Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260206_0600
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 02/06 06Z 0.02 1704  58   7 -10 0.00
Fri 02/06 09Z 0.02 1708  58  16 -11 0.00
Fri 02/06 12Z 0.04 1810  63  83 -11 0.00
Fri 02/06 15Z 0.04 1810  81  95 -12 0.00
Fri 02/06 18Z 0.09 1908  93  98 -12 0.00
Fri 02/06 21Z 0.07 1809  95  97 -11 0.01
Sat 02/07 00Z 0.06 1811  96  99 -11 0.02
Sat 02/07 03Z 0.06 1812  95  98 -11 0.02
Sat 02/07 06Z 0.05 1811  95  99 -11 0.03
Sat 02/07 09Z 0.03 1807  95 100 -11 0.03
Sat 02/07 12Z 0.08 3300  96  99 -12 0.05
Sat 02/07 15Z 0.13 3311  97  99 -16 0.07
Sat 02/07 18Z 0.15 3315  97  99 -17 0.02
Sat 02/07 21Z 0.20 3319  98  99 -19 0.01
Sun 02/08 00Z 0.22 3321  96  94 -20 0.01
Sun 02/08 03Z 0.24 3321  96  84 -21 0.01
Sun 02/08 06Z 0.20 3219  96  83 -21 0.01
Sun 02/08 09Z 0.19 3218  96  87 -21 0.00
Sun 02/08 12Z 0.17 3219  96  86 -21 0.00
Sun 02/08 15Z 0.11 3318  96  83 -19 0.00
Sun 02/08 18Z 0.12 3318  94  68 -18 0.00
Sun 02/08 21Z 0.16 3320  87  63 -17 0.00
Mon 02/09 00Z 0.14 3322  79  66 -15 0.00
Mon 02/09 03Z 0.13 3321  79  48 -14 0.00

Run total areal avg QPF for the greens is: 0.29 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