GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20250418_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 04/18 18Z 0.61 2023  38  75   5 0.00
Fri 04/18 21Z 0.61 2031  52  93   7 0.00
Sat 04/19 00Z 0.96 2237  78  98   6 0.02
Sat 04/19 03Z 0.83 2434  83  87   9 0.01
Sat 04/19 06Z 0.71 2331  84  93  10 0.02
Sat 04/19 09Z 0.56 2426  84  70  11 0.16
Sat 04/19 12Z 0.64 2428  93  83  11 0.00
Sat 04/19 15Z 0.69 2430  98  79  11 0.05
Sat 04/19 18Z 1.06 2435  97  87  10 0.10
Sat 04/19 21Z 1.50 2639  95  79  10 0.03
Sun 04/20 00Z 1.35 2736  95  31   7 0.02
Sun 04/20 03Z 1.11 2733  89  39   4 0.01
Sun 04/20 06Z 1.02 2931  87  49   0 0.02
Sun 04/20 09Z 1.14 3032  95  17  -4 0.01
Sun 04/20 12Z 1.07 3132  90  21  -7 0.01
Sun 04/20 15Z 1.48 3127  74  11  -7 0.00
Sun 04/20 18Z 2.08 3128  54   6  -4 0.00
Sun 04/20 21Z 6.10 3128  50   9  -3 0.00
Mon 04/21 00Z 0.60 3326  43  13  -2 0.00
Mon 04/21 03Z 0.23 3321  34  20  -1 0.00
Mon 04/21 06Z 0.09 3413  29  24   0 0.00
Mon 04/21 09Z 0.10 3208  23  19   0 0.00
Mon 04/21 12Z 0.05 3203  15   8   2 0.00
Mon 04/21 15Z 0.06 2005  22  67   3 0.00

Run total areal avg QPF for the greens is: 0.46 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