Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260418_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)
=== ===== === ==== ==== === === === ====
Sat 04/18 18Z 0.46 1521  79   4   7 0.00
Sat 04/18 19Z 0.32 1525  78  14   8 0.00
Sat 04/18 20Z 0.32 1527  77  11   8 0.00
Sat 04/18 21Z 0.33 1529  77   5   8 0.00
Sat 04/18 22Z 0.28 1630  74   5   8 0.00
Sat 04/18 23Z 0.19 1631  71   7   9 0.00
Sun 04/19 00Z 0.10 1733  68   6  10 0.00
Sun 04/19 01Z 0.04 1834  67  20  10 0.00
Sun 04/19 02Z 0.09 1834  67  32  11 0.00
Sun 04/19 03Z 0.17 1934  67  54  11 0.00
Sun 04/19 04Z 0.26 2035  68  66  11 0.00
Sun 04/19 05Z 0.32 2035  70  82  11 0.00
Sun 04/19 06Z 0.44 2136  90  86   8 0.01
Sun 04/19 07Z 0.49 2233  97  92   8 0.04
Sun 04/19 08Z 0.52 2327  98  89   6 0.03
Sun 04/19 09Z 0.43 2418  98  89   6 0.03
Sun 04/19 10Z 0.36 2513  99  94   4 0.04
Sun 04/19 11Z 0.58 2615  99  99   3 0.03
Sun 04/19 12Z 0.65 2819  98  99   2 0.08
Sun 04/19 13Z 0.56 3020  95  99   0 0.05
Sun 04/19 14Z 0.53 3023  96  99  -1 0.05
Sun 04/19 15Z 0.66 3025  97  99  -3 0.04
Sun 04/19 16Z 0.75 3026  96  99  -4 0.08
Sun 04/19 17Z 0.78 3024  96  96  -5 0.10
Sun 04/19 18Z 0.77 3022  96  94  -6 0.10
Sun 04/19 19Z 0.77 3021  95  91  -7 0.10
Sun 04/19 20Z 0.76 3120  93  89  -7 0.06
Sun 04/19 21Z 0.73 3020  85  63  -7 0.01
Sun 04/19 22Z 0.88 3020  81  18  -7 0.00
Sun 04/19 23Z 0.87 3023  75  20  -6 0.00
Mon 04/20 00Z 1.18 3025  70  25  -6 0.00
Mon 04/20 01Z 1.29 3025  66  24  -6 0.00
Mon 04/20 02Z 1.28 3025  67  30  -6 0.00
Mon 04/20 03Z 1.42 2923  69  40  -7 0.00
Mon 04/20 04Z 1.47 2922  70  48  -7 0.00
Mon 04/20 05Z 1.47 2921  74  48  -7 0.00
Mon 04/20 06Z 1.13 2820  77  45  -7 0.00
Mon 04/20 07Z 0.98 2719  80  51  -7 0.00
Mon 04/20 08Z 0.78 2717  85  69  -7 0.00
Mon 04/20 09Z 0.79 2619  90  71  -7 0.00
Mon 04/20 10Z 0.81 2719  93  65  -8 0.02
Mon 04/20 11Z 0.72 2918  94  72  -9 0.02
Mon 04/20 12Z 0.65 3017  94  63 -10 0.02
Mon 04/20 13Z 0.71 3116  94  60 -10 0.01
Mon 04/20 14Z 0.68 3215  87  55 -10 0.01
Mon 04/20 15Z 2.67 3114  82  47 -11 0.00
Mon 04/20 16Z 4.37 3113  77  45 -10 0.00
Mon 04/20 17Z 4.82 3012  73  43 -10 0.00
Mon 04/20 18Z 5.65 3012  73  46 -10 0.00
Mon 04/20 19Z 5.48 3012  74  51 -10 0.00
Mon 04/20 20Z 4.75 3011  74  52 -10 0.00
Mon 04/20 21Z 4.96 3112  73  49 -10 0.00
Mon 04/20 22Z 4.74 3112  71  44 -10 0.00
Mon 04/20 23Z 2.78 3113  70  33 -10 0.00
Tue 04/21 00Z 1.42 3113  67  21 -11 0.00
Tue 04/21 01Z 1.40 3113  64  17 -11 0.00
Tue 04/21 02Z 1.63 3114  61  15 -11 0.00
Tue 04/21 03Z 1.86 3115  59  12 -11 0.00
Tue 04/21 04Z 2.16 3115  58  11 -11 0.00
Tue 04/21 05Z 1.91 3114  56  13 -11 0.00

Run total areal avg QPF for the greens is: 0.93 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