GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20251118_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)
=== ===== === ==== ==== === === === ====
Tue 11/18 00Z 1.06 2930  94  88  -9 0.00
Tue 11/18 03Z 0.94 2927  93  82  -9 0.03
Tue 11/18 06Z 0.73 2926  91  82  -9 0.03
Tue 11/18 09Z 0.60 2924  92  76  -8 0.03
Tue 11/18 12Z 0.55 2923  90  69  -8 0.02
Tue 11/18 15Z 0.53 2918  92  52  -8 0.02
Tue 11/18 18Z 0.75 2817  87  51  -8 0.01
Tue 11/18 21Z 0.64 2818  85  39  -7 0.00
Wed 11/19 00Z 0.42 2915  84  31  -7 0.00
Wed 11/19 03Z 0.35 2812  85  27  -7 0.00
Wed 11/19 06Z 0.29 2810  78  16  -6 0.00
Wed 11/19 09Z 0.24 2910  75   7  -6 0.00
Wed 11/19 12Z 0.21 3009  74   6  -6 0.00
Wed 11/19 15Z 0.12 3107  66   8  -5 0.00
Wed 11/19 18Z 0.12 3105  66   8  -5 0.00
Wed 11/19 21Z 0.12 3107  57   9  -3 0.00
Thu 11/20 00Z 0.08 3206  52  14  -3 0.00
Thu 11/20 03Z 0.05 3203  50  16  -2 0.00
Thu 11/20 06Z 0.06 2702  47  38  -2 0.00
Thu 11/20 09Z 0.07 2603  44  51  -1 0.00
Thu 11/20 12Z 0.04 2202  40  56  -1 0.00
Thu 11/20 15Z 0.02 1905  37  66   0 0.00
Thu 11/20 18Z 0.05 1907  40  57   0 0.00
Thu 11/20 21Z 0.07 2008  38  42   0 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