GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20251230_0000
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 12/30 00Z 1.48 2626  83  83  -9 0.00
Tue 12/30 03Z 1.29 2527  92  91 -11 0.01
Tue 12/30 06Z 3.19 2728  90  95 -16 0.02
Tue 12/30 09Z 7.27 2733  92  89 -19 0.01
Tue 12/30 12Z 6.79 2831  92  91 -21 0.01
Tue 12/30 15Z 9.22 2730  93  83 -22 0.00
Tue 12/30 18Z 10.46 2731  92  86 -21 0.00
Tue 12/30 21Z 2.63 2732  92  92 -21 0.01
Wed 12/31 00Z 2.07 2732  92  88 -20 0.01
Wed 12/31 03Z 2.00 2730  92  90 -19 0.01
Wed 12/31 06Z 1.54 2731  83  70 -18 0.01
Wed 12/31 09Z 1.28 2630  74  34 -17 0.01
Wed 12/31 12Z 1.09 2527  81  50 -16 0.00
Wed 12/31 15Z 1.22 2425  89  81 -15 0.00
Wed 12/31 18Z 1.73 2423  90  94 -14 0.01
Wed 12/31 21Z 1.16 2322  93  98 -13 0.01
Thu 01/01 00Z 0.77 2320  92  98 -12 0.01
Thu 01/01 03Z 0.35 2116  96  99 -12 0.03
Thu 01/01 06Z 0.35 2113  96  99 -12 0.07
Thu 01/01 09Z 0.62 2814  96  99 -14 0.07
Thu 01/01 12Z 0.62 3018  94  87 -17 0.03
Thu 01/01 15Z 0.48 2915  90  55 -19 0.00
Thu 01/01 18Z 2.25 2815  93  81 -21 0.00
Thu 01/01 21Z 1.01 2819  85  65 -20 0.00

Run total areal avg QPF for the greens is: 0.33 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