GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260417_0600
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)
=== ===== === ==== ==== === === === ====
Fri 04/17 06Z 0.31 2911  88  89   9 0.00
Fri 04/17 09Z 0.13 3311  94  91   7 0.01
Fri 04/17 12Z 0.03 0011  93  62   7 0.01
Fri 04/17 15Z 0.02 3511  94  44   5 0.00
Fri 04/17 18Z 0.07 0011  84  22   5 0.00
Fri 04/17 21Z 0.20 0109  75  11   5 0.00
Sat 04/18 00Z 0.28 0409  67   7   6 0.00
Sat 04/18 03Z 0.23 0906  60   7   7 0.00
Sat 04/18 06Z 0.32 1414  70   6   6 0.00
Sat 04/18 09Z 0.18 1618  73   6   5 0.00
Sat 04/18 12Z 0.14 1619  76   4   5 0.00
Sat 04/18 15Z 0.25 1523  77   4   6 0.00
Sat 04/18 18Z 0.16 1624  86   5   4 0.00
Sat 04/18 21Z 0.08 1725  84  11   6 0.00
Sun 04/19 00Z 0.09 1826  82  83   8 0.00
Sun 04/19 03Z 0.17 1925  86  95   9 0.01
Sun 04/19 06Z 0.19 1928  92  97   8 0.01
Sun 04/19 09Z 0.27 2123  99  99   6 0.09
Sun 04/19 12Z 0.30 2711 100  99   1 0.19
Sun 04/19 15Z 0.45 2917  98  97  -1 0.16
Sun 04/19 18Z 0.53 2914  97  61  -4 0.06
Sun 04/19 21Z 0.62 2916  96  76  -4 0.03
Mon 04/20 00Z 1.40 2822  79  87  -5 0.04
Mon 04/20 03Z 1.33 2824  77  79  -6 0.01

Run total areal avg QPF for the greens is: 0.62 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