Snow in the GFS, NAM, and the NAM AK/CONUS nests is updated once per day 
using snow cover from the National Ice Center's Interactive
Multisensor Snow and Ice Mapping System (IMS) and snow depth
from the Air Force Weather Agency's SNODEP analysis model. 
The IMS product is a snow cover analysis (yes/no flag) at 4 km resolution.
It is northern hemisphere only and produced once per day at 22Z.
The AFWA product is a global physical snow depth analysis at 23 km.
It is produced daily (around 15Z).

The GFS snow is updated at the 00Z cycle.  The NAM and its nests at
the T-minus 6-hour point of the 06Z cycle.  The process is:

1) Create a snow depth analysis on the model physics grid:
Interpolate the IMS and AFWA data to the model grid using a 
"budget" interpolation method.  In the northern hemisphere, if the
IMS indicates snow cover, then the depth is set to 5 cm or the
AFWA depth, whichever is greater.  Otherwise, the depth is set to
zero.  In the southern hemisphere the depth is set to the AFWA depth.
This analysis is called the "external" analysis.

2) The model first-guess depth is then nudged to the external
analysis created in step (1).  If the model depth is greater than
twice the external analysis depth, the model depth is reduced to twice
the external depth.  If the model depth is less than one-half
the external analysis depth, then the model depth is increated to
one-half the external depth.  Otherwise the model snow depth
is not adjusted.  Two points about this update: (1) The model snow
is not adjusted unless it deviates greatly from the external analysis
(because we believe the model's snow pack evolution has skill).
(2) In the NH, the model coverage will always match the IMS (we
believe the IMS accurately depicts snow/no-snow areas).

3) GFS ONLY: Areas indicated as "permanant land ice/snow" according to the
model's vegetation type (like Greenland and Antarctica), are given
a depth of a least 1 meter.

4) Liquid equivalent is set according to a 10:1 ratio in GFS and a
5:1 ratio in NAM.

Between update times the models cycle the snow pack (i.e., it is accumulated
and melted thru the model's physical processes).