GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20251006_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)
=== ===== === ==== ==== === === === ====
Mon 10/06 12Z 0.57 2418  35  14  14 0.00
Mon 10/06 15Z 0.43 2414  46  13  13 0.00
Mon 10/06 18Z 0.63 2211  51  10  13 0.00
Mon 10/06 21Z 0.55 2216  49   7  14 0.00
Tue 10/07 00Z 1.10 2421  50  10  13 0.00
Tue 10/07 03Z 1.20 2422  54  18  13 0.00
Tue 10/07 06Z 0.86 2422  56  23  13 0.00
Tue 10/07 09Z 0.67 2419  58  22  13 0.00
Tue 10/07 12Z 0.66 2322  62  24  13 0.00
Tue 10/07 15Z 0.54 2219  65  52  13 0.00
Tue 10/07 18Z 1.27 2222  72  90  12 0.00
Tue 10/07 21Z 0.82 2427  86  92  11 0.18
Wed 10/08 00Z 0.67 2323  91  89  11 0.03
Wed 10/08 03Z 0.84 2329  94  98  10 0.09
Wed 10/08 06Z 0.50 2817  97  99   8 0.40
Wed 10/08 09Z 0.12 3414  85  94   6 0.16
Wed 10/08 12Z 0.12 3412  81  51   4 0.04
Wed 10/08 15Z 0.29 3313  89  13   2 0.00
Wed 10/08 18Z 0.70 3317  81   2   1 0.00
Wed 10/08 21Z 3.95 3322  69   3   0 0.00
Thu 10/09 00Z 0.65 3425  65   5  -1 0.00
Thu 10/09 03Z 0.27 3424  60   7  -3 0.00
Thu 10/09 06Z 0.31 3423  60   8  -4 0.00
Thu 10/09 09Z 0.30 3424  66   8  -5 0.00

Run total areal avg QPF for the greens is: 0.90 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