NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20261030_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)
=== ===== === ==== ==== === === === ====
Fri 10/30 18Z 1.78 2824  91  33   0 0.00
Fri 10/30 19Z 4.55 2929  91  34   0 0.01
Fri 10/30 20Z 3.94 2930  92  26   0 0.01
Fri 10/30 21Z 2.31 2931  94  25  -1 0.01
Fri 10/30 22Z 1.58 3031  94  24  -1 0.01
Fri 10/30 23Z 1.60 3032  96  26  -2 0.01
Sat 10/31 00Z 1.55 3033  95  30  -2 0.01
Sat 10/31 01Z 1.34 3133  95  35  -2 0.01
Sat 10/31 02Z 1.21 3132  95  42  -3 0.01
Sat 10/31 03Z 1.09 3131  94  37  -4 0.01
Sat 10/31 04Z 0.91 3230  93  38  -5 0.00
Sat 10/31 05Z 0.79 3229  94  34  -5 0.00
Sat 10/31 06Z 0.72 3228  94  21  -6 0.00
Sat 10/31 07Z 0.61 3228  92  12  -6 0.00
Sat 10/31 08Z 0.47 3327  91   7  -6 0.00
Sat 10/31 09Z 0.43 3325  91   9  -6 0.00
Sat 10/31 10Z 0.38 3323  89  13  -6 0.00
Sat 10/31 11Z 0.34 3323  84  19  -6 0.00
Sat 10/31 12Z 0.33 3322  79  21  -6 0.00
Sat 10/31 13Z 0.34 3222  77  22  -6 0.00
Sat 10/31 14Z 0.33 3220  77  24  -6 0.00
Sat 10/31 15Z 0.30 3217  73  27  -5 0.00
Sat 10/31 16Z 0.33 3116  69  32  -5 0.00
Sat 10/31 17Z 0.40 3116  68  33  -4 0.00
Sat 10/31 18Z 0.47 3117  71  27  -4 0.00
Sat 10/31 19Z 0.45 3016  72  16  -4 0.00
Sat 10/31 20Z 0.41 3015  69  16  -3 0.00
Sat 10/31 21Z 0.47 3017  70  26  -3 0.00
Sat 10/31 22Z 0.52 2918  69  32  -3 0.00
Sat 10/31 23Z 0.59 2919  73  33  -3 0.00
Sun 11/01 00Z 0.60 2919  76  32  -3 0.00
Sun 11/01 01Z 0.59 2919  78  21  -3 0.00
Sun 11/01 03Z 0.41 3018  75  15  -2 0.00
Sun 11/01 04Z 0.37 3017  71  11  -1 0.00
Sun 11/01 07Z 0.36 2918  60  12   0 0.00
Sun 11/01 08Z 0.36 2918  55  14   0 0.00
Sun 11/01 10Z 0.38 2920  45  13   0 0.00
Sun 11/01 12Z 0.42 3024  35   5   1 0.00
Sun 11/01 13Z 0.43 3126  30   6   1 0.00
Sun 11/01 14Z 0.44 3126  27   7   1 0.00
Sun 11/01 15Z 0.47 3126  25  14   1 0.00
Sun 11/01 16Z 0.53 3127  37  16   1 0.00
Sun 11/01 17Z 0.57 3126  67  20   1 0.00
Sun 11/01 18Z 0.58 3025  65  17   2 0.00
Sun 11/01 19Z 0.60 3025  59  13   2 0.00
Sun 11/01 20Z 0.67 2927  61   9   3 0.00
Sun 11/01 21Z 0.76 2928  57  11   3 0.00
Sun 11/01 22Z 0.81 2929  55  10   3 0.00
Sun 11/01 23Z 0.80 2929  54   8   3 0.00
Mon 11/02 00Z 0.79 2930  56   8   3 0.00
Mon 11/02 01Z 0.79 2929  53   9   4 0.00
Mon 11/02 02Z 0.78 2929  54  11   4 0.00
Mon 11/02 03Z 0.78 2928  55  16   4 0.00
Mon 11/02 04Z 0.81 2928  56  19   4 0.00
Mon 11/02 05Z 0.86 2928  55  19   4 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