National Weather Service United States Department of Commerce
Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20191021_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)
=== ===== === ==== ==== === === === ====
Mon 10/21 12Z 0.06 0108  67   2   7 0.00
Mon 10/21 15Z 0.10 0308  61   1   7 0.00
Mon 10/21 18Z 0.13 0507  55   0   8 0.00
Mon 10/21 21Z 0.15 0706  54   1   8 0.00
Tue 10/22 00Z 0.23 1110  45   1   9 0.00
Tue 10/22 03Z 0.18 1516  53   1   9 0.00
Tue 10/22 06Z 0.16 1520  54   1   9 0.00
Tue 10/22 09Z 0.10 1622  52   5   9 0.00
Tue 10/22 12Z 0.13 1628  46   8   9 0.00
Tue 10/22 15Z 0.18 1631  39  15   9 0.00
Tue 10/22 18Z 0.25 1534  42  26  10 0.00
Tue 10/22 21Z 0.24 1537  75  28   9 0.00
Wed 10/23 00Z 0.28 1535  94  46   8 0.01
Wed 10/23 03Z 0.29 1536  96  65   8 0.03
Wed 10/23 06Z 0.37 1437  94  95   9 0.09
Wed 10/23 09Z 0.14 1627  98  98   7 0.29
Wed 10/23 12Z 0.34 2512  98  94   4 0.10
Wed 10/23 15Z 0.70 2922  84  24   2 0.01
Wed 10/23 18Z 2.14 2820  85  21   0 0.00
Wed 10/23 21Z 3.96 2721  81   6   0 0.00
Thu 10/24 00Z 1.97 2628  85  60   0 0.00
Thu 10/24 03Z 1.84 2630  88  35   0 0.00
Thu 10/24 06Z 1.27 2726  89  18  -1 0.00
Thu 10/24 09Z 1.59 2628  87  37   0 0.00
Thu 10/24 12Z 0.76 2727  73  27   0 0.00
Thu 10/24 15Z 0.51 2721  45  53   3 0.00
Thu 10/24 18Z 0.53 2518  65  82   4 0.00
Thu 10/24 21Z 0.63 2324  62  83   5 0.00
Fri 10/25 00Z 0.82 2332  60  75   5 0.00

Run total areal avg QPF for the greens is: 0.53 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