GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260720_0600
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 07/20 06Z 0.71 3020  67  33   8 0.00
Mon 07/20 09Z 0.61 3019  68  53   8 0.00
Mon 07/20 12Z 0.43 3015  66  59   9 0.00
Mon 07/20 15Z 0.82 2810  63  62   9 0.00
Mon 07/20 18Z 4.17 2510  54  58  11 0.00
Mon 07/20 21Z 1.41 2210  53  49  13 0.00
Tue 07/21 00Z 0.61 2216  49  31  13 0.00
Tue 07/21 03Z 0.53 2115  50  19  13 0.00
Tue 07/21 06Z 0.38 2021  48  19  13 0.00
Tue 07/21 09Z 0.41 2125  57  26  14 0.00
Tue 07/21 12Z 0.25 2022  63  27  14 0.00
Tue 07/21 15Z 0.04 1720  73  85  13 0.00
Tue 07/21 18Z 0.19 1727  79  73  13 0.01
Tue 07/21 21Z 0.31 1630  90  89  13 0.09
Wed 07/22 00Z 0.21 1626  98  96  14 0.49
Wed 07/22 03Z 0.13 1914  94  89  14 0.88
Wed 07/22 06Z 0.46 2219  94  73  13 0.06
Wed 07/22 09Z 0.60 2323  95  68  13 0.00
Wed 07/22 12Z 0.54 2318  96  79  12 0.01
Wed 07/22 15Z 0.65 2320  93  83  11 0.03
Wed 07/22 18Z 1.50 2424  84  44  11 0.05
Wed 07/22 21Z 2.03 2521  72  29  11 0.01
Thu 07/23 00Z 1.03 2719  84  32   8 0.00
Thu 07/23 03Z 0.60 2816  89  37   6 0.01

Run total areal avg QPF for the greens is: 1.64 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