Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260206_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 02/06 18Z 0.08 1908  88  98 -12 0.00
Fri 02/06 21Z 0.09 1908  91  97 -12 0.00
Sat 02/07 00Z 0.09 1911  94  99 -12 0.00
Sat 02/07 03Z 0.09 1911  96  99 -12 0.02
Sat 02/07 06Z 0.08 1910  95  99 -12 0.03
Sat 02/07 09Z 0.04 1906  96 100 -12 0.03
Sat 02/07 12Z 0.06 3100  96  99 -12 0.05
Sat 02/07 15Z 0.12 3311  97  99 -16 0.07
Sat 02/07 18Z 0.15 3315  96  98 -17 0.02
Sat 02/07 21Z 0.18 3318  97  99 -19 0.01
Sun 02/08 00Z 0.19 3320  96  95 -20 0.01
Sun 02/08 03Z 0.21 3320  96  87 -21 0.01
Sun 02/08 06Z 0.16 3318  96  90 -20 0.01
Sun 02/08 09Z 0.13 3319  96  96 -19 0.00
Sun 02/08 12Z 0.11 3420  95  76 -18 0.00
Sun 02/08 15Z 0.10 3420  90  55 -17 0.00
Sun 02/08 18Z 0.12 3319  81  53 -15 0.00
Sun 02/08 21Z 0.18 3321  75  52 -14 0.00
Mon 02/09 00Z 0.19 3323  75  41 -14 0.00
Mon 02/09 03Z 0.20 3323  85  42 -15 0.00
Mon 02/09 06Z 0.17 3321  85  33 -14 0.00
Mon 02/09 09Z 0.16 3321  77  17 -13 0.00
Mon 02/09 12Z 0.15 3321  68  54 -13 0.00
Mon 02/09 15Z 0.12 3317  64  60 -13 0.00

Run total areal avg QPF for the greens is: 0.26 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