GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20250905_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)
=== ===== === ==== ==== === === === ====
Fri 09/05 18Z 1.35 2024  60  10  12 0.00
Fri 09/05 21Z 5.19 2027  64  25  12 0.00
Sat 09/06 00Z 1.24 2131  64  74  13 0.00
Sat 09/06 03Z 0.91 2134  69  59  13 0.00
Sat 09/06 06Z 1.13 2330  70  70  13 0.01
Sat 09/06 09Z 0.71 2420  77  50  12 0.00
Sat 09/06 12Z 0.44 2315  76  67  11 0.00
Sat 09/06 15Z 0.40 2410  83  96  10 0.01
Sat 09/06 18Z 0.28 2410  92  95   9 0.06
Sat 09/06 21Z 0.44 2616  89  87   8 0.10
Sun 09/07 00Z 0.39 2612  79  58   7 0.00
Sun 09/07 03Z 0.35 2611  73  53   7 0.00
Sun 09/07 06Z 0.44 2612  70  33   7 0.00
Sun 09/07 09Z 0.38 2510  70  28   7 0.00
Sun 09/07 12Z 0.41 2311  77  24   6 0.00
Sun 09/07 15Z 0.79 2413  76  31   5 0.01
Sun 09/07 18Z 2.07 2616  67  52   6 0.01
Sun 09/07 21Z 3.58 2715  69  74   6 0.01
Mon 09/08 00Z 1.12 2817  70  63   5 0.01
Mon 09/08 03Z 1.03 2717  64  44   4 0.00
Mon 09/08 06Z 0.79 2716  60  59   4 0.00
Mon 09/08 09Z 0.91 2720  68  54   4 0.00
Mon 09/08 12Z 0.74 2717  80  19   3 0.00
Mon 09/08 15Z 1.13 2713  82  24   2 0.00

Run total areal avg QPF for the greens is: 0.22 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