Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20251115_0000
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)
=== ===== === ==== ==== === === === ====
Sat 11/15 00Z 0.32 3315  91  92  -8 0.00
Sat 11/15 03Z 0.28 3314  93  65  -8 0.01
Sat 11/15 06Z 0.21 3314  89  37  -8 0.00
Sat 11/15 09Z 0.24 3213  90  22  -8 0.00
Sat 11/15 12Z 0.20 3212  86  10  -8 0.00
Sat 11/15 15Z 0.17 3108  78  12  -6 0.00
Sat 11/15 18Z 0.19 2706  68   9  -4 0.00
Sat 11/15 21Z 0.12 2505  52  84  -1 0.00
Sun 11/16 00Z 0.12 2110  54  96   0 0.00
Sun 11/16 03Z 0.05 1815  97  98   1 0.10
Sun 11/16 06Z 0.14 2017  99  98   2 0.24
Sun 11/16 09Z 0.42 2420  99  81   2 0.11
Sun 11/16 12Z 0.87 2625  99  73   0 0.02
Sun 11/16 15Z 1.11 2724  97  87  -4 0.03
Sun 11/16 18Z 4.68 2826  91  89  -5 0.03
Sun 11/16 21Z 1.55 2929  90  93  -7 0.03
Mon 11/17 00Z 1.49 2830  94  93  -8 0.04
Mon 11/17 03Z 1.24 2929  94  93  -9 0.06
Mon 11/17 06Z 1.23 2931  94  90  -9 0.06
Mon 11/17 09Z 1.15 2929  94  88 -10 0.06
Mon 11/17 12Z 0.97 3028  94  83  -9 0.05
Mon 11/17 15Z 1.13 3027  92  73  -9 0.04
Mon 11/17 18Z 1.67 3028  91  71  -9 0.04
Mon 11/17 21Z 1.31 3029  94  53  -9 0.02

Run total areal avg QPF for the greens is: 0.94 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