-W BOUNDARY. THIS FEATURE WILL 
       MOVE SLOWLY NWD AS THE LOW LEVEL JET STRENGTHENS...AND IS ALREADY 
       APPROACHING THE COAST NEAR APALACHICOLA. REMAINING PORTIONS OF THE 
       BOUNDARY WILL LIKELY NOT MOVE INLAND UNTIL AFTER 11Z. BOUNDARY LAYER  
       DESTABILIZATION AND POTENTIAL FOR SURFACE BASED STORMS WILL INCREASE 
       MARKEDLY WHERE THE MOIST WARM SECTOR MOVES ONSHORE....WHILE LARGE 
       HODOGRAPHS ACCOMPANYING THE LOW-LEVEL JET WILL PROMOTE SEVERE
       POTENTIAL. HOWEVER...THE COASTAL BOUNDARY WILL MAKE ONLY LIMITED 
       INLAND PROGRESS THIS MORNING SUGGESTING THE THREAT AREA SHOULD REMAIN      
       SMALL.
    
       ..DIAL/MEAD.. 11/16/2010
    
    

    Example of Severe Potential Mesoscale Discussion - Watch Likely

       MESOSCALE DISCUSSION 0589
       NWS STORM PREDICTION CENTER NORMAN OK
       0111 PM CDT TUE APR 26 2011
       
       AREAS AFFECTED...NERN TX AND FAR SERN OK/NRN LA/SRN AR
       
       CONCERNING...SEVERE POTENTIAL...TORNADO WATCH LIKELY 
       
       VALID 261811Z - 262015Z
       
       PROBABILITY OF WATCH ISSUANCE...80 PERCENT
    
       SUMMARY...DISCRETE SUPERCELLS EXPECTED TO FORM AFTER 20Z WITH AN 
       INCREASING LARGE HAIL/TORNADO THREAT INTO THIS EVENING. A RELATIVELY LONG 
       LEAD TIME/DURATION PDS TORNADO WATCH WILL BE ISSUED BY 20Z.
       
       DISCUSSION...WITH SURFACE CYCLOGENESIS UNDERWAY ACROSS THE SRN HIGH
       PLAINS...THE WARM SECTOR AIRMASS...EAST OF A NEAR I-35 CENTRAL TX
       DRYLINE...AND ALONG/SOUTH OF A SLOW NORTHWARD-RETURNING
       SOUTHWEST-NORTHEAST ORIENTED FRONTAL BOUNDARY FROM THE SOUTH DFW
       METRO INTO SOUTHERN AR...HAS ALREADY BECOME MODERATE TO STRONGLY
       UNSTABLE IN THE PRESENCE OF LITTLE CINH. AFTER A BRIEF BOUT OF
       SHORTWAVE RIDGING AT MIDDAY...THE LEADING EDGE OF AN UPPER JET
       STREAK APPEARS TO HAVE REACHED THE SOUTHERN HIGH PLAINS...WITH
       LIKELY-ASSOCIATED DEEP MIXED CUMULIFORM DEVELOPMENT NOW EVIDENT
       ACROSS EASTERN NM/NORTHWEST TX PER VISIBLE SATELLITE IMAGERY.
       INCREASING DEEP LAYER WIND FIELD ATOP THE MOIST/UNSTABLE WARM SECTOR
       WILL BE HIGHLY FAVORABLE FOR SUPERCELLS. STRENGTHENING LOWER
       TROPOSPHERIC WINDS THIS EVENING WILL ACCENTUATE THE RISK OF STRONG
       TORNADOES ESPECIALLY ACROSS THE ARKLATEX VICINITY/ADJACENT FAR
       SOUTHEAST OK.
       
       ..GUYER/HART.. 04/26/2011
    

    Example of Severe Potential Mesoscale Discussion - Watch Unlikely

       MESOSCALE DISCUSSION 0026
       NWS STORM PREDICTION CENTER NORMAN OK
       0345 PM CST TUE JAN 17 2012
       
       AREAS AFFECTED...PARTS OF CENTRAL/SRN LA AND SRN MS
       
       CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY 
       
       VALID 172145Z - 172245Z
    
       PROBABILITY OF WATCH ISSUANCE...20 PERCENT
       
       SUMMARY...POTENTIAL FOR ISOLATED STRONG TO SEVERE STORMS WILL PERSIST
       LATE THIS AFTERNOON. IN ADDITION...STORMS DEVELOPING OVER CENTRAL/SWRN
       LA WILL TEND TO TRACK ENEWD INTO FAR SRN MS WHERE STORMS ARE ALREADY IN  
       PROGRESS...WITH HEAVY RAINFALL RATES POSSIBLE WITH THESE TRAINING STORMS.
       
