Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260418_0600
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 06Z 0.14 1514  67   8   9 0.00
Sat 04/18 07Z 0.17 1518  66   7   9 0.00
Sat 04/18 08Z 0.14 1619  67   6   9 0.00
Sat 04/18 09Z 0.10 1719  69   6   8 0.00
Sat 04/18 10Z 0.10 1721  72   6   8 0.00
Sat 04/18 11Z 0.12 1723  70   6   8 0.00
Sat 04/18 12Z 0.11 1723  65   6   8 0.00
Sat 04/18 13Z 0.08 1722  58   6   9 0.00
Sat 04/18 14Z 0.07 1720  53   6   9 0.00
Sat 04/18 15Z 0.10 1621  49   6   9 0.00
Sat 04/18 16Z 0.14 1622  49   7   9 0.00
Sat 04/18 17Z 0.16 1623  51   8   9 0.00
Sat 04/18 18Z 0.16 1624  55  10   9 0.00
Sat 04/18 19Z 0.17 1626  60  10   9 0.00
Sat 04/18 20Z 0.16 1627  60   6  10 0.00
Sat 04/18 21Z 0.13 1629  59   3  10 0.00
Sat 04/18 22Z 0.09 1730  59   4  10 0.00
Sat 04/18 23Z 0.04 1730  61   5  10 0.01
Sun 04/19 00Z 0.03 1731  64   6  10 0.01
Sun 04/19 01Z 0.05 1832  68   7  10 0.01
Sun 04/19 02Z 0.06 1832  70  10  11 0.01
Sun 04/19 03Z 0.11 1933  72  27  11 0.00
Sun 04/19 04Z 0.15 1934  75  35  11 0.00
Sun 04/19 05Z 0.21 1936  79  42  11 0.00
Sun 04/19 06Z 0.24 2036  86  84  10 0.01
Sun 04/19 07Z 0.34 2038  97  96   8 0.10
Sun 04/19 08Z 0.47 2233  97  89   7 0.05
Sun 04/19 09Z 0.52 2327  97  93   6 0.02
Sun 04/19 10Z 0.48 2421  98  95   5 0.02
Sun 04/19 11Z 0.53 2417  99  95   3 0.03
Sun 04/19 12Z 0.55 2713  99  97   1 0.05
Sun 04/19 13Z 0.82 2919  98  98  -1 0.10
Sun 04/19 14Z 0.50 3022  97  99  -1 0.13
Sun 04/19 15Z 0.57 2925  97  99  -2 0.12
Sun 04/19 16Z 0.63 3025  97  99  -3 0.09
Sun 04/19 17Z 0.68 3025  96  96  -5 0.07
Sun 04/19 18Z 0.63 3024  89  92  -6 0.08
Sun 04/19 19Z 0.53 3122  69  82  -6 0.01
Sun 04/19 20Z 0.55 3122  54  34  -5 0.00
Sun 04/19 21Z 0.71 3021  63  12  -6 0.00
Sun 04/19 22Z 1.28 2920  80  15  -6 0.00
Sun 04/19 23Z 1.13 2920  80  14  -6 0.00
Mon 04/20 00Z 1.41 2820  77  21  -6 0.00
Mon 04/20 01Z 2.47 2822  75  29  -6 0.00
Mon 04/20 02Z 2.61 2823  76  39  -6 0.00
Mon 04/20 03Z 2.36 2823  77  48  -7 0.00
Mon 04/20 04Z 1.49 2823  80  48  -7 0.00
Mon 04/20 05Z 1.28 2921  72  42  -8 0.00
Mon 04/20 06Z 0.96 2919  71  44  -8 0.00
Mon 04/20 07Z 0.81 2918  71  53  -8 0.00
Mon 04/20 08Z 0.72 2917  75  65  -9 0.00
Mon 04/20 09Z 0.64 2915  78  60  -9 0.00
Mon 04/20 10Z 0.61 3014  82  39 -10 0.00
Mon 04/20 11Z 0.59 3113  85  38 -11 0.00
Mon 04/20 12Z 0.71 3112  85  41 -11 0.00
Mon 04/20 13Z 0.83 3111  83  46 -11 0.00
Mon 04/20 14Z 1.49 3111  83  50 -11 0.00
Mon 04/20 15Z 2.67 3111  81  53 -11 0.00
Mon 04/20 16Z 3.62 3010  75  54 -11 0.00
Mon 04/20 17Z 5.52 3010  69  55 -11 0.00

Run total areal avg QPF for the greens is: 0.92 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