NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260212_0000
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 00Z 0.82 3122  93  82 -13 0.00
Thu 02/12 01Z 0.70 3225  95  85 -13 0.00
Thu 02/12 02Z 0.57 3225  94  84 -13 0.01
Thu 02/12 03Z 0.48 3224  94  84 -13 0.01
Thu 02/12 04Z 0.44 3324  94  82 -13 0.01
Thu 02/12 05Z 0.43 3323  94  80 -13 0.01
Thu 02/12 06Z 0.43 3323  95  78 -13 0.01
Thu 02/12 07Z 0.45 3324  95  77 -13 0.01
Thu 02/12 08Z 0.46 3324  96  74 -13 0.01
Thu 02/12 09Z 0.37 3323  95  69 -13 0.01
Thu 02/12 10Z 0.30 3323  95  66 -13 0.01
Thu 02/12 11Z 0.27 3323  94  63 -13 0.01
Thu 02/12 12Z 0.24 3323  94  61 -13 0.01
Thu 02/12 13Z 0.21 3422  93  62 -13 0.00
Thu 02/12 14Z 0.19 3421  93  62 -12 0.00
Thu 02/12 15Z 0.21 3321  93  61 -12 0.00
Thu 02/12 16Z 0.25 3321  93  58 -12 0.00
Thu 02/12 17Z 0.28 3321  92  51 -12 0.00
Thu 02/12 18Z 0.29 3321  92  38 -12 0.00
Thu 02/12 19Z 0.31 3320  91  22 -12 0.00
Thu 02/12 20Z 0.32 3320  89  12 -12 0.00
Thu 02/12 21Z 0.32 3321  83   9 -12 0.00
Thu 02/12 22Z 0.28 3322  75  10 -11 0.00
Thu 02/12 23Z 0.22 3323  69  13 -10 0.00
Fri 02/13 00Z 0.15 3423  66  16 -10 0.00
Fri 02/13 01Z 0.11 3423  63  17 -10 0.00
Fri 02/13 02Z 0.09 3423  63  18 -10 0.00
Fri 02/13 03Z 0.09 3422  62  19  -9 0.00
Fri 02/13 04Z 0.10 3421  61  21  -9 0.00
Fri 02/13 05Z 0.11 3420  61  24  -9 0.00
Fri 02/13 06Z 0.12 3420  61  26  -9 0.00
Fri 02/13 07Z 0.13 3319  60  25  -9 0.00
Fri 02/13 08Z 0.14 3319  59  24  -9 0.00
Fri 02/13 09Z 0.17 3319  60  21  -9 0.00
Fri 02/13 10Z 0.19 3219  61  18  -9 0.00
Fri 02/13 11Z 0.21 3219  61  15 -10 0.00
Fri 02/13 12Z 0.24 3119  62  13 -10 0.00
Fri 02/13 13Z 0.27 3120  63  14 -10 0.00
Fri 02/13 14Z 0.28 3119  62  17 -10 0.00
Fri 02/13 15Z 0.29 3018  62  18 -10 0.00
Fri 02/13 16Z 0.30 3017  61  18 -10 0.00
Fri 02/13 17Z 0.31 3016  62  16 -10 0.00
Fri 02/13 18Z 0.34 3016  63  21 -10 0.00
Fri 02/13 19Z 0.37 2916  62  35 -10 0.00
Fri 02/13 20Z 0.39 2916  61  47 -10 0.00
Fri 02/13 21Z 0.42 2816  62  58 -10 0.00
Fri 02/13 22Z 0.51 2718  65  45 -10 0.00
Fri 02/13 23Z 0.48 2718  60  35 -10 0.00
Sat 02/14 00Z 0.43 2718  54  36  -9 0.00
Sat 02/14 02Z 0.39 2718  47  45  -8 0.00
Sat 02/14 03Z 0.40 2718  51  57  -8 0.00
Sat 02/14 04Z 0.42 2719  53  73  -8 0.00
Sat 02/14 05Z 0.41 2618  73  80  -8 0.00
Sat 02/14 06Z 0.42 2618  91  78  -8 0.00
Sat 02/14 07Z 0.44 2718  94  81  -9 0.01
Sat 02/14 08Z 0.39 2817  94  81  -9 0.01
Sat 02/14 09Z 0.35 2815  93  84  -9 0.01
Sat 02/14 10Z 0.33 2814  93  85  -9 0.01
Sat 02/14 11Z 0.33 2814  93  84  -9 0.01

Run total areal avg QPF for the greens is: 0.16 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