I wish I had kept the METARS but across northern Scotland last week, there was the scenario of SN and +ve air being reported simultaneously.
Isn't the reverse what you want to demonstrate your point -- that RASN can occur in negative temperatures? I don't think there's much doubt that SN can occur when the OAT is above zero -- it just hasn't had time to melt yet.
Let me help with the stats. Here are the northern Europe (codes beginning with E) METARs containing sleet for the last 30 days.
Code:
wx | temperature | count
----------------+-------------+-------
-RASN | -2 | 3
-RASN | -1 | 8
-RASN | 0 | 238
-RASN | 1 | 645
-RASN | 2 | 346
-RASN | 3 | 107
-RASN | 4 | 21
-RASN | 5 | 1
-RASN | 6 | 3
Code:
wx | temperature | count
----------------+-------------+-------
RASN | -1 | 6
RASN | 0 | 35
RASN | 1 | 116
RASN | 2 | 87
RASN | 3 | 24
RASN | 4 | 6
The vast majority of RASN occur in positive temperatures. Compare that with FZRA
Code:
wx | temperature | count
----------------+-------------+-------
-FZRA | -10 | 2
-FZRA | -9 | 3
-FZRA | -8 | 12
-FZRA | -7 | 11
-FZRA | -6 | 16
-FZRA | -5 | 8
-FZRA | -4 | 15
-FZRA | -3 | 11
-FZRA | -2 | 14
-FZRA | -1 | 38
-FZRA | -0 | 38
-FZRA | 1 | 1
-FZRA | 2 | 1
Is it worth paying attention to icing when there's RASN around? You bet it is. But most of the time, snow melting into sleet is not a problem.