NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260212_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)
=== ===== === ==== ==== === === === ====
Thu 02/12 12Z 0.38 3320  91  71 -14 0.00
Thu 02/12 13Z 0.36 3322  93  72 -13 0.00
Thu 02/12 14Z 0.31 3321  93  71 -13 0.00
Thu 02/12 15Z 0.28 3320  92  70 -13 0.00
Thu 02/12 16Z 0.29 3219  91  68 -13 0.00
Thu 02/12 17Z 0.30 3219  90  60 -12 0.00
Thu 02/12 18Z 0.30 3220  88  45 -12 0.00
Thu 02/12 19Z 0.30 3220  88  32 -12 0.00
Thu 02/12 20Z 0.32 3220  88  22 -12 0.00
Thu 02/12 21Z 0.30 3220  85  16 -11 0.00
Thu 02/12 22Z 0.26 3321  80  13 -11 0.00
Thu 02/12 23Z 0.20 3321  74  12 -10 0.00
Fri 02/13 02Z 0.10 3422  66  12  -9 0.00
Fri 02/13 03Z 0.08 3421  65  13  -9 0.00
Fri 02/13 04Z 0.06 3420  63  14  -9 0.00
Fri 02/13 05Z 0.07 3419  62  15  -9 0.00
Fri 02/13 06Z 0.09 3418  61  17  -9 0.00
Fri 02/13 07Z 0.10 3417  60  20  -9 0.00
Fri 02/13 08Z 0.12 3317  60  23  -9 0.00
Fri 02/13 09Z 0.15 3317  61  24  -9 0.00
Fri 02/13 10Z 0.17 3217  61  23  -9 0.00
Fri 02/13 11Z 0.19 3218  61  21  -9 0.00
Fri 02/13 12Z 0.22 3218  62  19  -9 0.00
Fri 02/13 13Z 0.24 3118  62  18  -9 0.00
Fri 02/13 14Z 0.24 3117  63  18  -9 0.00
Fri 02/13 15Z 0.25 3016  64  18 -10 0.00
Fri 02/13 16Z 0.28 3016  66  18 -10 0.00
Fri 02/13 17Z 0.30 3015  65  17 -10 0.00
Fri 02/13 18Z 0.30 2913  60  17 -10 0.00
Fri 02/13 20Z 0.37 2814  58  26 -10 0.00
Fri 02/13 21Z 0.40 2715  62  37 -10 0.00
Fri 02/13 22Z 0.45 2716  65  45 -10 0.00
Fri 02/13 23Z 0.48 2618  62  48 -10 0.00
Sat 02/14 00Z 0.47 2618  57  44  -9 0.00
Sat 02/14 01Z 0.44 2618  51  43  -9 0.00
Sat 02/14 02Z 0.41 2618  45  48  -8 0.00
Sat 02/14 03Z 0.41 2618  43  53  -8 0.00
Sat 02/14 04Z 0.42 2619  49  67  -9 0.00
Sat 02/14 05Z 0.43 2619  58  83  -9 0.00
Sat 02/14 06Z 0.38 2616  86  79  -9 0.00
Sat 02/14 07Z 0.41 2616  94  76 -10 0.00
Sat 02/14 08Z 0.45 2716  94  86 -10 0.00
Sat 02/14 09Z 0.42 2815  93  86 -10 0.01
Sat 02/14 10Z 0.36 2814  94  80 -10 0.01
Sat 02/14 11Z 0.38 2815  95  79 -10 0.01
Sat 02/14 12Z 0.43 2916  94  78 -10 0.01
Sat 02/14 13Z 0.48 2919  95  78 -10 0.01
Sat 02/14 14Z 0.49 3020  95  54 -11 0.00
Sat 02/14 15Z 0.46 3020  95  30 -11 0.01
Sat 02/14 16Z 0.49 3120  92  21 -11 0.01
Sat 02/14 17Z 0.52 3019  83  21 -11 0.01
Sat 02/14 18Z 0.58 3019  75  25 -11 0.00
Sat 02/14 19Z 0.65 2919  73  30 -11 0.00
Sat 02/14 20Z 0.71 2919  75  33 -12 0.00
Sat 02/14 21Z 0.84 2921  74  35 -12 0.00
Sat 02/14 22Z 0.91 2923  73  39 -12 0.00
Sat 02/14 23Z 0.98 2925  71  39 -12 0.00

Run total areal avg QPF for the greens is: 0.08 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