GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20250618_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)
=== ===== === ==== ==== === === === ====
Wed 06/18 00Z 0.26 2023  99  72  13 0.00
Wed 06/18 03Z 0.32 2123  99  73  13 0.00
Wed 06/18 06Z 0.44 2221  98  68  13 0.00
Wed 06/18 09Z 0.40 2317  98  69  13 0.00
Wed 06/18 12Z 0.31 2313  98  64  14 0.00
Wed 06/18 15Z 0.27 2212  96  53  14 0.00
Wed 06/18 18Z 0.36 2310  92  54  14 0.01
Wed 06/18 21Z 0.40 2309  88  67  15 0.04
Thu 06/19 00Z 0.25 2308  87  75  15 0.03
Thu 06/19 03Z 0.15 2206  86  79  15 0.01
Thu 06/19 06Z 0.11 2107  86  82  15 0.01
Thu 06/19 09Z 0.23 2112  88  85  15 0.02
Thu 06/19 12Z 0.33 2314  85  77  16 0.01
Thu 06/19 15Z 0.47 2215  79  88  16 0.00
Thu 06/19 18Z 1.64 2227  76  84  16 0.01
Thu 06/19 21Z 1.43 2328  82  70  16 0.05
Fri 06/20 00Z 1.09 2334  80  71  15 0.04
Fri 06/20 03Z 0.76 2426  87  49  11 0.00
Fri 06/20 06Z 0.65 2519  96  46   8 0.00
Fri 06/20 09Z 0.70 2923  99  98   8 0.03
Fri 06/20 12Z 0.72 3131  99  93   7 0.11
Fri 06/20 15Z 0.63 3127  97  65   8 0.03
Fri 06/20 18Z 1.77 3024  76  40  10 0.02
Fri 06/20 21Z 3.56 3121  60  53  11 0.01

Run total areal avg QPF for the greens is: 0.43 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