Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260116_1200
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 12Z 0.57 2920  97  85 -20 0.00
Fri 01/16 15Z 0.49 2820  90  53 -18 0.01
Fri 01/16 18Z 0.45 2819  62   7 -14 0.00
Fri 01/16 21Z 0.39 2716  47  18 -12 0.00
Sat 01/17 00Z 0.28 2512  43  50 -11 0.00
Sat 01/17 03Z 0.28 2215  53  94 -10 0.00
Sat 01/17 06Z 0.32 2120  86  94 -10 0.01
Sat 01/17 09Z 0.27 2120  92  92  -8 0.01
Sat 01/17 12Z 0.18 2019  95  96  -7 0.02
Sat 01/17 15Z 0.17 2020  96  96  -7 0.02
Sat 01/17 18Z 0.34 2119  96  96  -7 0.02
Sat 01/17 21Z 0.55 2418  95  82  -7 0.01
Sun 01/18 00Z 0.72 2520  93  71  -7 0.00
Sun 01/18 03Z 0.69 2420  94  79  -7 0.00
Sun 01/18 06Z 0.64 2319  89  76  -7 0.00
Sun 01/18 09Z 0.74 2225  88  65  -7 0.00
Sun 01/18 12Z 0.75 2421  94  39 -10 0.02
Sun 01/18 15Z 0.82 2518  82  12 -12 0.02
Sun 01/18 18Z 1.18 2515  86  21 -13 0.00
Sun 01/18 21Z 0.64 2510  85  36 -13 0.00
Mon 01/19 00Z 0.56 2708  85  53 -13 0.00
Mon 01/19 03Z 0.46 2910  89  61 -14 0.00
Mon 01/19 06Z 0.49 2812  87  57 -14 0.00
Mon 01/19 09Z 0.40 2610  87  45 -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