NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20251119_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)
=== ===== === ==== ==== === === === ====
Wed 11/19 00Z 0.69 2818  88  32  -7 0.00
Wed 11/19 03Z 0.41 2916  75  19  -6 0.00
Wed 11/19 06Z 0.40 2915  70   9  -6 0.00
Wed 11/19 09Z 0.40 2915  69   4  -7 0.00
Wed 11/19 12Z 0.33 3015  66   4  -7 0.00
Wed 11/19 15Z 0.23 3112  63   6  -6 0.00
Wed 11/19 18Z 0.20 3009  61  10  -5 0.00
Wed 11/19 21Z 0.20 3111  57  13  -5 0.00
Thu 11/20 00Z 0.13 3210  58  20  -4 0.00
Thu 11/20 03Z 0.07 3206  56  23  -2 0.00
Thu 11/20 06Z 0.05 3103  55  18  -2 0.00
Thu 11/20 09Z 0.08 2904  51  32  -3 0.00
Thu 11/20 12Z 0.07 2703  41  53  -1 0.00
Thu 11/20 15Z 0.07 2503  37  66  -1 0.00
Thu 11/20 18Z 0.07 2303  39  70  -1 0.00
Thu 11/20 21Z 0.03 2003  35  57   0 0.00
Fri 11/21 00Z 0.02 1807  34  70   1 0.00
Fri 11/21 03Z 0.06 1912  36  64   2 0.00
Fri 11/21 06Z 0.16 2117  36  49   2 0.00
Fri 11/21 09Z 0.23 2119  41  37   1 0.00
Fri 11/21 12Z 0.32 2221  57  69   0 0.00
Fri 11/21 15Z 0.42 2421  85  65  -1 0.00
Fri 11/21 18Z 0.60 2422  97  54  -2 0.01
Fri 11/21 21Z 0.43 2321  80  63   0 0.01
Sat 11/22 00Z 0.86 2623  95   8  -2 0.01
Sat 11/22 03Z 0.91 2719  96   6  -4 0.01
Sat 11/22 06Z 0.73 2717  94   9  -5 0.00
Sat 11/22 09Z 0.58 2917  91  14  -6 0.00
Sat 11/22 12Z 0.30 3216  87  12  -7 0.00

Run total areal avg QPF for the greens is: 0.04 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