       DISCUSSION...MID-AFTERNOON SURFACE ANALYSES SHOWED A COLD FRONT STEADILY
       ADVANCING EWD THROUGH THE LOWER MS VALLEY AND TOWARD THE TX COAST. 
       STORMS OVER CENTRAL LA WERE LOCATED ALONG AND E OF THE FRONT...WHILE
       THE STORMS OVER FAR SRN MS WERE LOCATED ALONG AN APPARENT
       DIFFERENTIAL HEATING BOUNDARY. DESPITE SURFACE DEWPOINTS IN THE 60S
       AND STEEP MIDLEVEL LAPSE RATES...EXTENSIVE CLOUDINESS HAS CONTINUED
       TO INHIBIT GREATER SURFACE HEATING AND THUS MAINTAINING WEAK
       INSTABILITY /MLCAPE 500-1000 J PER KG/. STRONG EFFECTIVE BULK SHEAR
       /AROUND 50 KT/ IS SUPPORTIVE FOR ORGANIZED STORMS...BUT VEERED
       PRE-FRONTAL LOW LEVEL WINDS WILL CONTINUE TO LIMIT CONVERGENCE AND
       OVERALL TSTM COVERAGE. GIVEN STEEP MIDLEVEL LAPSE RATES AND DEEP
       LAYER SHEAR...HAIL AND STRONG WIND GUSTS WILL BE POSSIBLE WITH THE
       MORE SUSTAINED UPDRAFTS.  
       
       CONCERN ALSO EXISTS FOR HEAVY RAINFALL RATES...AS STORMS TRAIN ENEWD
       FROM CENTRAL LA THROUGH EXTREME SRN MS...WITH THE GREATEST THREAT
       AREA EXTENDING WSW-ENE WITHIN A 30-40 MILE WIDE CORRIDOR FROM NRN
       EVANGELINE PARISH LA TO LAMAR COUNTY MS. PRECIPITABLE WATER VALUES
       TO AROUND 1.5 INCH WILL SUPPORT THIS THREAT.
       
       ..PETERS/RACY.. 01/17/2012
    

    Example of Severe Potential Mesoscale Discussion - Watch in Effect

       MESOSCALE DISCUSSION 2049
       NWS STORM PREDICTION CENTER NORMAN OK
       1027 PM CST TUE NOV 16 2010
       
       AREAS AFFECTED...PORTIONS CNTRL/ERN SC/NC/VA...SRN MD
       
       CONCERNING...TORNADO WATCH 741...
       
       VALID 170427Z - 170630Z
       
       THE SEVERE WEATHER THREAT FOR TORNADO WATCH 741 CONTINUES.
    
       SUMMARY...SVR POTENTIAL WILL CONTINUE OVER ERN PORTIONS WW WITH BOWS 
       EMBEDDED WITHIN SEVERAL BROKEN LINE SEGMENTS...AND MAY SHIFT SOMEWHAT    
       E OR NE OF WW INTO LOWER POTOMAC REGION AND COASTAL PLAIN SWD TO SC 
       BY 06-07Z.    
       
