GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260502_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)
=== ===== === ==== ==== === === === ====
Sat 05/02 18Z 0.29 2606  94  79  -3 0.00
Sat 05/02 21Z 0.60 2608  84  88  -2 0.02
Sun 05/03 00Z 0.91 2912  82  73  -3 0.02
Sun 05/03 03Z 0.35 3110  81  57  -4 0.01
Sun 05/03 06Z 0.20 3310  87  53  -4 0.00
Sun 05/03 09Z 0.16 3312  91  49  -5 0.00
Sun 05/03 12Z 0.21 3314  86  49  -5 0.00
Sun 05/03 15Z 1.86 3217  68  49  -4 0.00
Sun 05/03 18Z 2.35 3119  61  63  -3 0.00
Sun 05/03 21Z 6.29 3020  64  74  -2 0.00
Mon 05/04 00Z 1.50 2919  64  49  -2 0.00
Mon 05/04 03Z 0.95 2721  66  70  -1 0.00
Mon 05/04 06Z 0.58 2619  92  91  -1 0.01
Mon 05/04 09Z 0.64 2423  98  86   0 0.02
Mon 05/04 12Z 0.74 2425  98  91   0 0.06
Mon 05/04 15Z 0.97 2626  98  16   0 0.02
Mon 05/04 18Z 1.41 2625  88  38   2 0.01
Mon 05/04 21Z 1.45 2522  85  32   4 0.01
Tue 05/05 00Z 1.15 2423  77  42   5 0.00
Tue 05/05 03Z 0.94 2328  73  22   6 0.00
Tue 05/05 06Z 0.89 2332  78   7   6 0.00
Tue 05/05 09Z 0.66 2230  82  76   7 0.00
Tue 05/05 12Z 0.66 2328  88  81   7 0.00
Tue 05/05 15Z 0.84 2326  94  82   7 0.00

Run total areal avg QPF for the greens is: 0.18 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