Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260123_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 01/23 18Z 5.59 2616  81  62 -18 0.00
Fri 01/23 19Z 10.80 2719  79  67 -18 0.00
Fri 01/23 20Z 8.85 2822  77  76 -19 0.01
Fri 01/23 21Z 7.58 2923  77  78 -20 0.01
Fri 01/23 22Z 4.76 2924  76  76 -21 0.01
Fri 01/23 23Z 6.33 2925  72  71 -22 0.00
Sat 01/24 00Z 4.39 2827  74  70 -23 0.00
Sat 01/24 01Z 9.27 2931  71  58 -24 0.01
Sat 01/24 02Z 4.53 2831  66  36 -24 0.00
Sat 01/24 03Z 10.41 2929  73  36 -26 0.00
Sat 01/24 04Z 7.74 3030  76  33 -27 0.00
Sat 01/24 05Z 7.16 3030  81  25 -28 0.00
Sat 01/24 06Z 4.25 3029  74  25 -29 0.00
Sat 01/24 07Z 0.98 3030  61  25 -29 0.00
Sat 01/24 08Z 0.70 3029  62  23 -29 0.00
Sat 01/24 09Z 0.58 3129  64  22 -28 0.00
Sat 01/24 10Z 0.50 3128  64  21 -27 0.00
Sat 01/24 11Z 0.45 3127  63  20 -26 0.00
Sat 01/24 12Z 0.41 3127  60  19 -26 0.00
Sat 01/24 13Z 0.39 3127  57  19 -25 0.00
Sat 01/24 14Z 0.37 3126  54  19 -24 0.00
Sat 01/24 15Z 0.37 3125  51  17 -24 0.00
Sat 01/24 16Z 0.38 3024  49  17 -24 0.00
Sat 01/24 17Z 0.41 3024  47  18 -24 0.00
Sat 01/24 18Z 0.43 3024  48  18 -24 0.00
Sat 01/24 19Z 0.44 3022  51  17 -24 0.00
Sat 01/24 21Z 0.49 2921  55  13 -24 0.00
Sat 01/24 22Z 0.48 2922  53  11 -23 0.00
Sat 01/24 23Z 0.43 3022  50  11 -23 0.00
Sun 01/25 00Z 0.40 3022  49  12 -22 0.00
Sun 01/25 01Z 0.37 3022  48  12 -21 0.00
Sun 01/25 02Z 0.35 3021  46  13 -21 0.00
Sun 01/25 03Z 0.31 3020  42  12 -20 0.00
Sun 01/25 04Z 0.28 3019  38  10 -20 0.00
Sun 01/25 05Z 0.26 3018  36   9 -19 0.00
Sun 01/25 06Z 0.24 3016  34   8 -19 0.00
Sun 01/25 07Z 0.22 3116  34   7 -18 0.00
Sun 01/25 08Z 0.20 3114  35   8 -18 0.00
Sun 01/25 09Z 0.18 3012  36  10 -18 0.00
Sun 01/25 10Z 0.17 2910  35  13 -18 0.00
Sun 01/25 11Z 0.14 2908  33  21 -17 0.00
Sun 01/25 12Z 0.14 2807  31  30 -17 0.00
Sun 01/25 13Z 0.10 2705  28  42 -16 0.00
Sun 01/25 14Z 0.06 2403  27  57 -16 0.00
Sun 01/25 15Z 0.04 2103  27  68 -16 0.00
Sun 01/25 16Z 0.03 1805  30  77 -16 0.00
Sun 01/25 17Z 0.02 1806  40  85 -16 0.00
Sun 01/25 18Z 0.04 1607  59  87 -17 0.00
Sun 01/25 19Z 0.08 1508  69  87 -16 0.00
Sun 01/25 20Z 0.11 1410  74  87 -15 0.01
Sun 01/25 21Z 0.18 1415  82  88 -15 0.01
Sun 01/25 22Z 0.21 1418  85  89 -15 0.02
Sun 01/25 23Z 0.22 1421  85  91 -15 0.02
Mon 01/26 00Z 0.23 1523  85  92 -15 0.03
Mon 01/26 01Z 0.32 1426  85  93 -15 0.03
Mon 01/26 02Z 0.38 1429  86  92 -14 0.03
Mon 01/26 03Z 0.41 1330  86  91 -14 0.04
Mon 01/26 04Z 0.39 1329  86  91 -12 0.04
Mon 01/26 05Z 0.39 1227  87  91 -11 0.04

Run total areal avg QPF for the greens is: 0.31 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