Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260118_1200
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)
=== ===== === ==== ==== === === === ====
Sun 01/18 12Z 0.86 2423  67  86 -10 0.00
Sun 01/18 15Z 0.65 2417  86  74 -11 0.00
Sun 01/18 18Z 0.67 2510  91  53 -12 0.00
Sun 01/18 21Z 0.23 2504  88  59 -11 0.00
Mon 01/19 00Z 0.06 3001  84  68 -11 0.00
Mon 01/19 03Z 0.05 3303  88  86 -11 0.00
Mon 01/19 06Z 0.04 3403  96  91 -11 0.00
Mon 01/19 09Z 0.08 2303  96  85 -12 0.01
Mon 01/19 12Z 0.19 2307  96  60 -13 0.01
Mon 01/19 15Z 0.38 2212  94  96 -14 0.00
Mon 01/19 18Z 1.05 2316  92  87 -14 0.00
Mon 01/19 21Z 1.29 2323  93  93 -14 0.01
Tue 01/20 00Z 1.09 2326  86  97 -13 0.00
Tue 01/20 03Z 2.96 2435  81  77 -17 0.02
Tue 01/20 06Z 2.87 2633  91  67 -18 0.00
Tue 01/20 09Z 1.74 2632  81  43 -19 0.01
Tue 01/20 12Z 1.18 2627  83  65 -20 0.00
Tue 01/20 15Z 1.69 2521  92  90 -22 0.00
Tue 01/20 18Z 4.21 2524  93  92 -21 0.01
Tue 01/20 21Z 2.79 2625  89  87 -21 0.01
Wed 01/21 00Z 1.68 2626  92  82 -21 0.01
Wed 01/21 03Z 1.25 2727  88  70 -21 0.00
Wed 01/21 06Z 0.90 2625  86  64 -20 0.00
Wed 01/21 09Z 0.72 2522  89  31 -19 0.00

Run total areal avg QPF for the greens is: 0.09 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