       DISCUSSION...SFC ANALYSIS INDICATES COLD FRONT CONTINUING TO MOVE EWD 
       ACROSS W-CENTRAL PORTIONS SC/NC AND FROM SWRN INTO CENTRAL  
       VA...IMMEDIATELY BEHIND LAST BANDS OF CONVECTION IN THESE AREAS. 
       TSTMS HAVE PRODUCED SEVERAL REPORTS OF TREES DOWN DURING PAST 2 HOURS.  
       MEASURED GUSTS UP TO 58 KT ALSO WERE REPORTED IN APPOMATTOX COUNTY VA
       AT 0331Z...WITH BOW NOW MOVING NEWD ABOUT 50 KT TOWARD TIDAL POTOMAC 
       REGION. AIR MASS AHEAD OF TSTM BANDS HAS BEEN DESTABILIZING TO AT 
       LEAST MRGL LEVELS FOR CONTINUE CONVECTIVE POTENTIAL...AS LOW-LEVEL 
       THETAE ADVECTION OFFSETS SFC DIABATIC COOLING...WHICH ALSO IS KEEPING 
       LIFTED PARCELS EITHER SFC-BASED OR NEARLY SO IN IMMEDIATE INFLOW 
       REGION.  MODIFIED RAOBS AND FCST SOUNDINGS SUGGEST ONLY 200-500 J/KG 
       MLCAPE IN NARROW STRIP FROM CENTRAL NC TO CENTRAL/ERN VA.  45-55 KT  
       EFFECTIVE SHEAR MAGNITUDE SHOULD BE MAINTAINED ALONG CORRESPONDING 
       INSTABILITY AXIS.  WHILE BUOYANCY GENERALLY WILL DIMINISH WITH TIME 
       AND WITH EWD EXTENT...THIS PROCESS WILL BE GRADUAL ENOUGH THAT SVR 
       POTENTIAL MAY EXTEND SOMEWHAT PAST CURRENT ERN BOUNDS OF WW.
       
       PRIND SVR POTENTIAL WILL CONTINUE TO CLEAR OUT OF WW FROM W-E BEHIND
       COLD FRONT AS LOW-LEVEL FLOW VEERS...AND AS BOUNDARY LAYER AIR MASS
       STABILIZES DUE TO COMBINATION OF CAA AND DIABATIC SFC COOLING.
       
       ..EDWARDS.. 11/17/2010
    

    Example of Severe Potential Mesoscale Discussion - Winter Weather

       MESOSCALE DISCUSSION 0088
       NWS STORM PREDICTION CENTER NORMAN OK
       1222 PM CST SAT FEB 05 2011
       
       AREAS AFFECTED...S-CNTRL/SERN NY
       
       CONCERNING...FREEZING RAIN 
       
       VALID 051822Z - 052215Z
       
       SUMMARY...FREEZING RAIN RATES ARE EXPECTED TO INCREASE TO 0.05-0.10
       IN/HR AFTER 20Z. EMBEDDED CONVECTIVE BURSTS MAY YIELD LOCALLY
       HIGHER RATES AT TIMES.
       
       DISCUSSION...LARGE SWATH OF LIGHT-MODERATE PRECIPITATION CONTINUES TO     
       EXPAND N/EWD INTO CNTRL/ERN NY AHEAD OF AN APPROACHING MIDLEVEL CYCLONE
       POSITIONED OVER THE CNTRL OH RIVER VALLEY. STRENGTHENING LOW-LEVEL
       WAA PROFILES /AHEAD OF 50-60 KT 850 MB LLJ/ AND ISENTROPIC UPGLIDE
       MAXIMIZED IN THE 285-295 K LAYER ATOP SUBFREEZING SURFACE
       TEMPERATURES...COMBINED WITH INCREASING MID-LEVEL DCVA...WILL ACT TO
       INCREASE PRECIPITATION RATES ACROSS THE REGION.
       
       ADDITIONALLY...MODERATELY STEEP MIDLEVEL LAPSE RATES OVER THE REGION
       WILL FAVOR OCCASIONAL EMBEDDED CONVECTIVE ELEMENTS LEADING TO
       LOCALLY HIGHER RATES. NAMKF AND RUC POINT FORECAST SOUNDINGS
       INDICATE SIGNIFICANT WARMING WILL OCCUR IN THE 2-6 KFT AGL LAYER
       ACROSS SERN NY...REINFORCING A FAVORED P-TYPE OF FREEZING RAIN. SURFACE
       FREEZING LINE IS EXPECTED TO SHIFT NWD WITH TIME...AND WILL YIELD
       POCKETS OF ABOVE FREEZING TEMPERATURES ACROSS SRN PORTIONS OF THE
       AREA BY THIS AFTERNOON.
       
       ..ROGERS.. 02/05/2011
    
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    Page last modified: March 22, 2012
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