GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260908_0000
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 09/08 00Z 0.75 3218  64  26  10 0.00
Tue 09/08 03Z 0.40 3317  67  43  10 0.00
Tue 09/08 06Z 0.36 3215  70  56  10 0.00
Tue 09/08 09Z 0.45 3014  74  74   9 0.00
Tue 09/08 12Z 0.33 3114  80  87   9 0.03
Tue 09/08 15Z 0.28 3212  85  76   8 0.02
Tue 09/08 18Z 0.32 3210  86  43   8 0.00
Tue 09/08 21Z 0.22 3307  75  14   9 0.00
Wed 09/09 00Z 0.06 3203  61   9  10 0.00
Wed 09/09 03Z 0.11 2303  68  19  10 0.00
Wed 09/09 06Z 0.14 2008  75  12  10 0.00
Wed 09/09 09Z 0.27 2114  81  74  11 0.00
Wed 09/09 12Z 0.30 2116  76  79  11 0.00
Wed 09/09 15Z 0.27 2117  72  67  12 0.00
Wed 09/09 18Z 0.32 2022  77  86  12 0.00
Wed 09/09 21Z 0.47 2128  98  99  12 0.12
Thu 09/10 00Z 0.63 2231  97  97  13 0.23
Thu 09/10 03Z 0.78 2332  99  96  14 0.19
Thu 09/10 06Z 0.82 2528  99  94  14 0.26
Thu 09/10 09Z 0.76 2722  96  57  13 0.08
Thu 09/10 12Z 0.78 2722  90  49  12 0.02
Thu 09/10 15Z 1.35 2725  86  44  11 0.02
Thu 09/10 18Z 16.97 2731  57  15  12 0.00
Thu 09/10 21Z 13.53 2829  59  10  11 0.00

Run total areal avg QPF for the greens is: 0.97 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