GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20250501_1800
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 05/01 18Z 0.36 2106  32  18   5 0.00
Thu 05/01 21Z 0.22 2007  34  43   7 0.00
Fri 05/02 00Z 0.07 1712  38  81   9 0.00
Fri 05/02 03Z 0.14 1921  52  95   9 0.00
Fri 05/02 06Z 0.27 2026  68  98   8 0.02
Fri 05/02 09Z 0.25 2026  86  94  10 0.01
Fri 05/02 12Z 0.30 2125  96  95  10 0.01
Fri 05/02 15Z 0.46 2322  97  72   9 0.04
Fri 05/02 18Z 0.73 2421  96  55   8 0.00
Fri 05/02 21Z 1.21 2423  87  78   9 0.01
Sat 05/03 00Z 1.05 2425  82  75  10 0.03
Sat 05/03 03Z 0.99 2524  88  75   9 0.01
Sat 05/03 06Z 0.86 2521  91  79   8 0.00
Sat 05/03 09Z 0.62 2617  96  55   7 0.01
Sat 05/03 12Z 0.44 2613  94  73   6 0.00
Sat 05/03 15Z 0.39 2610  93  95   6 0.02
Sat 05/03 18Z 0.23 2607  90  99   6 0.04
Sat 05/03 21Z 0.12 2905  90  97   7 0.10
Sun 05/04 00Z 0.03 3001  91  97   7 0.07
Sun 05/04 03Z 0.03 1805  94  97   7 0.04
Sun 05/04 06Z 0.06 1907  98  98   8 0.02
Sun 05/04 09Z 0.06 1908  99  99   8 0.10
Sun 05/04 12Z 0.08 2007  99  95   8 0.08
Sun 05/04 15Z 0.09 2105  99  87   8 0.07

Run total areal avg QPF for the greens is: 0.68 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