Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20251108_1200
Using the MB925 to MB850 layer.
Assumes greens are orthogonal to 270 degree winds.

925-850 mb Layer Average Wind/RH. Avg QPF over the greens.
Date   Time        Wind RH% 700 850T QPF
Day mm/dd hhZ Frd# DDFF (%) RH% (C) (in)
=== ===== === ==== ==== === === === ====
Sat 11/08 12Z 1.28 2724  93  50   0 0.00
Sat 11/08 15Z 1.18 2823  93  16  -1 0.01
Sat 11/08 18Z 1.93 2917  82   2  -2 0.01
Sat 11/08 21Z 0.78 2915  80   4  -3 0.00
Sun 11/09 00Z 0.28 3010  70   7  -4 0.00
Sun 11/09 03Z 0.12 2704  60  41  -3 0.00
Sun 11/09 06Z 0.11 2306  62  92  -2 0.00
Sun 11/09 09Z 0.07 2008  89  93  -1 0.01
Sun 11/09 12Z 0.04 1814  97  93   0 0.04
Sun 11/09 15Z 0.03 1718  99  93   0 0.06
Sun 11/09 18Z 0.10 1620  99  97   0 0.10
Sun 11/09 21Z 0.39 1426  99  91   0 0.04
Mon 11/10 00Z 0.34 1529  99  92   1 0.02
Mon 11/10 03Z 0.21 1523  99  89   3 0.05
Mon 11/10 06Z 0.12 1616  99  58   3 0.03
Mon 11/10 09Z 0.03 1812  98  88   2 0.03
Mon 11/10 12Z 0.04 1808  99  89   1 0.12
Mon 11/10 15Z 0.08 1506  99  93   1 0.04
Mon 11/10 18Z 0.07 2105  99  94   0 0.08
Mon 11/10 21Z 0.09 2703 100  84   0 0.04
Tue 11/11 00Z 0.11 3005  99  94  -1 0.03
Tue 11/11 03Z 0.24 3010  96  83  -4 0.05
Tue 11/11 06Z 0.28 2909  97  85  -6 0.02
Tue 11/11 09Z 0.32 2811  96  60  -8 0.01

Run total areal avg QPF for the greens is: 0.79 inches.

Froude Number (Frd#): 
Frd# < 0.5 Flow is subcritical and blocked.
         Upslope clouds/precip backed farther upwind of and up to mtn crest.
         W Upslope snow favors eastern Champlain Valley, BTV, and Western slopes.
         Gap winds possible.
Frd# 0.5-1 Flow is subcritical/slow moving/blocked.
         Upslope clouds/precip falls immediately upwind of mtn crest.
         W Upslope snow favors western slopes to the Green Mt. spine including:
         Montgomery,Underhill,Jericho,Bolton,Richmond,Huntington,Lincoln.
         Gap winds possible.
Frd# 1-2 Flow is critical.
         With strong winds, Mountain waves/downslope winds possible.
         Precip falls close to mountain ridge crests and on lee side.
         W Upslope snow favors Green Mt Spine,Jay,Stowe,Waterbury,NE Kingdom.
Froude > 2 Flow is supercritical/unblocked(rapid flow).
         Air flows freely over terrain.
         Persistent upslope snow not favored. Scattered snow showers and flurries.

Relative Humidity (RH): 
         925-850 mb RH > 90% needed for upslope precipitation.
         with 700mb RH > 70% favors upslope snow.
         with 700mb RH > 90% greater amounts of W upslope snow possible.

850mb Temperature (T degC) Westerly Upslope Snow Ratios: (Avg=28:1)
        T > -11       10-15:1
        T -11 to -15  25-35:1
        T < -15       15-20:1