Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260223_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)
=== ===== === ==== ==== === === === ====
Mon 02/23 00Z 0.46 1013  86  74  -8 0.00
Mon 02/23 01Z 0.59 0916  86  72  -8 0.00
Mon 02/23 02Z 0.64 0919  85  75  -8 0.00
Mon 02/23 03Z 0.63 0920  84  74  -8 0.00
Mon 02/23 05Z 0.61 0821  78  54  -8 0.00
Mon 02/23 06Z 0.68 0822  77  32  -9 0.00
Mon 02/23 07Z 0.67 0822  76  47  -9 0.00
Mon 02/23 08Z 0.63 0822  73  67  -8 0.00
Mon 02/23 09Z 0.56 0721  72  77  -8 0.00
Mon 02/23 12Z 0.46 0525  69  76  -8 0.00
Mon 02/23 13Z 0.50 0526  61  75  -8 0.00
Mon 02/23 15Z 0.45 0426  47  69  -8 0.00
Mon 02/23 16Z 0.37 0323  49  63  -9 0.01
Mon 02/23 17Z 0.33 0222  56  63  -9 0.00
Mon 02/23 18Z 0.26 0222  61  67 -10 0.00
Mon 02/23 19Z 0.17 0122  64  75 -10 0.00
Mon 02/23 20Z 0.10 0023  70  81 -10 0.00
Mon 02/23 21Z 0.09 3525  71  71 -10 0.00
Mon 02/23 22Z 0.12 3528  74  56 -10 0.00
Mon 02/23 23Z 0.13 3531  73  45 -11 0.00
Tue 02/24 00Z 0.12 3534  76  32 -12 0.00
Tue 02/24 01Z 0.17 3533  87  23 -13 0.00
Tue 02/24 02Z 0.31 3433  92  17 -13 0.00
Tue 02/24 04Z 0.28 3428  92  19 -13 0.00
Tue 02/24 05Z 0.36 3427  95  18 -14 0.00
Tue 02/24 06Z 0.41 3328  95  20 -14 0.00
Tue 02/24 07Z 0.52 3328  95  23 -15 0.00
Tue 02/24 08Z 0.61 3326  96  29 -16 0.00
Tue 02/24 09Z 0.55 3224  93  30 -16 0.00
Tue 02/24 10Z 0.44 3224  90  44 -16 0.01
Tue 02/24 11Z 0.41 3123  89  56 -16 0.01
Tue 02/24 12Z 0.37 3123  89  58 -16 0.01
Tue 02/24 13Z 0.32 3122  88  60 -16 0.01
Tue 02/24 14Z 0.31 3121  89  62 -16 0.01
Tue 02/24 15Z 0.31 3120  89  64 -16 0.01
Tue 02/24 16Z 0.31 3118  88  62 -16 0.01
Tue 02/24 17Z 0.34 3017  89  61 -16 0.01
Tue 02/24 18Z 0.38 3015  88  62 -15 0.00
Tue 02/24 19Z 0.42 2915  88  62 -15 0.00
Tue 02/24 20Z 0.44 2914  87  53 -15 0.00
Tue 02/24 21Z 0.46 2814  86  36 -14 0.00
Tue 02/24 22Z 0.63 2815  88  20 -15 0.00
Tue 02/24 23Z 0.51 2715  86   7 -14 0.00
Wed 02/25 00Z 0.38 2714  83   3 -14 0.00
Wed 02/25 01Z 0.30 2713  79   3 -13 0.00
Wed 02/25 02Z 0.26 2612  73   4 -13 0.00
Wed 02/25 03Z 0.22 2412  63  10 -12 0.00
Wed 02/25 04Z 0.21 2313  55  36 -12 0.00
Wed 02/25 05Z 0.21 2214  48  64 -11 0.00
Wed 02/25 06Z 0.22 2216  39  84 -11 0.00
Wed 02/25 07Z 0.27 2218  45  87 -12 0.00
Wed 02/25 09Z 0.21 2022  86  86 -11 0.00
Wed 02/25 10Z 0.17 1925  90  84 -10 0.00
Wed 02/25 11Z 0.21 2026  92  87 -10 0.00

Run total areal avg QPF for the greens is: 0.09 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