GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20250813_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)
=== ===== === ==== ==== === === === ====
Wed 08/13 18Z 0.89 2411  63  91  18 0.00
Wed 08/13 21Z 0.43 2711  72  81  17 0.24
Thu 08/14 00Z 0.36 2609  73  93  16 0.11
Thu 08/14 03Z 0.32 2708  83  86  15 0.01
Thu 08/14 06Z 0.20 3107  83  71  14 0.00
Thu 08/14 09Z 0.12 3307  81  61  14 0.01
Thu 08/14 12Z 0.09 3307  82  55  14 0.00
Thu 08/14 15Z 0.11 3308  83  32  14 0.01
Thu 08/14 18Z 0.34 3410  68  43  13 0.00
Thu 08/14 21Z 0.14 3509  59  33  14 0.00
Fri 08/15 00Z 0.06 3511  56  19  13 0.00
Fri 08/15 03Z 0.06 0010  51  12  13 0.00
Fri 08/15 06Z 0.07 0107  46  10  12 0.00
Fri 08/15 09Z 0.02 0004  42   6  12 0.00
Fri 08/15 12Z 0.04 0302  38   5  12 0.00
Fri 08/15 15Z 0.02 1501  42   5  12 0.00
Fri 08/15 18Z 0.67 2002  43   6  13 0.00
Fri 08/15 21Z 0.54 2302  43   7  14 0.00
Sat 08/16 00Z 0.07 1905  46   8  15 0.00
Sat 08/16 03Z 0.15 2008  54  11  15 0.00
Sat 08/16 06Z 0.25 2210  63  13  14 0.00
Sat 08/16 09Z 0.32 2311  73  15  14 0.00
Sat 08/16 12Z 0.24 2408  75  14  14 0.00
Sat 08/16 15Z 0.21 2206  78  13  14 0.00

Run total areal avg QPF for the greens is: 0.38 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