Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260418_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)
=== ===== === ==== ==== === === === ====
Sat 04/18 12Z 0.16 1620  70   3   8 0.00
Sat 04/18 13Z 0.24 1524  71   3   8 0.00
Sat 04/18 14Z 0.18 1522  67   5   8 0.00
Sat 04/18 15Z 0.18 1522  64   7   8 0.00
Sat 04/18 16Z 0.26 1525  65  11   8 0.00
Sat 04/18 17Z 0.25 1526  65  18   8 0.00
Sat 04/18 18Z 0.26 1528  67  23   8 0.00
Sat 04/18 19Z 0.24 1529  66  15   8 0.00
Sat 04/18 20Z 0.22 1630  65   3   9 0.00
Sat 04/18 21Z 0.17 1630  64   2   9 0.00
Sat 04/18 22Z 0.12 1629  61   5   9 0.00
Sat 04/18 23Z 0.07 1729  63   7  10 0.00
Sun 04/19 00Z 0.05 1729  68  10  11 0.00
Sun 04/19 01Z 0.04 1829  71  34  11 0.00
Sun 04/19 02Z 0.10 1831  71  53  11 0.00
Sun 04/19 03Z 0.15 1932  71  47  11 0.00
Sun 04/19 04Z 0.20 1933  70  30  12 0.00
Sun 04/19 05Z 0.29 2034  75  49  11 0.00
Sun 04/19 06Z 0.29 2033  80  53  11 0.00
Sun 04/19 07Z 0.30 2034  91  86  10 0.01
Sun 04/19 08Z 0.35 2132  98  95   8 0.09
Sun 04/19 09Z 0.38 2227  98  90   7 0.05
Sun 04/19 10Z 0.37 2222  98  91   6 0.03
Sun 04/19 11Z 0.40 2320  99  97   5 0.06
Sun 04/19 12Z 0.52 2416  99  99   3 0.08
Sun 04/19 13Z 0.65 2819  98  98   1 0.10
Sun 04/19 14Z 0.53 3022  97  99   0 0.13
Sun 04/19 15Z 0.60 3025  97  99  -2 0.12
Sun 04/19 16Z 0.62 2924  97  99  -3 0.14
Sun 04/19 17Z 0.72 2925  97  99  -4 0.10
Sun 04/19 18Z 0.76 2925  96  96  -5 0.09
Sun 04/19 19Z 0.69 3022  95  93  -6 0.07
Sun 04/19 20Z 0.57 3022  92  87  -6 0.05
Sun 04/19 21Z 0.53 3122  62  15  -6 0.01
Sun 04/19 22Z 0.66 3023  67  11  -6 0.00
Sun 04/19 23Z 0.85 2924  80  13  -6 0.00
Mon 04/20 00Z 1.02 2924  71  16  -6 0.00
Mon 04/20 01Z 1.22 2924  66  19  -6 0.00
Mon 04/20 02Z 1.27 2923  66  51  -6 0.00
Mon 04/20 03Z 1.48 2923  71  57  -6 0.00
Mon 04/20 04Z 1.37 2922  73  53  -6 0.00
Mon 04/20 05Z 1.26 2920  74  51  -7 0.00
Mon 04/20 06Z 1.00 2919  75  53  -7 0.00
Mon 04/20 07Z 0.74 2918  77  43  -7 0.00
Mon 04/20 08Z 0.60 2917  83  60  -8 0.00
Mon 04/20 09Z 0.53 3015  86  70  -8 0.00
Mon 04/20 10Z 0.56 3014  87  81  -9 0.00
Mon 04/20 11Z 0.62 3015  88  87 -10 0.02
Mon 04/20 12Z 0.63 3017  89  83 -10 0.02
Mon 04/20 13Z 0.81 3116  86  54 -11 0.01
Mon 04/20 14Z 1.67 3114  81  46 -11 0.00
Mon 04/20 15Z 4.03 3113  75  43 -11 0.00
Mon 04/20 16Z 6.24 3012  73  44 -11 0.00
Mon 04/20 17Z 7.19 3012  72  46 -10 0.00
Mon 04/20 18Z 6.94 2913  78  51 -10 0.00
Mon 04/20 19Z 6.45 3012  77  53 -10 0.00
Mon 04/20 20Z 5.77 3013  77  55 -10 0.00
Mon 04/20 21Z 5.15 3013  76  52 -10 0.00
Mon 04/20 22Z 4.54 3112  73  40 -10 0.00
Mon 04/20 23Z 2.42 3112  71  23 -11 0.00

Run total areal avg QPF for the greens is: 1.18 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