Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260116_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 01/16 18Z 0.45 2819  65   9 -15 0.00
Fri 01/16 21Z 0.36 2815  51  21 -13 0.00
Sat 01/17 00Z 0.24 2511  42  51 -11 0.00
Sat 01/17 03Z 0.24 2214  46  94 -10 0.00
Sat 01/17 06Z 0.32 2121  86  94 -10 0.00
Sat 01/17 09Z 0.29 2120  90  94  -9 0.01
Sat 01/17 12Z 0.21 2019  94  95  -7 0.01
Sat 01/17 15Z 0.20 2020  95  95  -7 0.03
Sat 01/17 18Z 0.27 2118  96  95  -6 0.03
Sat 01/17 21Z 0.40 2415  96  93  -7 0.02
Sun 01/18 00Z 0.61 2518  91  79  -7 0.01
Sun 01/18 03Z 0.63 2420  90  67  -7 0.00
Sun 01/18 06Z 0.59 2320  86  72  -7 0.00
Sun 01/18 09Z 0.75 2225  95  39  -8 0.00
Sun 01/18 12Z 0.72 2423  80  27 -10 0.02
Sun 01/18 15Z 0.86 2520  83  14 -13 0.01
Sun 01/18 18Z 1.02 2514  89  30 -14 0.00
Sun 01/18 21Z 0.68 2513  88  36 -13 0.00
Mon 01/19 00Z 0.33 2607  87  36 -13 0.00
Mon 01/19 03Z 0.25 2705  86  46 -13 0.00
Mon 01/19 06Z 0.24 2805  90  44 -13 0.00
Mon 01/19 09Z 0.23 2506  93  38 -14 0.00
Mon 01/19 12Z 0.28 2210  85  41 -14 0.00
Mon 01/19 15Z 0.62 2215  88  91 -14 0.00

Run total areal avg QPF for the greens is: 0.14 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