Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260313_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)
=== ===== === ==== ==== === === === ====
Fri 03/13 06Z 1.06 3021  91  65 -14 0.00
Fri 03/13 09Z 0.62 2917  90  67 -14 0.00
Fri 03/13 12Z 0.58 2815  93  18 -14 0.00
Fri 03/13 15Z 1.00 2509  80   4 -13 0.00
Fri 03/13 18Z 1.51 2213  59  17 -11 0.00
Fri 03/13 21Z 0.20 1817  69  97  -9 0.00
Sat 03/14 00Z 0.13 1722  94  96  -8 0.02
Sat 03/14 03Z 0.23 1623  95  95  -8 0.06
Sat 03/14 06Z 0.16 1921  96  90  -6 0.12
Sat 03/14 09Z 0.76 2323  94  98  -6 0.03
Sat 03/14 12Z 0.83 2421  94  96  -7 0.02
Sat 03/14 15Z 1.03 2518  92  93  -7 0.02
Sat 03/14 18Z 1.05 2718  92  96  -6 0.02
Sat 03/14 21Z 1.48 2932  94  95  -9 0.08
Sun 03/15 00Z 1.33 3033  93  83 -10 0.03
Sun 03/15 03Z 0.87 3127  93  24 -12 0.02
Sun 03/15 06Z 0.49 3019  82  12 -12 0.01
Sun 03/15 09Z 0.27 3115  59  11 -10 0.00
Sun 03/15 12Z 0.13 3007  38  56  -8 0.00
Sun 03/15 15Z 0.03 1905  40  94  -7 0.00
Sun 03/15 18Z 0.03 1713  67  97  -8 0.00
Sun 03/15 21Z 0.15 1725  62  91  -6 0.00
Mon 03/16 00Z 0.03 1833  47  76  -3 0.00
Mon 03/16 03Z 0.12 1837  52  53  -1 0.00

Run total areal avg QPF for the greens is: 0.43 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