GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260914_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)
=== ===== === ==== ==== === === === ====
Mon 09/14 06Z 0.77 2923  67  92  10 0.00
Mon 09/14 09Z 0.76 3025  87  35   6 0.00
Mon 09/14 12Z 0.59 3124  92  14   3 0.00
Mon 09/14 15Z 0.78 3220  87   9   2 0.01
Mon 09/14 18Z 1.64 3221  65  18   3 0.00
Mon 09/14 21Z 1.69 3220  68  19   3 0.00
Tue 09/15 00Z 0.49 3319  76  23   3 0.00
Tue 09/15 03Z 0.17 3415  73  11   3 0.00
Tue 09/15 06Z 0.05 3510  63   8   5 0.00
Tue 09/15 09Z 0.03 3503  56  11   6 0.00
Tue 09/15 12Z 0.02 1803  47  14   7 0.00
Tue 09/15 15Z 0.07 1908  48  15   8 0.00
Tue 09/15 18Z 0.23 1912  51  19   7 0.00
Tue 09/15 21Z 0.21 1914  48  17   9 0.00
Wed 09/16 00Z 0.30 2119  44  63  11 0.00
Wed 09/16 03Z 0.42 2221  45  92  12 0.00
Wed 09/16 06Z 0.47 2322  45  89  13 0.00
Wed 09/16 09Z 0.47 2520  49  93  13 0.00
Wed 09/16 12Z 0.48 2618  62  72  13 0.00
Wed 09/16 15Z 0.40 2715  79  75  13 0.00
Wed 09/16 18Z 0.35 2710  84  56  12 0.00
Wed 09/16 21Z 0.44 2810  85  20  11 0.00
Thu 09/17 00Z 0.46 2810  82  13  11 0.00
Thu 09/17 03Z 0.31 3008  79   9  10 0.00

Run total areal avg QPF for the greens is: 0.01 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