GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260108_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)
=== ===== === ==== ==== === === === ====
Thu 01/08 00Z 0.31 3115  98  74  -5 0.00
Thu 01/08 03Z 0.34 3117  97  52  -5 0.01
Thu 01/08 06Z 0.28 3116  96  54  -3 0.01
Thu 01/08 09Z 0.35 2917  96  50  -4 0.01
Thu 01/08 12Z 0.40 3019  96  10  -4 0.01
Thu 01/08 15Z 0.39 3016  97  27  -5 0.01
Thu 01/08 18Z 0.31 3012  91  13  -4 0.00
Thu 01/08 21Z 0.21 3011  73   8  -1 0.00
Fri 01/09 00Z 0.10 3107  56  12   0 0.00
Fri 01/09 03Z 0.06 2403  40  49   2 0.00
Fri 01/09 06Z 0.08 2009  35  45   3 0.00
Fri 01/09 09Z 0.12 2016  32  33   4 0.00
Fri 01/09 12Z 0.13 1923  35  45   5 0.00
Fri 01/09 15Z 0.24 1938  41  33   5 0.00
Fri 01/09 18Z 0.39 2041  74  97   3 0.02
Fri 01/09 21Z 0.45 2140  96  98   4 0.08
Sat 01/10 00Z 0.66 2241  98  79   4 0.02
Sat 01/10 03Z 1.54 2546  98  19   0 0.00
Sat 01/10 06Z 0.65 2932  66   3   0 0.00
Sat 01/10 09Z 0.41 3126  57   5  -1 0.00
Sat 01/10 12Z 0.15 3319  45   6  -1 0.00
Sat 01/10 15Z 0.02 0012  23   7   0 0.00
Sat 01/10 18Z 0.12 0806  21   9   1 0.00
Sat 01/10 21Z 0.24 1315  27  55   0 0.01

Run total areal avg QPF for the greens is: 0.18 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