GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20250318_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)
=== ===== === ==== ==== === === === ====
Tue 03/18 18Z 0.05 2302  37  47   2 0.00
Tue 03/18 21Z 0.14 2305  40  34   2 0.00
Wed 03/19 00Z 0.24 2310  35  19   3 0.00
Wed 03/19 03Z 0.30 2411  37  42   3 0.00
Wed 03/19 06Z 0.32 2511  41  43   4 0.00
Wed 03/19 09Z 0.19 2308  43  42   5 0.00
Wed 03/19 12Z 0.12 2012  41  39   6 0.00
Wed 03/19 15Z 0.12 1919  40  34   8 0.00
Wed 03/19 18Z 0.13 1921  44  27   8 0.00
Wed 03/19 21Z 0.13 1922  46  22   8 0.00
Thu 03/20 00Z 0.17 1927  41  17   8 0.00
Thu 03/20 03Z 0.15 1930  37  10   9 0.00
Thu 03/20 06Z 0.12 1930  37   7   8 0.00
Thu 03/20 09Z 0.10 1832  36  10   9 0.00
Thu 03/20 12Z 0.11 1835  51   9   9 0.00
Thu 03/20 15Z 0.11 1932  68  10   8 0.00
Thu 03/20 18Z 0.05 1828  74   9   7 0.00
Thu 03/20 21Z 0.05 1825  81  15   5 0.00
Fri 03/21 00Z 0.04 1823  85  45   5 0.00
Fri 03/21 03Z 0.07 1817  93  54   3 0.00
Fri 03/21 06Z 0.05 1705  97  67   3 0.00
Fri 03/21 09Z 0.11 0306  99  83   1 0.01
Fri 03/21 12Z 0.10 0017  99  95   0 0.05
Fri 03/21 15Z 0.03 0023  99  95  -1 0.18

Run total areal avg QPF for the greens is: 0.24 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