The following are National Weather Service criteria for issuing Advisories and Warnings for various weather events. Watches generally are issued with longer lead times in expectation of meeting Warning criteria.

TYPE OF ISSUANCE              WHEN ISSUED FOR SOUTHERN NEW ENGLAND

WINTER WEATHER ADVISORY       4", 5", or 6" of snow mentioned in 24 hours;
                              OR any accretion of freezing rain or freezing
                              drizzle on road surfaces; OR a mix of
                              snow/sleet totaling 4", 5", or 6"; OR
                              blowing/drifting snow occasionally reducing
                              visibility to < 1/4 mile

WINTER STORM WARNING          > 7" of snow or snow/sleet combination
                              mentioned in next 24 hours; OR > 1/2" accretion
                              of freezing rain

BLIZZARD WARNING              For at least 3 hours, falling and/or blowing
                              snow frequently reduces visibility to < 1/4
                              mile AND sustained winds or frequent gusts> 
                              35 mph

WIND CHILL ADVISORY           Wind chill index between -25F and -39F for at
                              least 3 hours using only the sustained wind

WIND CHILL WARNING            Wind chill index > -40F for at least 3 hours
                              using only the sustained wind

WIND ADVISORY                 Sustained winds 31-39 mph for at least 1 hour;
                              OR any gusts to 46-57 mph 
                                   
HIGH WIND WARNING             Sustained winds 40-73 mph for at least 1 hour;
                              OR any gusts > 58 mph

INLAND HIGH WIND WARNING      Sustained winds > 74 mph 
FOR HURRICANE FORCE WINDS

DENSE FOG ADVISORY            Widespread visibility < 1/4 mile for at least
                              3 hours

SMALL CRAFT ADVISORY          Sustained wind 25-33 knots AND/OR Seas > 7 \
                              feet within 12 hours                    

GALE WARNING                  Sustained wind 34-47 knots within 24 hours from
                              a non-tropical system

STORM WARNING                 Sustained wind > 48 knots within 24 hours from
                              a non-tropical system

TROPICAL STORM WARNING        Sustained winds of 39-73 mph (34-63 knots)
                              within 24 hours

HURRICANE WARNING             Sustained winds > 74 mph (64 knots) within 24
                              hours; implies dangerous storm surge

SPECIAL MARINE WARNING        Brief/sudden occurrence of sustained wind or
                              frequent gusts > 34 knots, usually associated
                              with thunderstorms; also issued for waterspouts

SEVERE THUNDERSTORM WARNING   Thunderstorms with wind gusts > 58 mph AND/OR
                              hail 3/4" in diameter  

TORNADO WARNING               Likelihood of a tornado within the given area
                              based on radar or actual sighting; usually
                              accompanied by conditions indicated above
                              for "Severe Thunderstorm Warning"

FLASH FLOOD WARNING           A flood that occurs within a few hours of heavy
                              rainfall, a dam or levee failure, or water
                              released from an ice jam

FLOOD WARNING                 Expected inundation of a normally dry area near
                              a stream or other watercourse; OR unusually
                              severe ponding of water expected

RIVER FLOOD WARNING           Expected flooding of main stem rivers (such as
                              the Merrimack, Charles, Connecticut, etc.)

COASTAL FLOOD WARNING         Widespread coastal flooding expected within 12
                              hours; more than just typical overwash