Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20251116_1800
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)
=== ===== === ==== ==== === === === ====
Sun 11/16 18Z 1.52 2817  95  83  -6 0.00
Sun 11/16 19Z 2.86 2922  94  79  -6 0.01
Sun 11/16 20Z 1.54 2924  95  77  -6 0.02
Sun 11/16 21Z 1.96 2927  94  69  -7 0.02
Sun 11/16 22Z 1.60 2928  95  67  -8 0.01
Sun 11/16 23Z 1.54 2929  95  70  -8 0.01
Mon 11/17 00Z 1.48 2929  95  76  -9 0.01
Mon 11/17 01Z 1.56 2929  95  75  -9 0.01
Mon 11/17 02Z 1.41 2929  94  81  -9 0.02
Mon 11/17 03Z 1.40 2929  93  85 -10 0.02
Mon 11/17 04Z 1.42 2928  93  80 -10 0.03
Mon 11/17 05Z 1.38 2929  93  77 -10 0.03
Mon 11/17 06Z 1.33 2928  93  80 -10 0.03
Mon 11/17 07Z 1.27 2928  93  81 -10 0.03
Mon 11/17 08Z 1.31 2929  93  80 -10 0.02
Mon 11/17 09Z 1.34 2930  94  82 -10 0.02
Mon 11/17 10Z 1.31 3030  94  82 -10 0.03
Mon 11/17 11Z 1.29 3031  94  79 -10 0.02
Mon 11/17 12Z 1.27 3030  94  77 -10 0.02
Mon 11/17 13Z 1.26 3030  94  73 -10 0.02
Mon 11/17 14Z 1.33 3029  94  70 -10 0.02
Mon 11/17 15Z 1.59 2928  93  71 -10 0.02
Mon 11/17 16Z 2.72 2927  92  72 -10 0.02
Mon 11/17 17Z 3.71 2926  91  72 -10 0.02
Mon 11/17 18Z 3.51 2926  90  71  -9 0.01
Mon 11/17 19Z 3.06 2925  91  70  -9 0.01
Mon 11/17 20Z 1.58 2926  92  76  -9 0.01
Mon 11/17 21Z 1.38 2928  93  76  -9 0.02
Mon 11/17 22Z 1.32 2930  94  70 -10 0.02
Mon 11/17 23Z 1.28 2932  95  67 -10 0.01
Tue 11/18 00Z 1.30 2932  96  66 -10 0.01
Tue 11/18 01Z 1.24 2932  96  66 -10 0.01
Tue 11/18 02Z 1.15 3031  96  67 -10 0.01
Tue 11/18 03Z 1.06 2929  96  71 -10 0.01
Tue 11/18 04Z 0.93 3029  96  74  -9 0.01
Tue 11/18 05Z 0.84 2928  96  76  -9 0.01
Tue 11/18 06Z 0.80 2928  96  74  -9 0.01
Tue 11/18 07Z 0.82 2928  95  74  -9 0.01
Tue 11/18 08Z 0.81 2928  95  74  -9 0.01
Tue 11/18 09Z 0.77 2928  94  73  -9 0.01
Tue 11/18 10Z 0.74 2929  94  72  -8 0.01
Tue 11/18 11Z 0.73 2929  93  64  -8 0.01
Tue 11/18 12Z 0.67 2929  93  57  -8 0.01
Tue 11/18 13Z 0.62 2928  92  54  -8 0.01
Tue 11/18 14Z 0.60 2927  89  56  -7 0.01
Tue 11/18 15Z 0.56 2925  87  57  -7 0.01
Tue 11/18 16Z 0.58 2925  86  56  -6 0.00
Tue 11/18 17Z 0.62 2924  83  49  -6 0.00
Tue 11/18 18Z 0.63 2923  81  48  -6 0.00
Tue 11/18 19Z 0.63 2922  80  47  -6 0.00
Tue 11/18 20Z 0.69 2922  81  46  -6 0.00
Tue 11/18 21Z 0.77 2823  84  42  -6 0.00
Tue 11/18 22Z 0.79 2823  85  36  -6 0.00
Tue 11/18 23Z 0.69 2923  84  34  -6 0.00
Wed 11/19 00Z 0.56 2922  83  33  -5 0.00
Wed 11/19 01Z 0.48 2921  81  31  -5 0.00
Wed 11/19 02Z 0.43 2920  80  31  -5 0.00
Wed 11/19 03Z 0.36 2918  78  30  -4 0.00
Wed 11/19 04Z 0.34 2918  77  31  -4 0.00
Wed 11/19 05Z 0.30 2916  75  30  -3 0.00

Run total areal avg QPF for the greens is: 0.70 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