GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260508_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)
=== ===== === ==== ==== === === === ====
Fri 05/08 18Z 7.93 2816  55  98  -1 0.00
Fri 05/08 21Z 4.54 2814  53  95  -1 0.00
Sat 05/09 00Z 0.68 2810  53  63  -1 0.00
Sat 05/09 03Z 0.60 2709  57  53  -1 0.00
Sat 05/09 06Z 0.44 2310  48  55  -1 0.00
Sat 05/09 09Z 0.36 2310  46  32   0 0.00
Sat 05/09 12Z 0.13 1910  55  39   0 0.00
Sat 05/09 15Z 0.08 1810  70  57   0 0.00
Sat 05/09 18Z 0.11 1713  85  75   1 0.04
Sat 05/09 21Z 0.06 1815  97  68   1 0.01
Sun 05/10 00Z 0.15 1917  97  54   3 0.00
Sun 05/10 03Z 0.22 2020  96  52   3 0.00
Sun 05/10 06Z 0.34 2026  95  73   4 0.00
Sun 05/10 09Z 0.48 2127  93  51   4 0.00
Sun 05/10 12Z 0.73 2325  94  62   3 0.01
Sun 05/10 15Z 1.56 2424  86  51   2 0.04
Sun 05/10 18Z 5.98 2521  52  42   3 0.01
Sun 05/10 21Z 4.65 2616  52  50   3 0.00
Mon 05/11 00Z 0.75 2915  64  33   1 0.00
Mon 05/11 03Z 0.40 3010  73  24   0 0.00
Mon 05/11 06Z 0.29 2806  82  29  -1 0.00
Mon 05/11 09Z 0.41 2709  78  28  -1 0.00
Mon 05/11 12Z 0.40 2810  67  47  -1 0.00
Mon 05/11 15Z 2.54 2814  64  52  -1 0.00

Run total areal avg QPF for the greens is: 0.11 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