With the air colder than ISA the column of air between the airfield and QNH will be shorter and so QNH will be found at a higher level/lower pressure:

                        Airfield level (QFE)

 

                                                            Cold

                                                            Air

                                                                               ISA           Warm

                                         Cold QNH                                       Air

                                         (less than ISA)

                 ISA QNH

 

 

                                                                        Warm QNH

                                                                        (more than ISA)