Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20241222_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)
=== ===== === ==== ==== === === === ====
Sun 12/22 00Z 0.64 3223  96  81 -20 0.00
Sun 12/22 03Z 0.44 3220  93  65 -21 0.01
Sun 12/22 06Z 0.34 3219  92  32 -21 0.00
Sun 12/22 09Z 0.29 3219  90  24 -21 0.00
Sun 12/22 12Z 0.21 3319  84  20 -21 0.00
Sun 12/22 15Z 0.11 3417  85  13 -20 0.00
Sun 12/22 18Z 0.11 3414  86  20 -19 0.00
Sun 12/22 21Z 0.08 3414  73  13 -18 0.00
Mon 12/23 00Z 0.04 3513  58  10 -17 0.00
Mon 12/23 03Z 0.01 3507  48   9 -15 0.00
Mon 12/23 06Z 0.04 3203  41   8 -14 0.00
Mon 12/23 09Z 0.10 2605  35   6 -13 0.00
Mon 12/23 12Z 0.13 2307  29  14 -12 0.00
Mon 12/23 15Z 0.14 2110  24  65 -11 0.00
Mon 12/23 18Z 0.18 2114  28  67 -10 0.00
Mon 12/23 21Z 0.21 2117  28  86  -8 0.00
Tue 12/24 00Z 0.26 2120  59  95  -9 0.01
Tue 12/24 03Z 0.29 2124  72  95  -7 0.06
Tue 12/24 06Z 0.44 2325  95  95  -9 0.08
Tue 12/24 09Z 0.40 2518  95  94  -8 0.05
Tue 12/24 12Z 0.31 2813  95  95  -8 0.04
Tue 12/24 15Z 0.18 3008  96  61  -9 0.01
Tue 12/24 18Z 0.14 3208  97   6  -9 0.00
Tue 12/24 21Z 0.06 3408  92   1  -8 0.00

Run total areal avg QPF for the greens is: 0.26 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