NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260423_0600
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)
=== ===== === ==== ==== === === === ====
Thu 04/23 06Z 0.35 2812  82  72   2 0.00
Thu 04/23 07Z 0.42 2814  82  78   2 0.00
Thu 04/23 08Z 0.43 2916  81  82   2 0.00
Thu 04/23 09Z 0.43 3019  81  87   1 0.00
Thu 04/23 10Z 0.45 3122  84  81   0 0.00
Thu 04/23 11Z 0.40 3223  86  78   0 0.00
Thu 04/23 12Z 0.38 3324  87  59  -1 0.01
Thu 04/23 13Z 0.43 3224  88  37  -2 0.00
Thu 04/23 14Z 0.51 3224  89  17  -3 0.00
Thu 04/23 15Z 0.87 3224  84   9  -3 0.00
Thu 04/23 16Z 2.41 3222  81   8  -3 0.00
Thu 04/23 17Z 4.85 3220  80   6  -3 0.00
Thu 04/23 18Z 5.75 3219  75   5  -2 0.00
Thu 04/23 19Z 5.70 3219  71   5  -2 0.00
Thu 04/23 20Z 5.87 3219  67   5  -2 0.00
Thu 04/23 21Z 5.96 3219  64   4  -2 0.00
Thu 04/23 22Z 5.04 3220  63   4  -2 0.00
Thu 04/23 23Z 2.55 3320  62   4  -2 0.00
Fri 04/24 00Z 1.37 3322  62   4  -2 0.00
Fri 04/24 01Z 0.98 3323  62   5  -3 0.00
Fri 04/24 02Z 0.70 3422  64   5  -3 0.00
Fri 04/24 03Z 0.42 3420  66   6  -4 0.00
Fri 04/24 04Z 0.21 3517  69   7  -4 0.00
Fri 04/24 05Z 0.13 3516  73   9  -5 0.00
Fri 04/24 06Z 0.13 3414  75   9  -5 0.00
Fri 04/24 07Z 0.16 3414  77   9  -6 0.00
Fri 04/24 08Z 0.18 3415  77   9  -6 0.00
Fri 04/24 09Z 0.12 3417  77   9  -6 0.00
Fri 04/24 10Z 0.09 3517  77   9  -6 0.00
Fri 04/24 11Z 0.08 3416  79  10  -6 0.00
Fri 04/24 12Z 0.10 3415  81  15  -6 0.00
Fri 04/24 13Z 0.13 3414  81  23  -6 0.00
Fri 04/24 14Z 0.19 3313  78  37  -5 0.00
Fri 04/24 15Z 0.40 3313  74  49  -5 0.00
Fri 04/24 16Z 1.10 3313  72  46  -5 0.00
Fri 04/24 17Z 1.74 3313  68  44  -5 0.00
Fri 04/24 18Z 2.31 3314  63  36  -4 0.00
Fri 04/24 19Z 1.87 3414  58  21  -4 0.00
Fri 04/24 20Z 1.56 3413  57  13  -4 0.00
Fri 04/24 21Z 1.10 3412  59   9  -3 0.00
Fri 04/24 22Z 0.43 0011  64   6  -3 0.00
Fri 04/24 23Z 0.41 0111  68   7  -4 0.00
Sat 04/25 00Z 0.32 0111  73   9  -4 0.00
Sat 04/25 01Z 0.48 0213  77  11  -5 0.00
Sat 04/25 02Z 0.67 0317  77  15  -5 0.00
Sat 04/25 03Z 0.73 0419  74  18  -5 0.00
Sat 04/25 04Z 0.74 0520  75  22  -5 0.00
Sat 04/25 05Z 0.61 0518  75  24  -5 0.00
Sat 04/25 06Z 0.48 0515  74  24  -5 0.00
Sat 04/25 07Z 0.36 0614  70  25  -4 0.00
Sat 04/25 08Z 0.29 0612  67  25  -4 0.00
Sat 04/25 09Z 0.26 0711  64  25  -3 0.00
Sat 04/25 10Z 0.24 0710  61  26  -3 0.00
Sat 04/25 11Z 0.24 0810  58  29  -3 0.00
Sat 04/25 12Z 0.23 0810  55  30  -3 0.00
Sat 04/25 13Z 0.26 0910  55  30  -3 0.00
Sat 04/25 14Z 0.32 0910  55  29  -3 0.00
Sat 04/25 15Z 0.46 1008  55  28  -3 0.00
Sat 04/25 16Z 1.46 1007  52  29  -3 0.00
Sat 04/25 17Z 2.86 0905  52  31  -2 0.00

Run total areal avg QPF for the greens is: 0.01 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