The question of what season the gullies form may be decided by THEMIS on Mars Odyssey. BTW, the images by Bleier here are around 50S, though there are gullies in the 70S range of latitudes.

At least three released THEMIS images show gullies at the instruments visible imaging scale:

Gullied Craters 41°S (Released 28 March 2002)
http://themis.asu.edu/zoom-20020328a.html

Nirgal Vallis (Released 27 March 2002)
http://themis.la.asu.edu/zoom-20020327a.html

Gullies of Gorgonus Chaos (Released 11 June 2002)
http://themis.la.asu.edu/zoom-20020611a.html

It is interesting that the second describes a haze over the area where the gullies form:

"Some gullies appear to originate in the uppermost layers and others in lower layers. An atmospheric haze is also visible in the poleward facing slopes of the craters. This haze is visible in the original data but has been enhanced by image processing."

The gullies are likewise known to be poleward facing.
"Haze" is a rather indistinct term that can mean either low-lying atmospheric dust or fog.
However, it is known that low lying fogs emanate diurnally from the ground on Mars as the temperature changes. Depending on altitude and the surrounding atmospheric pressure, it is my view that low lying fogs on Mars consist at least in part of supercooled liquid water droplets at "temperate" latitudes.
It is now local Summer at this southern hemispheric site. It would be interesting to find out what was the THEMIS measured temperature at the site when the image was taken.

Bob Clark