Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20251125_1200
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)
=== ===== === ==== ==== === === === ====
Tue 11/25 12Z 0.59 2423  32  79   1 0.00
Tue 11/25 15Z 0.43 2319  38  76   1 0.00
Tue 11/25 18Z 0.33 2218  48  76   2 0.00
Tue 11/25 21Z 0.31 2119  54  72   2 0.00
Wed 11/26 00Z 0.29 2119  71  88   2 0.00
Wed 11/26 03Z 0.23 2020  97  97   1 0.08
Wed 11/26 06Z 0.25 2120  99  99   2 0.07
Wed 11/26 09Z 0.26 2215  99  96   4 0.03
Wed 11/26 12Z 0.20 2113  99  96   4 0.02
Wed 11/26 15Z 0.28 2117  99  95   5 0.02
Wed 11/26 18Z 0.27 2214  97  91   5 0.02
Wed 11/26 21Z 0.15 2012  97  96   5 0.00
Thu 11/27 00Z 0.29 2018  97  94   4 0.01
Thu 11/27 03Z 0.56 2221  97  96   3 0.07
Thu 11/27 06Z 1.24 2625  89  22  -4 0.10
Thu 11/27 09Z 1.43 2525  85  45  -6 0.00
Thu 11/27 12Z 1.24 2426  85  52  -7 0.00
Thu 11/27 15Z 1.76 2326  88  48  -8 0.00
Thu 11/27 18Z 7.23 2327  83  69  -7 0.01
Thu 11/27 21Z 2.97 2329  79  83  -7 0.01
Fri 11/28 00Z 1.83 2330  86  87  -8 0.00
Fri 11/28 03Z 1.46 2330  91  89  -8 0.01
Fri 11/28 06Z 1.29 2329  93  92  -9 0.01
Fri 11/28 09Z 1.12 2327  95  97  -9 0.02

Run total areal avg QPF for the greens is: 0.48 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