GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20251120_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)
=== ===== === ==== ==== === === === ====
Thu 11/20 12Z 0.04 2601  54  48  -2 0.00
Thu 11/20 15Z 0.03 2102  49  57  -2 0.00
Thu 11/20 18Z 0.05 2103  48  57  -1 0.00
Thu 11/20 21Z 0.04 2004  43  58  -1 0.00
Fri 11/21 00Z 0.05 2006  38  55   0 0.00
Fri 11/21 03Z 0.09 2009  34  61   0 0.00
Fri 11/21 06Z 0.16 2112  38  63   0 0.00
Fri 11/21 09Z 0.23 2214  49  42   0 0.00
Fri 11/21 12Z 0.35 2317  66  71  -1 0.00
Fri 11/21 15Z 0.33 2219  88  54   0 0.00
Fri 11/21 18Z 0.55 2421  96  46   0 0.00
Fri 11/21 21Z 0.79 2525  96   8   0 0.00
Sat 11/22 00Z 1.04 2624  94   9  -2 0.01
Sat 11/22 03Z 0.86 2721  93   9  -2 0.01
Sat 11/22 06Z 0.61 2716  92  10  -3 0.00
Sat 11/22 09Z 0.51 2813  96  11  -4 0.00
Sat 11/22 12Z 0.35 3111  86   6  -6 0.00
Sat 11/22 15Z 0.25 3210  78   7  -6 0.00
Sat 11/22 18Z 0.58 3111  79  12  -7 0.00
Sat 11/22 21Z 0.53 3014  90  21  -7 0.00
Sun 11/23 00Z 0.52 3117  96  10  -8 0.02
Sun 11/23 03Z 0.32 3115  81  31  -7 0.00
Sun 11/23 06Z 0.26 3013  73  33  -6 0.00
Sun 11/23 09Z 0.25 2912  69  18  -5 0.00

Run total areal avg QPF for the greens is: 0.04 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