Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260117_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 01/17 06Z 0.31 2122  84  88 -10 0.00
Sat 01/17 09Z 0.50 2224  82  79 -10 0.01
Sat 01/17 12Z 0.33 2219  83  82  -8 0.00
Sat 01/17 15Z 0.25 2021  92  87  -7 0.01
Sat 01/17 18Z 0.31 2123  93  85  -6 0.04
Sat 01/17 21Z 0.42 2221  94  86  -6 0.08
Sun 01/18 00Z 0.61 2721  90  62  -7 0.05
Sun 01/18 03Z 0.61 2620  82  54  -7 0.00
Sun 01/18 06Z 0.95 2426  91  46  -8 0.00
Sun 01/18 09Z 1.20 2425  90  29  -9 0.00
Sun 01/18 12Z 1.05 2425  82  54 -11 0.00
Sun 01/18 15Z 0.79 2423  81  76 -11 0.00
Sun 01/18 18Z 6.18 2416  84  53 -12 0.00
Sun 01/18 21Z 1.56 2409  80  26 -12 0.00
Mon 01/19 00Z 0.36 2706  72  26 -12 0.00
Mon 01/19 03Z 0.20 2904  74  48 -12 0.00
Mon 01/19 06Z 0.23 2804  78  44 -13 0.00
Mon 01/19 09Z 0.24 2506  76  31 -13 0.00
Mon 01/19 12Z 0.24 2110  76  35 -12 0.00
Mon 01/19 15Z 0.17 1919  87  78 -11 0.00
Mon 01/19 18Z 0.79 2121  93  80 -12 0.03
Mon 01/19 21Z 4.25 2323  89  72 -13 0.01
Tue 01/20 00Z 1.52 2328  86  67 -13 0.00
Tue 01/20 03Z 1.31 2224  89  73 -13 0.00
Tue 01/20 06Z 1.16 2433  82  55 -15 0.00
Tue 01/20 09Z 2.68 2630  78  32 -18 0.00
Tue 01/20 12Z 2.04 2629  76  28 -20 0.00
Tue 01/20 15Z 1.38 2724  71  30 -21 0.00
Tue 01/20 18Z 7.26 2619  75  42 -21 0.00

Run total areal avg QPF for the greens is: 0.23 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