Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260109_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 01/09 18Z 0.27 2038  65  94   3 0.00
Fri 01/09 21Z 0.37 2040  95  99   3 0.07
Sat 01/10 00Z 0.59 2143  98  52   3 0.01
Sat 01/10 03Z 1.84 2550  98  27  -1 0.01
Sat 01/10 06Z 0.81 2936  68   3   0 0.00
Sat 01/10 09Z 0.53 3029  57   8   0 0.00
Sat 01/10 12Z 0.23 3222  46  11   0 0.00
Sat 01/10 15Z 0.03 3510  31  14   0 0.00
Sat 01/10 18Z 0.02 0901  24  10   1 0.00
Sat 01/10 21Z 0.13 1413  22  19   1 0.00
Sun 01/11 00Z 0.16 1621  40  92  -1 0.01
Sun 01/11 03Z 0.25 1527  96  97  -2 0.06
Sun 01/11 06Z 0.15 1624  98  97  -1 0.08
Sun 01/11 09Z 0.14 1920  98  96   0 0.15
Sun 01/11 12Z 0.25 2213  99  73  -2 0.03
Sun 01/11 15Z 0.37 2511  98  86  -4 0.04
Sun 01/11 18Z 0.57 2612  90  31  -7 0.02
Sun 01/11 21Z 1.24 2822  83  92  -9 0.02
Mon 01/12 00Z 1.17 2722  77  73 -10 0.01
Mon 01/12 03Z 0.92 2722  95  96 -11 0.02
Mon 01/12 06Z 0.88 3027  95  80 -11 0.05
Mon 01/12 09Z 0.70 3031  90  51 -11 0.03
Mon 01/12 12Z 0.54 3028  88  36 -11 0.01
Mon 01/12 15Z 0.36 3020  81   9 -10 0.00

Run total areal avg QPF for the greens is: 0.62 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