GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260720_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)
=== ===== === ==== ==== === === === ====
Mon 07/20 00Z 1.39 3022  56  21   8 0.00
Mon 07/20 03Z 0.86 3023  62  32   9 0.00
Mon 07/20 06Z 0.66 3020  66  29   8 0.00
Mon 07/20 09Z 0.51 3016  65  53   8 0.00
Mon 07/20 12Z 0.42 2914  63  62   9 0.00
Mon 07/20 15Z 0.66 2810  60  64   9 0.00
Mon 07/20 18Z 3.01 2509  54  56  11 0.00
Mon 07/20 21Z 1.51 2309  53  45  12 0.00
Tue 07/21 00Z 0.43 2114  52  38  13 0.00
Tue 07/21 03Z 0.46 2116  50  22  13 0.00
Tue 07/21 06Z 0.40 2022  48  18  13 0.00
Tue 07/21 09Z 0.33 2023  49  24  13 0.00
Tue 07/21 12Z 0.17 1922  59  23  13 0.00
Tue 07/21 15Z 0.03 1821  75  76  11 0.00
Tue 07/21 18Z 0.07 1728  79  91  12 0.01
Tue 07/21 21Z 0.30 1630  82  84  12 0.01
Wed 07/22 00Z 0.18 1627  96  94  13 0.15
Wed 07/22 03Z 0.10 1720  96  95  13 0.48
Wed 07/22 06Z 0.34 2119  93  72  13 0.13
Wed 07/22 09Z 0.50 2222  96  77  13 0.00
Wed 07/22 12Z 0.58 2418  96  72  12 0.01
Wed 07/22 15Z 0.56 2317  95  70  11 0.03
Wed 07/22 18Z 0.85 2322  84  47  11 0.03
Wed 07/22 21Z 6.60 2423  64  13  11 0.00

Run total areal avg QPF for the greens is: 0.85 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