GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20251006_0000
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)
=== ===== === ==== ==== === === === ====
Mon 10/06 00Z 0.50 2312  56  38  15 0.00
Mon 10/06 03Z 0.62 2316  55  27  14 0.00
Mon 10/06 06Z 0.72 2320  53  19  14 0.00
Mon 10/06 09Z 0.65 2420  43  17  14 0.00
Mon 10/06 12Z 0.56 2417  36  14  14 0.00
Mon 10/06 15Z 0.41 2413  44  13  13 0.00
Mon 10/06 18Z 0.57 2311  50  11  13 0.00
Mon 10/06 21Z 0.51 2215  48   7  14 0.00
Tue 10/07 00Z 0.91 2320  49   9  13 0.00
Tue 10/07 03Z 1.12 2422  54  16  13 0.00
Tue 10/07 06Z 0.84 2422  54  18  13 0.00
Tue 10/07 09Z 0.68 2419  58  15  13 0.00
Tue 10/07 12Z 0.63 2321  64  26  13 0.00
Tue 10/07 15Z 0.55 2220  69  48  13 0.00
Tue 10/07 18Z 1.20 2223  77  81  12 0.00
Tue 10/07 21Z 0.80 2525  91  98  11 0.25
Wed 10/08 00Z 0.77 2325  93  86  10 0.17
Wed 10/08 03Z 0.75 2325  97  98  10 0.11
Wed 10/08 06Z 0.51 2716  98  98   9 0.35
Wed 10/08 09Z 0.27 3317  91  95   6 0.17
Wed 10/08 12Z 0.26 3316  81  53   4 0.02
Wed 10/08 15Z 0.26 3315  79   7   2 0.00
Wed 10/08 18Z 0.81 3317  72   2   0 0.00
Wed 10/08 21Z 2.80 3321  63   3   0 0.00

Run total areal avg QPF for the greens is: 1.07 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