GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260328_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)
=== ===== === ==== ==== === === === ====
Sat 03/28 06Z 0.44 3214  89  18 -16 0.00
Sat 03/28 09Z 0.43 3211  92  63 -17 0.00
Sat 03/28 12Z 0.37 3109  89  97 -17 0.00
Sat 03/28 15Z 1.00 2909  72  87 -16 0.00
Sat 03/28 18Z 2.91 2715  62  99 -14 0.00
Sat 03/28 21Z 2.47 2821  71  99 -13 0.01
Sun 03/29 00Z 1.92 2922  72  73 -14 0.01
Sun 03/29 03Z 1.54 2821  74  22 -14 0.00
Sun 03/29 06Z 1.22 2720  76  90 -13 0.00
Sun 03/29 09Z 0.96 2722  90  90 -13 0.00
Sun 03/29 12Z 0.80 2622  88  89 -12 0.00
Sun 03/29 15Z 1.40 2520  82  84 -10 0.00
Sun 03/29 18Z 3.10 2523  66  86  -7 0.00
Sun 03/29 21Z 15.18 2525  67  93  -5 0.00
Mon 03/30 00Z 1.70 2429  66  91  -4 0.00
Mon 03/30 03Z 1.29 2531  56  83  -2 0.00
Mon 03/30 06Z 1.11 2530  54  81  -1 0.00
Mon 03/30 09Z 0.93 2524  60  92  -1 0.00
Mon 03/30 12Z 0.76 2423  61  88   0 0.00
Mon 03/30 15Z 0.75 2325  68  60   0 0.00
Mon 03/30 18Z 1.36 2430  79  76   0 0.00
Mon 03/30 21Z 1.47 2532  88  86   2 0.00
Tue 03/31 00Z 0.86 2627  97  94   3 0.02
Tue 03/31 03Z 0.61 2621  99  93   3 0.06

Run total areal avg QPF for the greens is: 0.10 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