GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20250205_0600
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)
=== ===== === ==== ==== === === === ====
Wed 02/05 06Z 0.55 3122  93  11 -19 0.00
Wed 02/05 09Z 0.46 3021  90  13 -19 0.00
Wed 02/05 12Z 0.35 3018  86  13 -19 0.00
Wed 02/05 15Z 0.29 3016  85  18 -19 0.00
Wed 02/05 18Z 0.26 3014  82  14 -17 0.00
Wed 02/05 21Z 0.27 2813  72  12 -15 0.00
Thu 02/06 00Z 0.27 2813  59   9 -14 0.00
Thu 02/06 03Z 0.20 2610  49   9 -12 0.00
Thu 02/06 06Z 0.13 2308  32   9 -10 0.00
Thu 02/06 09Z 0.08 2011  23  15  -8 0.00
Thu 02/06 12Z 0.10 1918  26  49  -8 0.00
Thu 02/06 15Z 0.06 1823  75  96  -8 0.01
Thu 02/06 18Z 0.08 1829  95  98  -7 0.14
Thu 02/06 21Z 0.18 1929  96  98  -4 0.07
Fri 02/07 00Z 0.29 2127  99  25  -2 0.01
Fri 02/07 03Z 0.77 2333  98   7  -3 0.01
Fri 02/07 06Z 2.85 2539  89  15  -7 0.03
Fri 02/07 09Z 2.77 2638  86  55 -11 0.01
Fri 02/07 12Z 2.87 2736  85  75 -14 0.01
Fri 02/07 15Z 3.31 2836  88  54 -15 0.00
Fri 02/07 18Z 5.28 2834  92  65 -14 0.01
Fri 02/07 21Z 1.95 2934  93  43 -13 0.01
Sat 02/08 00Z 1.38 2931  94  31 -13 0.00
Sat 02/08 03Z 1.15 2827  94  24 -14 0.00

Run total areal avg QPF for the greens is: 0.31 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