Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260210_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)
=== ===== === ==== ==== === === === ====
Tue 02/10 12Z 0.12 2906  82  87  -7 0.00
Tue 02/10 15Z 0.10 2305  76  71  -7 0.00
Tue 02/10 18Z 0.12 2011  61  64  -6 0.00
Tue 02/10 21Z 0.05 1815  80  96  -6 0.02
Wed 02/11 00Z 0.10 1920  97  96  -4 0.14
Wed 02/11 03Z 0.29 2219  97  97  -5 0.16
Wed 02/11 06Z 0.34 2415  97  43  -5 0.05
Wed 02/11 09Z 0.43 2615  98  34  -6 0.03
Wed 02/11 12Z 0.46 2914  96  70  -8 0.03
Wed 02/11 15Z 0.52 2913  96  92  -9 0.02
Wed 02/11 18Z 0.66 2914  95  95  -9 0.02
Wed 02/11 21Z 0.55 3016  95  92  -9 0.03
Thu 02/12 00Z 0.43 3217  95  93 -11 0.03
Thu 02/12 03Z 0.28 3317  92  95 -11 0.02
Thu 02/12 06Z 0.25 3318  95  93 -11 0.01
Thu 02/12 09Z 0.18 3418  95  85 -11 0.01
Thu 02/12 12Z 0.13 3418  94  75 -11 0.01
Thu 02/12 15Z 0.17 3317  95  67 -11 0.01
Thu 02/12 18Z 0.27 3317  94  18 -12 0.00
Thu 02/12 21Z 0.20 3319  75  10 -10 0.00
Fri 02/13 00Z 0.14 3419  67  14 -10 0.00
Fri 02/13 03Z 0.13 3417  80  16 -10 0.00
Fri 02/13 06Z 0.12 3314  78  22  -9 0.00
Fri 02/13 09Z 0.18 3213  76  32  -9 0.00

Run total areal avg QPF for the greens is: 0.59 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