Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260128_1200
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)
=== ===== === ==== ==== === === === ====
Wed 01/28 12Z 0.47 2917  81  35 -19 0.00
Wed 01/28 13Z 0.51 2917  83  31 -19 0.00
Wed 01/28 14Z 0.47 2916  84  31 -19 0.00
Wed 01/28 15Z 0.49 2816  82  39 -19 0.00
Wed 01/28 16Z 0.48 2814  76  45 -19 0.00
Wed 01/28 17Z 0.54 2813  73  53 -19 0.00
Wed 01/28 18Z 0.58 2812  72  62 -18 0.00
Wed 01/28 19Z 0.63 2812  74  66 -18 0.00
Wed 01/28 20Z 0.68 2811  79  61 -18 0.00
Wed 01/28 21Z 0.79 2711  86  58 -19 0.00
Wed 01/28 22Z 0.73 2812  83  56 -19 0.00
Wed 01/28 23Z 0.76 2914  78  56 -18 0.00
Thu 01/29 00Z 0.67 2914  77  57 -18 0.00
Thu 01/29 01Z 0.56 2912  79  60 -18 0.00
Thu 01/29 02Z 0.51 2911  84  60 -18 0.00
Thu 01/29 03Z 0.48 2810  88  62 -18 0.00
Thu 01/29 04Z 0.45 2810  90  65 -18 0.00
Thu 01/29 05Z 0.43 2810  90  67 -18 0.00
Thu 01/29 06Z 0.42 2811  88  66 -18 0.00
Thu 01/29 07Z 0.43 2811  84  65 -19 0.00
Thu 01/29 08Z 0.42 2912  85  61 -19 0.00
Thu 01/29 09Z 0.40 2811  86  57 -19 0.00
Thu 01/29 10Z 0.41 2812  86  56 -19 0.00
Thu 01/29 11Z 0.41 2812  86  57 -19 0.00
Thu 01/29 12Z 0.39 2813  84  59 -19 0.00
Thu 01/29 13Z 0.37 2913  84  64 -19 0.00
Thu 01/29 14Z 0.35 3012  85  67 -19 0.00
Thu 01/29 15Z 0.36 3012  88  69 -19 0.00
Thu 01/29 16Z 0.43 3012  89  67 -19 0.00
Thu 01/29 17Z 0.62 3013  90  65 -19 0.00
Thu 01/29 18Z 1.27 3014  90  61 -19 0.00
Thu 01/29 19Z 1.96 3016  92  58 -19 0.00
Thu 01/29 20Z 1.53 3016  93  58 -19 0.00
Thu 01/29 21Z 1.34 3017  93  56 -19 0.01
Thu 01/29 22Z 1.40 3019  93  51 -20 0.01
Thu 01/29 23Z 1.25 3022  92  47 -21 0.01
Fri 01/30 00Z 0.93 3024  88  43 -21 0.00
Fri 01/30 01Z 0.88 3025  86  39 -22 0.00
Fri 01/30 02Z 0.96 3026  86  38 -23 0.00
Fri 01/30 03Z 0.95 3027  84  36 -23 0.00
Fri 01/30 04Z 0.88 3027  83  36 -24 0.00
Fri 01/30 05Z 0.83 2927  82  34 -24 0.00
Fri 01/30 06Z 0.79 2927  80  33 -24 0.00
Fri 01/30 07Z 0.71 2929  76  32 -24 0.00
Fri 01/30 08Z 0.63 2930  72  36 -23 0.00
Fri 01/30 09Z 0.56 3029  71  42 -23 0.00
Fri 01/30 10Z 0.50 3029  70  46 -23 0.00
Fri 01/30 11Z 0.45 3028  69  52 -22 0.00
Fri 01/30 12Z 0.41 3028  70  61 -22 0.00
Fri 01/30 13Z 0.38 3027  72  69 -21 0.00
Fri 01/30 14Z 0.35 3025  74  76 -21 0.00
Fri 01/30 15Z 0.34 3023  76  80 -20 0.00
Fri 01/30 16Z 0.33 3022  79  82 -20 0.00
Fri 01/30 17Z 0.33 3020  80  81 -20 0.00
Fri 01/30 18Z 0.33 3020  80  79 -19 0.00
Fri 01/30 19Z 0.34 2919  80  76 -19 0.00
Fri 01/30 20Z 0.36 2919  82  71 -19 0.00
Fri 01/30 21Z 0.38 2919  83  66 -19 0.00
Fri 01/30 22Z 0.38 3020  84  63 -18 0.00
Fri 01/30 23Z 0.34 3021  85  62 -18 0.00

Run total areal avg QPF for the greens is: 0.03 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