GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260111_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)
=== ===== === ==== ==== === === === ====
Sun 01/11 18Z 1.02 2616  83  73  -8 0.00
Sun 01/11 21Z 1.40 2522  88  91  -8 0.01
Mon 01/12 00Z 1.23 2624  87  90 -10 0.01
Mon 01/12 03Z 1.36 2830  95  94 -13 0.03
Mon 01/12 06Z 1.30 2934  94  85 -15 0.02
Mon 01/12 09Z 0.83 3033  91  32 -14 0.01
Mon 01/12 12Z 0.60 2929  65  31 -12 0.00
Mon 01/12 15Z 0.61 2826  86  78 -10 0.00
Mon 01/12 18Z 0.86 2626  95  81 -10 0.00
Mon 01/12 21Z 1.17 2529  97  89  -9 0.02
Tue 01/13 00Z 1.17 2529  94  86  -9 0.02
Tue 01/13 03Z 0.98 2627  95  64  -9 0.01
Tue 01/13 06Z 0.85 2625  94  63  -8 0.01
Tue 01/13 09Z 0.67 2721  95  66  -7 0.01
Tue 01/13 12Z 0.55 2619  94  34  -7 0.01
Tue 01/13 15Z 0.39 2515  91  22  -6 0.00
Tue 01/13 18Z 0.30 2314  87  16  -4 0.00
Tue 01/13 21Z 0.26 2118  78  51  -2 0.00
Wed 01/14 00Z 0.27 2025  61  87   0 0.00
Wed 01/14 03Z 0.44 2134  62  92  -1 0.00
Wed 01/14 06Z 0.68 2132  86  94  -3 0.02
Wed 01/14 09Z 0.61 2227  96  94  -3 0.03
Wed 01/14 12Z 0.53 2322  95  86  -3 0.02
Wed 01/14 15Z 0.48 2319  98  80  -2 0.00

Run total areal avg QPF for the greens is: 0.23 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