GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260510_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)
=== ===== === ==== ==== === === === ====
Sun 05/10 12Z 0.40 2317  95  75   3 0.00
Sun 05/10 15Z 0.71 2419  94  89   3 0.07
Sun 05/10 18Z 1.13 2517  82  46   2 0.04
Sun 05/10 21Z 8.19 2722  46  59   3 0.01
Mon 05/11 00Z 1.39 2821  54  27   0 0.00
Mon 05/11 03Z 0.98 3017  68  29  -1 0.00
Mon 05/11 06Z 0.52 3012  73  24  -2 0.00
Mon 05/11 09Z 0.41 2910  74  21  -2 0.00
Mon 05/11 12Z 0.40 2909  76  35  -3 0.00
Mon 05/11 15Z 2.10 2911  61  62  -2 0.00
Mon 05/11 18Z 3.98 2812  59  98   0 0.00
Mon 05/11 21Z 0.85 3012  81  94  -1 0.02
Tue 05/12 00Z 0.47 3213  75  83  -1 0.01
Tue 05/12 03Z 0.38 3313  68  76  -2 0.00
Tue 05/12 06Z 0.27 3413  68  67  -2 0.00
Tue 05/12 09Z 0.26 3312  68  47  -3 0.00
Tue 05/12 12Z 0.37 3214  71  31  -4 0.00
Tue 05/12 15Z 1.55 3114  66  33  -3 0.00
Tue 05/12 18Z 3.41 3015  57  49  -1 0.00
Tue 05/12 21Z 1.23 3113  57  42   0 0.00
Wed 05/13 00Z 0.37 3209  59  29   0 0.00
Wed 05/13 03Z 0.18 2904  58  22   0 0.00
Wed 05/13 06Z 0.13 2007  58  20   0 0.00
Wed 05/13 09Z 0.19 2013  56  16   1 0.00

Run total areal avg QPF for the greens is: 0.15 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