Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260403_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)
=== ===== === ==== ==== === === === ====
Fri 04/03 12Z 0.45 2238  98  97   8 0.00
Fri 04/03 15Z 0.54 2336  91  91  10 0.04
Fri 04/03 18Z 0.58 2432  92  69  10 0.00
Fri 04/03 21Z 0.84 2434  95  53   9 0.00
Sat 04/04 00Z 1.25 2539  97  23   9 0.01
Sat 04/04 03Z 1.51 2738  96  17   5 0.01
Sat 04/04 06Z 0.75 2930  91  26   1 0.00
Sat 04/04 09Z 0.22 3219  92  16   0 0.00
Sat 04/04 12Z 0.07 0210  73  10   2 0.00
Sat 04/04 15Z 0.13 0808  63  15   4 0.00
Sat 04/04 18Z 0.25 1413  71  57   2 0.00
Sat 04/04 21Z 0.48 1524  80  59   1 0.00
Sun 04/05 00Z 0.11 1629  88  94   3 0.01
Sun 04/05 03Z 0.08 1733  92  98   4 0.01
Sun 04/05 06Z 0.05 1837  98  93   7 0.01
Sun 04/05 09Z 0.13 1938  99  97   7 0.01
Sun 04/05 12Z 0.26 2032  99  94   6 0.06
Sun 04/05 15Z 0.46 2324  97  96   5 0.17
Sun 04/05 18Z 1.00 2527  98  25   1 0.07
Sun 04/05 21Z 1.48 2629  91  25  -2 0.02
Mon 04/06 00Z 2.00 2629  88  19  -4 0.00
Mon 04/06 03Z 1.66 2730  87  34  -6 0.00
Mon 04/06 06Z 1.32 2830  91  59  -7 0.00
Mon 04/06 09Z 1.16 2927  95  63  -8 0.01

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