Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20251204_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)
=== ===== === ==== ==== === === === ====
Thu 12/04 18Z 1.59 2925  93  95 -15 0.00
Thu 12/04 21Z 2.87 2931  88  52 -20 0.02
Fri 12/05 00Z 6.49 3033  88  24 -24 0.00
Fri 12/05 03Z 0.54 3126  83  10 -22 0.00
Fri 12/05 06Z 0.23 3217  73  22 -19 0.00
Fri 12/05 09Z 0.12 3210  53  56 -17 0.00
Fri 12/05 12Z 0.13 2407  37  77 -15 0.00
Fri 12/05 15Z 0.20 2215  27  28 -14 0.00
Fri 12/05 18Z 0.25 2216  32  32 -12 0.00
Fri 12/05 21Z 0.29 2316  37  24 -11 0.00
Sat 12/06 00Z 0.34 2416  59  40 -11 0.00
Sat 12/06 03Z 0.26 2313  64  56  -9 0.00
Sat 12/06 06Z 0.22 2212  68  55  -9 0.00
Sat 12/06 09Z 0.20 2212  72  48  -8 0.00
Sat 12/06 12Z 0.25 2116  82  74  -8 0.00
Sat 12/06 15Z 0.30 2218  70  87  -7 0.00
Sat 12/06 18Z 0.38 2217  76  91  -7 0.00
Sat 12/06 21Z 0.50 2317  87  93  -7 0.00
Sun 12/07 00Z 0.68 2521  94  87  -7 0.00
Sun 12/07 03Z 0.88 2721  95  25  -8 0.00
Sun 12/07 06Z 0.82 2819  93  51  -9 0.01
Sun 12/07 09Z 0.48 2916  94  56 -10 0.01
Sun 12/07 12Z 0.26 3012  85  45 -11 0.00
Sun 12/07 15Z 0.17 3008  68  50 -10 0.00

Run total areal avg QPF for the greens is: 0.04 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