NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20251007_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)
=== ===== === ==== ==== === === === ====
Tue 10/07 06Z 0.78 2618  60  26  14 0.00
Tue 10/07 09Z 0.58 2515  54  26  14 0.00
Tue 10/07 12Z 0.69 2416  58  14  13 0.00
Tue 10/07 15Z 0.69 2221  65  40  13 0.00
Tue 10/07 18Z 0.92 2226  75  83  14 0.00
Tue 10/07 21Z 0.79 2328  95  92  12 0.05
Wed 10/08 00Z 0.74 2326  96  88  12 0.12
Wed 10/08 03Z 0.71 2133  95  92  12 0.11
Wed 10/08 06Z 0.52 2321  99  98  11 0.52
Wed 10/08 09Z 0.36 3117  97  97   6 0.66
Wed 10/08 12Z 0.14 3418  84  86   6 0.22
Wed 10/08 15Z 0.19 3419  76  26   3 0.00
Wed 10/08 18Z 1.68 3318  81   2   1 0.00
Wed 10/08 21Z 4.14 3320  78  16   1 0.00
Thu 10/09 00Z 1.16 3323  85  50   0 0.01
Thu 10/09 03Z 0.19 3422  89  23  -2 0.00
Thu 10/09 06Z 0.20 3423  77   6  -4 0.00
Thu 10/09 09Z 0.35 3426  65   4  -6 0.00
Thu 10/09 12Z 0.33 3530  72   7  -7 0.00
Thu 10/09 15Z 1.25 3420  75   7  -7 0.00
Thu 10/09 18Z 3.48 3317  69   8  -6 0.00
Thu 10/09 21Z 1.02 3314  61   7  -5 0.00
Fri 10/10 00Z 0.10 3416  40   5  -2 0.00
Fri 10/10 03Z 0.24 3415  57   7  -4 0.00
Fri 10/10 06Z 0.11 3107  28   9   0 0.00
Fri 10/10 09Z 0.19 2809  23   7   1 0.00
Fri 10/10 12Z 0.22 2710  22   7   2 0.00
Fri 10/10 15Z 0.21 2609  21  11   3 0.00
Fri 10/10 18Z 0.21 2408  23   9   3 0.00

Run total areal avg QPF for the greens is: 1.69 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