Current Heathrow TAF is as follows:
EGLL 121056Z 1212/1318 23008KT 9999 -RA BKN020 TEMPO 1212/1216 4000 RA BKN008 BECMG 1215/1217 33016G26KT TEMPO 1216/1220 1200 SNRA BKN004 PROB30 TEMPO 1217/1220 33022G36KT 0500 SN BKN001 BECMG 1219/1222 29012KT 9999 NSW FEW020 TEMPO 1307/1309 30018G28KT 1200 -SN BKN004 BECMG 1309/1311 31020G32KT
So they expect rain until 4pm then rain/snow until 8pm with lowish (1.2km) visibility and a chance snow will drop visibility to 500m. If that happened they'd definitely need to space out flights more and thus cancel a few.