NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20250701_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)
=== ===== === ==== ==== === === === ====
Tue 07/01 12Z 0.48 2124  96  96  16 0.00
Tue 07/01 15Z 0.67 2418  95  87  16 0.37
Tue 07/01 18Z 0.56 2514  91  73  16 0.13
Tue 07/01 21Z 0.94 2418  93  59  17 0.03
Wed 07/02 00Z 1.38 2624  84  62  16 0.04
Wed 07/02 03Z 0.83 2621  82  49  15 0.00
Wed 07/02 06Z 0.63 2819  87  51  14 0.00
Wed 07/02 09Z 0.54 2917  82  35  14 0.00
Wed 07/02 12Z 0.47 2714  71  22  14 0.00
Wed 07/02 15Z 0.49 2713  69  18  14 0.00
Wed 07/02 18Z 1.86 2709  78  12  13 0.00
Wed 07/02 21Z 5.21 2709  77  15  14 0.00
Thu 07/03 00Z 0.92 2711  79  18  14 0.00
Thu 07/03 03Z 0.99 2812  71  23  14 0.00
Thu 07/03 06Z 0.63 2812  64  38  14 0.00
Thu 07/03 09Z 0.79 2714  68  38  14 0.00
Thu 07/03 12Z 0.77 2616  71  46  13 0.00
Thu 07/03 15Z 1.02 2715  60  87  13 0.00
Thu 07/03 18Z 3.43 2713  75  76  13 0.00
Thu 07/03 21Z 5.30 2913  75  52  12 0.09
Fri 07/04 00Z 1.30 3114  77  55  11 0.01
Fri 07/04 03Z 0.95 3216  70  56  10 0.00
Fri 07/04 06Z 0.85 3216  67  66   9 0.00
Fri 07/04 09Z 0.86 3115  71  59  10 0.00
Fri 07/04 12Z 0.73 3113  72  42   7 0.00
Fri 07/04 15Z 3.79 2913  76  32   7 0.00
Fri 07/04 18Z 8.87 2814  76  28   9 0.00
Fri 07/04 21Z 3.22 2913  72  29  10 0.00
Sat 07/05 00Z 0.72 3013  67  26  10 0.00

Run total areal avg QPF for the greens is: 0.67 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