GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260531_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)
=== ===== === ==== ==== === === === ====
Sun 05/31 06Z 0.08 3511  47  65   4 0.00
Sun 05/31 09Z 0.34 3012  56  92   4 0.00
Sun 05/31 12Z 0.45 2917  62  87   5 0.00
Sun 05/31 15Z 0.50 3115  77  87   3 0.02
Sun 05/31 18Z 0.98 3018  82  90   3 0.03
Sun 05/31 21Z 1.04 3022  83  87   2 0.06
Mon 06/01 00Z 0.79 3119  77  74   3 0.03
Mon 06/01 03Z 0.41 3216  76  76   3 0.01
Mon 06/01 06Z 0.20 3312  86  74   2 0.00
Mon 06/01 09Z 0.18 0110  92  74   2 0.00
Mon 06/01 12Z 0.27 0411  91  55   1 0.00
Mon 06/01 15Z 0.59 0509  85  41   1 0.01
Mon 06/01 18Z 0.92 0207  67  68   3 0.01
Mon 06/01 21Z 0.12 0008  61  79   4 0.01
Tue 06/02 00Z 0.10 3408  61  61   5 0.01
Tue 06/02 03Z 0.10 3509  63  55   4 0.00
Tue 06/02 06Z 0.18 3207  63  61   4 0.00
Tue 06/02 09Z 0.33 3112  68  75   4 0.00
Tue 06/02 12Z 0.25 3214  64  62   5 0.00
Tue 06/02 15Z 0.37 3313  60  44   4 0.00
Tue 06/02 18Z 1.29 3416  49  52   5 0.00
Tue 06/02 21Z 0.37 3516  51  54   5 0.00
Wed 06/03 00Z 0.11 0018  53  73   6 0.00
Wed 06/03 03Z 0.29 0116  51  78   6 0.00

Run total areal avg QPF for the greens is: 0.19 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