Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260118_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)
=== ===== === ==== ==== === === === ====
Sun 01/18 18Z 0.35 2309  88  66 -10 0.00
Sun 01/18 21Z 0.14 2203  82  69 -10 0.00
Mon 01/19 00Z 0.05 0600  82  71 -10 0.00
Mon 01/19 03Z 0.06 0005  88  77 -11 0.00
Mon 01/19 06Z 0.08 3506  94  63 -11 0.00
Mon 01/19 09Z 0.13 3205  93  22 -12 0.01
Mon 01/19 12Z 0.31 2707  93  24 -13 0.01
Mon 01/19 15Z 0.50 2210  91  68 -14 0.00
Mon 01/19 18Z 2.33 2213  80  75 -14 0.00
Mon 01/19 21Z 1.52 2220  88  92 -14 0.00
Tue 01/20 00Z 1.73 2328  86  83 -14 0.01
Tue 01/20 03Z 9.04 2432  83  67 -16 0.00
Tue 01/20 06Z 1.64 2533  85  59 -17 0.00
Tue 01/20 09Z 5.17 2529  84  54 -18 0.00
Tue 01/20 12Z 1.85 2626  78  36 -20 0.00
Tue 01/20 15Z 3.15 2619  76  59 -21 0.00
Tue 01/20 18Z 9.53 2518  78  74 -21 0.00
Tue 01/20 21Z 9.38 2623  79  74 -20 0.00
Wed 01/21 00Z 2.27 2625  81  41 -21 0.00
Wed 01/21 03Z 1.35 2727  80  50 -21 0.00
Wed 01/21 06Z 0.98 2725  78  54 -20 0.00
Wed 01/21 09Z 0.91 2625  82  21 -20 0.00
Wed 01/21 12Z 0.70 2424  78  32 -18 0.00
Wed 01/21 15Z 0.84 2624  79  35 -19 0.00
Wed 01/21 18Z 0.41 2122  55  85 -14 0.00
Wed 01/21 21Z 0.24 1929  83  88 -13 0.01
Thu 01/22 00Z 0.49 2137  83  86 -11 0.03
Thu 01/22 03Z 0.59 2239  80  73  -8 0.00
Thu 01/22 06Z 0.68 2336  86  46  -8 0.00

Run total areal avg QPF for the greens is: 0.07 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