GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260216_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)
=== ===== === ==== ==== === === === ====
Mon 02/16 18Z 0.07 2007  96  65  -5 0.00
Mon 02/16 21Z 0.11 2107  96  49  -5 0.00
Tue 02/17 00Z 0.15 2109  95  42  -4 0.00
Tue 02/17 03Z 0.27 2213  97  82  -5 0.00
Tue 02/17 06Z 0.31 2314  97  91  -4 0.01
Tue 02/17 09Z 0.35 2414  98  88  -3 0.01
Tue 02/17 12Z 0.25 2311  97  96  -3 0.01
Tue 02/17 15Z 0.22 2310  98  87  -3 0.04
Tue 02/17 18Z 0.28 2610  99  38  -3 0.01
Tue 02/17 21Z 0.36 2813  99  36  -3 0.01
Wed 02/18 00Z 0.50 2815  95  24  -4 0.00
Wed 02/18 03Z 0.68 2719  91  32  -4 0.00
Wed 02/18 06Z 0.69 2820  89  18  -5 0.00
Wed 02/18 09Z 0.55 2919  88  17  -6 0.01
Wed 02/18 12Z 0.30 3016  73  23  -6 0.00
Wed 02/18 15Z 0.19 3011  59  32  -6 0.00
Wed 02/18 18Z 0.06 3305  50  30  -4 0.00
Wed 02/18 21Z 0.04 0303  51  35  -3 0.01
Thu 02/19 00Z 0.05 0207  56  37  -4 0.02
Thu 02/19 03Z 0.04 0010  49  45  -6 0.01
Thu 02/19 06Z 0.06 0112  42   8  -6 0.00
Thu 02/19 09Z 0.02 0009  38   6  -6 0.00
Thu 02/19 12Z 0.04 0006  29   9  -6 0.00
Thu 02/19 15Z 0.03 0202  28  11  -5 0.00

Run total areal avg QPF for the greens is: 0.14 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