GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20251225_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 12/25 12Z 0.61 2723  82  70  -7 0.00
Thu 12/25 15Z 0.66 3021  95  65  -9 0.04
Thu 12/25 18Z 0.87 3227  94  26 -13 0.03
Thu 12/25 21Z 0.78 3332  96  21 -17 0.01
Fri 12/26 00Z 0.35 3331  94  14 -18 0.01
Fri 12/26 03Z 0.18 3427  87   6 -18 0.00
Fri 12/26 06Z 0.11 3426  79   6 -17 0.00
Fri 12/26 09Z 0.03 3522  62   4 -15 0.00
Fri 12/26 12Z 0.02 0017  53   4 -14 0.00
Fri 12/26 15Z 0.05 0111  36   5 -12 0.00
Fri 12/26 18Z 0.04 0205  29   6  -9 0.00
Fri 12/26 21Z 0.05 1003  24  38  -7 0.00
Sat 12/27 00Z 0.06 1406  23  70  -8 0.00
Sat 12/27 03Z 0.02 1606  33  94 -10 0.01
Sat 12/27 06Z 0.02 1706  63  95 -10 0.02
Sat 12/27 09Z 0.03 1902  77  94 -10 0.02
Sat 12/27 12Z 0.03 3201  75  91  -9 0.00
Sat 12/27 15Z 0.03 3504  69  88  -9 0.00
Sat 12/27 18Z 0.02 3505  65  77  -8 0.00
Sat 12/27 21Z 0.05 3408  51  55  -7 0.00
Sun 12/28 00Z 0.05 3412  30  33  -5 0.00
Sun 12/28 03Z 0.05 3413  19  24  -5 0.00
Sun 12/28 06Z 0.06 3413  15  42  -4 0.00
Sun 12/28 09Z 0.08 3413  22  13  -3 0.00

Run total areal avg QPF for the greens is: 0.14 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