GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260902_1200
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)
=== ===== === ==== ==== === === === ====
Wed 09/02 12Z 0.03 1812  92  38  12 0.00
Wed 09/02 15Z 0.04 1812  92  41  12 0.00
Wed 09/02 18Z 0.04 1714  90  57  11 0.00
Wed 09/02 21Z 0.11 1819  87  63  11 0.00
Thu 09/03 00Z 0.06 1823  88  89  12 0.06
Thu 09/03 03Z 0.21 1926  94  95  12 0.06
Thu 09/03 06Z 0.32 2125  98  92  13 0.14
Thu 09/03 09Z 0.45 2223  99  85  13 0.09
Thu 09/03 12Z 0.51 2518  99  63  13 0.02
Thu 09/03 15Z 0.47 2614  94  29  12 0.01
Thu 09/03 18Z 1.50 2715  76  27  13 0.02
Thu 09/03 21Z 3.84 2916  61  32  14 0.00
Fri 09/04 00Z 0.98 3017  68  32  13 0.00
Fri 09/04 03Z 0.48 3010  70  36  12 0.00
Fri 09/04 06Z 0.42 2909  71  42  12 0.00
Fri 09/04 09Z 0.40 2810  77  36  12 0.00
Fri 09/04 12Z 0.33 2809  81  37  12 0.00
Fri 09/04 15Z 0.59 2910  81  37  11 0.01
Fri 09/04 18Z 1.31 3013  73  38  12 0.01
Fri 09/04 21Z 0.75 3216  73  51  11 0.00
Sat 09/05 00Z 0.51 3220  78  49  10 0.01
Sat 09/05 03Z 0.31 3317  83  71   9 0.00
Sat 09/05 06Z 0.20 3315  91  61   8 0.00
Sat 09/05 09Z 0.10 3414  94  65   8 0.00

Run total areal avg QPF for the greens is: 0.43 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