Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20200123_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)
=== ===== === ==== ==== === === === ====
Thu 01/23 12Z 0.35 2817  23   7   1 0.00
Thu 01/23 15Z 0.22 2710  21   9   1 0.00
Thu 01/23 18Z 0.28 2314  12   8   1 0.00
Thu 01/23 21Z 0.40 2618   8  11   2 0.00
Fri 01/24 00Z 0.34 2816   9  12   2 0.00
Fri 01/24 03Z 0.20 3012   8  11   2 0.00
Fri 01/24 06Z 0.03 3303  10  15   1 0.00
Fri 01/24 09Z 0.06 1303  11  23   0 0.00
Fri 01/24 12Z 0.02 1706  10  23   0 0.00
Fri 01/24 15Z 0.04 1608  13  24   1 0.00
Fri 01/24 18Z 0.04 1710  24  29   0 0.00
Fri 01/24 21Z 0.04 1712  36  64   0 0.00
Sat 01/25 00Z 0.05 1613  39  76   1 0.00
Sat 01/25 03Z 0.04 1715  39  63   1 0.00
Sat 01/25 06Z 0.04 1714  35  47   1 0.00
Sat 01/25 09Z 0.06 1714  31  39   1 0.00
Sat 01/25 12Z 0.10 1518  28  38   2 0.00
Sat 01/25 15Z 0.18 1523  26  34   2 0.00
Sat 01/25 18Z 0.19 1526  30  39   3 0.00
Sat 01/25 21Z 0.27 1430  45  90   2 0.00
Sun 01/26 00Z 0.41 1335  73  96   2 0.05
Sun 01/26 03Z 0.53 1342  94  96   2 0.14
Sun 01/26 06Z 0.49 1336  98  94   0 0.10
Sun 01/26 09Z 0.47 1338  88  95   1 0.05
Sun 01/26 12Z 0.02 1709  98  28  -1 0.10
Sun 01/26 15Z 0.06 2006  98  53  -2 0.01
Sun 01/26 18Z 0.33 2412  98  90  -3 0.02
Sun 01/26 21Z 0.75 2720  96  86  -6 0.03
Mon 01/27 00Z 0.85 2622  96  94  -6 0.02

Run total areal avg QPF for the greens is: 0.52 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