NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260228_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)
=== ===== === ==== ==== === === === ====
Sat 02/28 12Z 0.49 2135  70  50  -1 0.00
Sat 02/28 13Z 0.56 2138  70  64  -1 0.00
Sat 02/28 14Z 0.54 2139  70  83   0 0.00
Sat 02/28 15Z 0.63 2141  72  91   0 0.00
Sat 02/28 16Z 0.65 2139  81  96   0 0.01
Sat 02/28 17Z 0.76 2238  88  96  -1 0.02
Sat 02/28 18Z 0.75 2236  88  92   0 0.01
Sat 02/28 19Z 0.79 2333  90  72  -1 0.01
Sat 02/28 20Z 0.85 2430  91  65  -1 0.01
Sat 02/28 21Z 0.97 2528  92  45  -2 0.00
Sat 02/28 22Z 1.03 2527  90  25  -3 0.01
Sat 02/28 23Z 0.91 2624  88  23  -3 0.01
Sun 03/01 00Z 0.81 2723  84  23  -4 0.01
Sun 03/01 01Z 0.65 2821  77  25  -5 0.01
Sun 03/01 02Z 0.46 2918  74  32  -5 0.01
Sun 03/01 03Z 0.33 3015  69  42  -6 0.00
Sun 03/01 04Z 0.28 3014  69  52  -6 0.00
Sun 03/01 05Z 0.27 3014  74  75  -7 0.00
Sun 03/01 06Z 0.27 3015  75  84  -8 0.00
Sun 03/01 07Z 0.25 3115  73  93  -9 0.00
Sun 03/01 08Z 0.27 3116  74  92 -10 0.00
Sun 03/01 09Z 0.25 3115  82  89 -11 0.00
Sun 03/01 10Z 0.25 3115  84  86 -12 0.00
Sun 03/01 11Z 0.21 3215  83  83 -13 0.00
Sun 03/01 12Z 0.20 3215  79  85 -13 0.00
Sun 03/01 13Z 0.23 3115  75  87 -14 0.00
Sun 03/01 14Z 0.29 3116  72  85 -15 0.00
Sun 03/01 15Z 0.30 3116  64  82 -15 0.00
Sun 03/01 16Z 0.36 3117  59  79 -16 0.00
Sun 03/01 17Z 0.40 3118  56  61 -16 0.00
Sun 03/01 18Z 0.46 3118  60  47 -17 0.00
Sun 03/01 19Z 0.61 3118  66  27 -18 0.00
Sun 03/01 20Z 0.58 3118  67  24 -18 0.00
Sun 03/01 21Z 0.58 3118  68  19 -18 0.00
Sun 03/01 22Z 0.60 3119  70  17 -19 0.00
Sun 03/01 23Z 0.58 3120  70  14 -19 0.00
Mon 03/02 00Z 0.55 3121  66  13 -20 0.00
Mon 03/02 01Z 0.50 3121  63  13 -20 0.00
Mon 03/02 02Z 0.45 3121  61  14 -20 0.00
Mon 03/02 03Z 0.39 3120  58  15 -20 0.00
Mon 03/02 05Z 0.34 3119  52  15 -20 0.00
Mon 03/02 06Z 0.33 3119  47  14 -20 0.00
Mon 03/02 07Z 0.30 3118  42  14 -20 0.00
Mon 03/02 08Z 0.31 3118  42  13 -20 0.00
Mon 03/02 09Z 0.31 3117  42  12 -20 0.00
Mon 03/02 10Z 0.32 3017  42  12 -20 0.00
Mon 03/02 11Z 0.33 3018  42  12 -20 0.00
Mon 03/02 12Z 0.35 3018  42  15 -20 0.00
Mon 03/02 13Z 0.37 2918  42  16 -20 0.00
Mon 03/02 14Z 0.36 2917  44  19 -20 0.00
Mon 03/02 15Z 0.38 2916  46  17 -20 0.00
Mon 03/02 16Z 0.38 2914  45  20 -19 0.00
Mon 03/02 17Z 0.38 2913  42  19 -19 0.00
Mon 03/02 18Z 0.38 2912  43  19 -18 0.00
Mon 03/02 19Z 0.36 2811  43  22 -17 0.00
Mon 03/02 20Z 0.35 2711  45  15 -16 0.00
Mon 03/02 21Z 0.35 2712  47  28 -15 0.00
Mon 03/02 22Z 0.36 2613  48  43 -14 0.00
Mon 03/02 23Z 0.35 2514  45  49 -13 0.00

Run total areal avg QPF for the greens is: 0.11 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