NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260317_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 03/17 06Z 0.34 2021  98  98   5 0.00
Tue 03/17 07Z 1.80 2631  97  92  -3 0.11
Tue 03/17 08Z 1.37 2633  95  95  -6 0.05
Tue 03/17 09Z 1.09 2530  92  88  -7 0.02
Tue 03/17 10Z 2.24 2432  92  36  -9 0.00
Tue 03/17 11Z 7.01 2533  86  27 -11 0.00
Tue 03/17 12Z 10.22 2433  78  28 -12 0.00
Tue 03/17 13Z 14.36 2535  74  23 -13 0.00
Tue 03/17 14Z 20.03 2533  75  19 -15 0.00
Tue 03/17 15Z 19.92 2531  76  17 -15 0.00
Tue 03/17 16Z 18.49 2529  78  18 -15 0.00
Tue 03/17 17Z 17.15 2428  77  23 -15 0.00
Tue 03/17 18Z 17.88 2528  73  25 -15 0.00
Tue 03/17 19Z 18.95 2528  70  31 -15 0.00
Tue 03/17 20Z 18.90 2628  72  35 -15 0.00
Tue 03/17 21Z 16.79 2627  75  39 -15 0.00
Tue 03/17 22Z 13.33 2626  77  46 -16 0.00
Tue 03/17 23Z 12.63 2727  78  52 -17 0.00
Wed 03/18 00Z 10.07 2726  82  48 -18 0.00
Wed 03/18 01Z 7.32 2824  88  42 -18 0.00
Wed 03/18 02Z 5.18 2823  85  46 -19 0.00
Wed 03/18 03Z 3.10 2822  85  54 -19 0.00
Wed 03/18 04Z 2.49 2823  86  54 -19 0.00
Wed 03/18 05Z 2.67 2823  84  41 -19 0.00
Wed 03/18 06Z 2.21 2823  83  26 -19 0.00
Wed 03/18 07Z 1.75 2823  82  16 -19 0.00
Wed 03/18 08Z 1.42 2823  79  10 -19 0.00
Wed 03/18 09Z 1.16 2823  73   7 -19 0.00
Wed 03/18 10Z 0.99 2822  71   6 -19 0.00
Wed 03/18 11Z 0.78 2920  69   5 -20 0.00
Wed 03/18 12Z 0.59 2917  65   5 -19 0.00
Wed 03/18 13Z 0.53 2916  63   6 -19 0.00
Wed 03/18 14Z 0.46 2914  56   7 -18 0.00
Wed 03/18 15Z 0.41 2811  49   8 -18 0.00
Wed 03/18 16Z 0.42 2809  44   8 -17 0.00
Wed 03/18 17Z 0.72 2707  40   9 -16 0.00
Wed 03/18 18Z 0.84 2606  37   9 -16 0.00
Wed 03/18 19Z 0.82 2505  35   9 -15 0.00
Wed 03/18 20Z 0.74 2404  36   9 -14 0.00
Wed 03/18 21Z 0.34 2104  36   8 -13 0.00
Wed 03/18 22Z 0.15 1906  37   8 -13 0.00
Wed 03/18 23Z 0.09 1810  38   7 -13 0.00
Thu 03/19 00Z 0.10 1813  39   9 -12 0.00
Thu 03/19 01Z 0.18 1916  38  12 -12 0.00
Thu 03/19 02Z 0.28 2017  38  23 -12 0.00
Thu 03/19 03Z 0.34 2019  42  39 -12 0.00
Thu 03/19 04Z 0.45 2020  48  71 -11 0.00
Thu 03/19 05Z 0.49 2121  53  88 -11 0.00
Thu 03/19 06Z 0.49 2121  58  87 -11 0.00
Thu 03/19 09Z 0.52 2121  66  87 -10 0.00
Thu 03/19 10Z 0.48 2121  73  86 -11 0.00
Thu 03/19 11Z 0.41 2121  79  89 -10 0.00
Thu 03/19 12Z 0.34 2022  83  88 -10 0.00
Thu 03/19 13Z 0.32 2022  86  90  -9 0.00
Thu 03/19 14Z 0.32 2021  88  89  -9 0.00
Thu 03/19 15Z 0.34 2020  87  81  -9 0.00
Thu 03/19 16Z 0.58 2018  84  69  -8 0.00
Thu 03/19 17Z 0.91 2018  80  47  -8 0.00

Run total areal avg QPF for the greens is: 0.18 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