Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260207_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)
=== ===== === ==== ==== === === === ====
Sat 02/07 06Z 0.08 1909  89  84 -12 0.00
Sat 02/07 07Z 0.05 1809  91  88 -12 0.00
Sat 02/07 08Z 0.06 1908  91  86 -12 0.00
Sat 02/07 09Z 0.07 1906  90  83 -12 0.01
Sat 02/07 10Z 0.08 2104  91  83 -12 0.01
Sat 02/07 11Z 0.13 2603  92  83 -13 0.02
Sat 02/07 12Z 0.15 3108  91  80 -13 0.02
Sat 02/07 13Z 0.14 3310  89  81 -14 0.02
Sat 02/07 15Z 0.10 3315  84  81 -16 0.01
Sat 02/07 16Z 0.11 3315  84  83 -16 0.01
Sat 02/07 17Z 0.14 3316  83  82 -17 0.01
Sat 02/07 18Z 0.15 3217  82  82 -18 0.01
Sat 02/07 19Z 0.17 3218  82  82 -18 0.01
Sat 02/07 20Z 0.19 3219  82  81 -19 0.01
Sat 02/07 21Z 0.21 3321  83  80 -20 0.01
Sat 02/07 22Z 0.23 3323  83  81 -20 0.01
Sat 02/07 23Z 0.22 3324  83  81 -20 0.01
Sun 02/08 00Z 0.21 3324  83  80 -20 0.01
Sun 02/08 01Z 0.18 3324  83  79 -20 0.01
Sun 02/08 02Z 0.15 3324  84  79 -20 0.01
Sun 02/08 03Z 0.13 3423  84  78 -19 0.00
Sun 02/08 04Z 0.11 3422  83  76 -19 0.00
Sun 02/08 05Z 0.10 3422  84  76 -18 0.00
Sun 02/08 06Z 0.09 3421  85  76 -18 0.00
Sun 02/08 07Z 0.10 3420  84  80 -18 0.00
Sun 02/08 08Z 0.11 3421  83  81 -18 0.00
Sun 02/08 09Z 0.10 3421  81  78 -18 0.00
Sun 02/08 10Z 0.11 3422  80  70 -17 0.00
Sun 02/08 11Z 0.11 3425  80  61 -17 0.00
Sun 02/08 12Z 0.10 3427  77  48 -16 0.00
Sun 02/08 13Z 0.09 3428  74  34 -16 0.00
Sun 02/08 14Z 0.08 3427  70  34 -15 0.00
Sun 02/08 15Z 0.07 3426  69  36 -15 0.00
Sun 02/08 16Z 0.07 3425  68  44 -14 0.00
Sun 02/08 17Z 0.07 3425  65  56 -13 0.00
Sun 02/08 18Z 0.07 3425  62  55 -12 0.00
Sun 02/08 19Z 0.07 3425  60  42 -12 0.00
Sun 02/08 20Z 0.08 3425  56  33 -11 0.00
Sun 02/08 21Z 0.09 3426  53  26 -11 0.00
Sun 02/08 22Z 0.10 3427  50  19 -11 0.00
Sun 02/08 23Z 0.10 3428  47  19 -11 0.00
Mon 02/09 00Z 0.11 3427  46  24 -11 0.00
Mon 02/09 01Z 0.11 3426  47  26 -11 0.00
Mon 02/09 02Z 0.13 3426  49  38 -12 0.00
Mon 02/09 03Z 0.14 3326  52  38 -12 0.00
Mon 02/09 04Z 0.16 3326  55  35 -12 0.00
Mon 02/09 05Z 0.18 3327  60  37 -12 0.00
Mon 02/09 06Z 0.19 3328  64  33 -12 0.00
Mon 02/09 07Z 0.19 3328  68  32 -13 0.00
Mon 02/09 08Z 0.18 3328  66  30 -12 0.00
Mon 02/09 09Z 0.18 3328  66  28 -12 0.00
Mon 02/09 10Z 0.19 3329  65  23 -12 0.00
Mon 02/09 11Z 0.17 3328  61  15 -12 0.00
Mon 02/09 12Z 0.18 3329  58  11 -12 0.00
Mon 02/09 13Z 0.17 3328  54   7 -12 0.00
Mon 02/09 14Z 0.16 3327  52   9 -12 0.00
Mon 02/09 15Z 0.15 3325  49  19 -11 0.00
Mon 02/09 16Z 0.16 3324  47  30 -11 0.00
Mon 02/09 17Z 0.16 3322  43  40 -11 0.00

Run total areal avg QPF for the greens is: 0.20 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