Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260129_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 01/29 00Z 0.47 2812  78  52 -18 0.00
Thu 01/29 01Z 0.54 2913  81  52 -18 0.00
Thu 01/29 02Z 0.58 2914  83  58 -18 0.00
Thu 01/29 03Z 0.60 2914  84  62 -19 0.00
Thu 01/29 04Z 0.64 2914  83  60 -19 0.00
Thu 01/29 05Z 0.66 2915  84  58 -19 0.00
Thu 01/29 06Z 0.54 2915  82  56 -19 0.00
Thu 01/29 07Z 0.53 3015  79  54 -19 0.00
Thu 01/29 08Z 0.56 3016  81  51 -19 0.00
Thu 01/29 09Z 0.56 3017  81  46 -19 0.00
Thu 01/29 10Z 0.50 3015  79  44 -19 0.00
Thu 01/29 11Z 0.45 3014  79  45 -19 0.00
Thu 01/29 12Z 0.45 2914  83  47 -19 0.00
Thu 01/29 13Z 0.43 2914  83  51 -19 0.00
Thu 01/29 14Z 0.40 2913  80  59 -19 0.00
Thu 01/29 15Z 0.41 2913  80  61 -19 0.00
Thu 01/29 16Z 0.46 3013  82  60 -19 0.00
Thu 01/29 17Z 0.63 3014  85  56 -19 0.00
Thu 01/29 18Z 0.88 2914  87  56 -19 0.00
Thu 01/29 19Z 1.13 2915  90  58 -19 0.00
Thu 01/29 20Z 1.14 2916  93  61 -19 0.00
Thu 01/29 21Z 1.11 2918  94  60 -19 0.01
Thu 01/29 22Z 1.25 2921  92  55 -20 0.01
Thu 01/29 23Z 1.68 3024  88  53 -20 0.01
Fri 01/30 00Z 1.32 3026  85  49 -21 0.00
Fri 01/30 01Z 1.10 3027  82  43 -22 0.00
Fri 01/30 02Z 1.02 3027  82  40 -23 0.00
Fri 01/30 03Z 0.87 3028  80  37 -23 0.00
Fri 01/30 04Z 0.78 3028  78  35 -24 0.00
Fri 01/30 05Z 0.69 3028  76  34 -24 0.00
Fri 01/30 06Z 0.62 3029  73  33 -23 0.00
Fri 01/30 07Z 0.57 3030  71  35 -23 0.00
Fri 01/30 08Z 0.52 3030  70  40 -23 0.00
Fri 01/30 09Z 0.49 3029  70  42 -22 0.00
Fri 01/30 10Z 0.46 3029  69  47 -22 0.00
Fri 01/30 11Z 0.44 3029  69  55 -22 0.00
Fri 01/30 12Z 0.40 3028  70  64 -21 0.00
Fri 01/30 13Z 0.37 3027  72  74 -21 0.00
Fri 01/30 14Z 0.35 3026  75  80 -20 0.00
Fri 01/30 15Z 0.33 3024  77  80 -19 0.00
Fri 01/30 16Z 0.32 3022  79  78 -19 0.00
Fri 01/30 17Z 0.31 3021  80  76 -18 0.00
Fri 01/30 18Z 0.32 3020  81  74 -18 0.00
Fri 01/30 19Z 0.33 2919  81  72 -18 0.00
Fri 01/30 20Z 0.36 2920  82  70 -18 0.00
Fri 01/30 21Z 0.37 2920  83  69 -17 0.00
Fri 01/30 22Z 0.36 2920  84  66 -17 0.00
Fri 01/30 23Z 0.34 3021  85  65 -17 0.00
Sat 01/31 00Z 0.30 3021  86  65 -16 0.00
Sat 01/31 01Z 0.25 3120  86  64 -16 0.00
Sat 01/31 02Z 0.22 3119  86  62 -15 0.00
Sat 01/31 03Z 0.20 3119  86  61 -15 0.00
Sat 01/31 04Z 0.18 3117  86  61 -15 0.00
Sat 01/31 05Z 0.17 3117  86  59 -15 0.00
Sat 01/31 06Z 0.17 3116  86  57 -15 0.00
Sat 01/31 07Z 0.18 3116  85  55 -15 0.00
Sat 01/31 08Z 0.19 3116  85  54 -15 0.00
Sat 01/31 09Z 0.20 3117  86  53 -15 0.00
Sat 01/31 10Z 0.20 3117  86  51 -15 0.00
Sat 01/31 11Z 0.19 3117  85  49 -15 0.00

Run total areal avg QPF for the greens is: 0.03 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