GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260709_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)
=== ===== === ==== ==== === === === ====
Thu 07/09 12Z 0.37 2810  80  41  14 0.00
Thu 07/09 15Z 0.73 2509  76  69  15 0.01
Thu 07/09 18Z 2.31 2412  71  84  16 0.03
Thu 07/09 21Z 1.22 2512  67  82  17 0.05
Fri 07/10 00Z 0.62 2814  79  79  16 0.11
Fri 07/10 03Z 0.29 3010  80  74  15 0.07
Fri 07/10 06Z 0.26 2807  87  80  14 0.02
Fri 07/10 09Z 0.32 2911  94  91  13 0.02
Fri 07/10 12Z 0.19 3211  93  79  12 0.01
Fri 07/10 15Z 0.24 3309  85  66  12 0.01
Fri 07/10 18Z 0.48 3309  68  58  13 0.01
Fri 07/10 21Z 0.21 3409  59  63  14 0.00
Sat 07/11 00Z 0.03 0008  59  74  13 0.00
Sat 07/11 03Z 0.07 0006  60  73  13 0.00
Sat 07/11 06Z 0.06 3506  64  64  12 0.00
Sat 07/11 09Z 0.02 3505  68  44  11 0.00
Sat 07/11 12Z 0.04 3507  69  29  11 0.00
Sat 07/11 15Z 0.17 3507  64  23  11 0.01
Sat 07/11 18Z 0.28 3408  51  35  12 0.00
Sat 07/11 21Z 0.06 0010  52  27  12 0.00
Sun 07/12 00Z 0.39 0215  56  21  11 0.00
Sun 07/12 03Z 0.37 0313  57  24  10 0.00
Sun 07/12 06Z 0.33 0508  62  27   9 0.00
Sun 07/12 09Z 0.22 0605  64  21   9 0.00

Run total areal avg QPF for the greens is: 0.35 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