GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260410_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)
=== ===== === ==== ==== === === === ====
Fri 04/10 18Z 0.58 2223  66  78   7 0.00
Fri 04/10 21Z 0.70 2224  69  58   7 0.00
Sat 04/11 00Z 0.71 2522  91  97   5 0.09
Sat 04/11 03Z 0.48 2817  99  94   2 0.19
Sat 04/11 06Z 0.46 3019  99  55  -1 0.03
Sat 04/11 09Z 0.63 3022  98  13  -5 0.01
Sat 04/11 12Z 0.88 3024  94   7  -7 0.01
Sat 04/11 15Z 1.43 3125  87  12  -8 0.01
Sat 04/11 18Z 2.11 3125  68  16  -6 0.00
Sat 04/11 21Z 3.88 3126  67  17  -5 0.00
Sun 04/12 00Z 1.29 3225  77  13  -6 0.00
Sun 04/12 03Z 0.91 3224  77  12  -7 0.00
Sun 04/12 06Z 0.58 3221  76  18  -7 0.00
Sun 04/12 09Z 0.35 3218  72  24  -6 0.00
Sun 04/12 12Z 0.17 3211  66  29  -4 0.00
Sun 04/12 15Z 0.13 2603  60  34  -3 0.00
Sun 04/12 18Z 0.21 2008  53  42  -1 0.00
Sun 04/12 21Z 0.23 2113  53  93   0 0.00
Mon 04/13 00Z 0.50 2126  58  97   1 0.00
Mon 04/13 03Z 0.54 2231  82  99   2 0.01
Mon 04/13 06Z 0.48 2236  96  99   6 0.09
Mon 04/13 09Z 0.62 2338  90  88   8 0.10
Mon 04/13 12Z 0.76 2437  89  72   7 0.02
Mon 04/13 15Z 0.84 2340  96  99   7 0.03

Run total areal avg QPF for the greens is: 0.59 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