GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20250524_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)
=== ===== === ==== ==== === === === ====
Sat 05/24 06Z 0.29 2708 100  95   0 0.00
Sat 05/24 09Z 0.31 2910  99  94   0 0.05
Sat 05/24 12Z 0.29 3112  99  93  -1 0.02
Sat 05/24 15Z 0.29 3213  98  94  -2 0.01
Sat 05/24 18Z 0.34 3113  98  95  -1 0.01
Sat 05/24 21Z 0.38 3014  99  94  -1 0.02
Sun 05/25 00Z 0.41 3116  99  92  -1 0.02
Sun 05/25 03Z 0.36 3217  99  88  -2 0.01
Sun 05/25 06Z 0.40 3116  99  81  -2 0.01
Sun 05/25 09Z 0.37 3217  99  76  -2 0.01
Sun 05/25 12Z 0.26 3316  99  68  -2 0.00
Sun 05/25 15Z 0.23 3315  99  68  -1 0.00
Sun 05/25 18Z 0.29 3316  98  71   0 0.01
Sun 05/25 21Z 0.32 3315  94  75   1 0.01
Mon 05/26 00Z 0.25 3416  85  71   1 0.01
Mon 05/26 03Z 0.18 3413  82  76   1 0.00
Mon 05/26 06Z 0.23 3311  90  77   1 0.00
Mon 05/26 09Z 0.12 3412  86  79   2 0.00
Mon 05/26 12Z 0.07 3411  85  76   2 0.00
Mon 05/26 15Z 0.15 3508  84  80   2 0.02
Mon 05/26 18Z 0.12 3508  76  78   4 0.03
Mon 05/26 21Z 0.10 3509  72  72   5 0.03
Tue 05/27 00Z 0.06 3508  68  68   5 0.01
Tue 05/27 03Z 0.15 3308  62  72   6 0.00

Run total areal avg QPF for the greens is: 0.28 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