Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260317_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)
=== ===== === ==== ==== === === === ====
Tue 03/17 00Z 0.05 1840  92  96   9 0.00
Tue 03/17 01Z 0.04 1742  94  94   9 0.02
Tue 03/17 02Z 0.04 1741  96  87   9 0.01
Tue 03/17 03Z 0.04 1842  95  89   9 0.01
Tue 03/17 04Z 0.05 1742  96  86   9 0.02
Tue 03/17 05Z 0.11 1741  96  92   9 0.01
Tue 03/17 06Z 0.16 1738  98  97   9 0.06
Tue 03/17 07Z 0.60 2525  96  90   0 0.18
Tue 03/17 08Z 1.33 2533  96  96  -5 0.03
Tue 03/17 09Z 1.21 2534  94  92  -7 0.03
Tue 03/17 10Z 1.44 2431  93  21  -9 0.01
Tue 03/17 11Z 5.85 2531  90   9 -11 0.00
Tue 03/17 12Z 15.83 2433  75  11 -13 0.00
Tue 03/17 13Z 20.50 2434  71  16 -14 0.00
Tue 03/17 14Z 20.06 2432  74  21 -15 0.00
Tue 03/17 15Z 20.80 2532  74  19 -15 0.00
Tue 03/17 16Z 20.08 2531  76  18 -15 0.00
Tue 03/17 17Z 20.23 2530  74  18 -16 0.00
Tue 03/17 18Z 20.02 2530  71  20 -16 0.00
Tue 03/17 19Z 19.72 2530  71  31 -16 0.00
Tue 03/17 20Z 19.43 2529  71  38 -16 0.00
Tue 03/17 21Z 18.28 2628  73  47 -17 0.00
Tue 03/17 22Z 18.15 2628  77  50 -17 0.00
Tue 03/17 23Z 13.12 2727  81  47 -18 0.00
Wed 03/18 00Z 9.58 2726  86  47 -19 0.00
Wed 03/18 01Z 5.49 2725  85  49 -19 0.00
Wed 03/18 02Z 3.61 2724  85  50 -19 0.00
Wed 03/18 03Z 2.79 2823  85  54 -19 0.00
Wed 03/18 04Z 2.75 2822  84  55 -19 0.00
Wed 03/18 05Z 2.65 2722  84  36 -18 0.00
Wed 03/18 06Z 2.21 2722  81  24 -18 0.00
Wed 03/18 07Z 2.18 2722  81  16 -19 0.00
Wed 03/18 08Z 1.85 2823  77  11 -19 0.00
Wed 03/18 09Z 1.42 2822  73   8 -19 0.00
Wed 03/18 10Z 1.02 2821  71   6 -19 0.00
Wed 03/18 11Z 0.77 2920  70   5 -20 0.00
Wed 03/18 12Z 0.54 2916  68   5 -19 0.00
Wed 03/18 13Z 0.44 2913  65   5 -19 0.00
Wed 03/18 14Z 0.42 2812  60   5 -19 0.00
Wed 03/18 15Z 0.42 2810  53   6 -18 0.00
Wed 03/18 16Z 0.55 2708  47   7 -17 0.00
Wed 03/18 17Z 0.72 2606  42   8 -17 0.00
Wed 03/18 18Z 0.72 2504  37   8 -16 0.00
Wed 03/18 19Z 0.79 2404  35   8 -15 0.00
Wed 03/18 20Z 0.59 2204  34   8 -14 0.00
Wed 03/18 21Z 0.33 2105  34   7 -13 0.00
Wed 03/18 22Z 0.18 1906  35   7 -13 0.00
Wed 03/18 23Z 0.08 1810  36   9 -13 0.00
Thu 03/19 00Z 0.08 1813  37  20 -12 0.00
Thu 03/19 01Z 0.17 1916  35  39 -12 0.00
Thu 03/19 02Z 0.24 1918  34  68 -12 0.00
Thu 03/19 03Z 0.31 1920  40  89 -12 0.00
Thu 03/19 04Z 0.43 2022  48  90 -11 0.00
Thu 03/19 05Z 0.52 2123  55  86 -11 0.00
Thu 03/19 06Z 0.53 2123  60  87 -11 0.00
Thu 03/19 07Z 0.55 2123  63  87 -11 0.00
Thu 03/19 08Z 0.56 2221  66  86 -11 0.00
Thu 03/19 09Z 0.51 2120  71  86 -11 0.00
Thu 03/19 10Z 0.38 2119  81  84 -11 0.00
Thu 03/19 11Z 0.27 2019  87  83 -10 0.00

Run total areal avg QPF for the greens is: 0.38 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