NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260407_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)
=== ===== === ==== ==== === === === ====
Tue 04/07 06Z 0.52 2708  83  68  -9 0.00
Tue 04/07 07Z 0.45 2707  80  66  -9 0.00
Tue 04/07 08Z 0.31 2406  79  63  -8 0.00
Tue 04/07 09Z 0.20 2106  80  66  -8 0.00
Tue 04/07 10Z 0.18 2107  85  73  -8 0.00
Tue 04/07 11Z 0.08 1907  87  78  -7 0.00
Tue 04/07 12Z 0.04 1708  90  81  -7 0.01
Tue 04/07 13Z 0.04 1710  92  87  -7 0.03
Tue 04/07 14Z 0.07 1712  94  85  -8 0.04
Tue 04/07 15Z 0.16 1911  96  88  -7 0.03
Tue 04/07 16Z 0.32 2109  98  78  -7 0.03
Tue 04/07 17Z 1.11 2409  97  83  -7 0.04
Tue 04/07 18Z 1.21 2610  97  81  -7 0.03
Tue 04/07 19Z 1.51 2713  96  74  -8 0.02
Tue 04/07 20Z 0.72 3123  95  71  -9 0.04
Tue 04/07 21Z 0.45 3227  91  80 -10 0.03
Tue 04/07 22Z 0.54 3229  90  62 -11 0.01
Tue 04/07 23Z 0.68 3128  91  29 -12 0.00
Wed 04/08 00Z 0.83 3127  90  11 -12 0.00
Wed 04/08 01Z 1.33 3127  79  21 -13 0.00
Wed 04/08 02Z 1.08 3127  66  26 -13 0.00
Wed 04/08 03Z 0.77 3126  60  23 -13 0.00
Wed 04/08 04Z 0.60 3124  53  21 -13 0.00
Wed 04/08 05Z 0.48 3122  48  22 -13 0.00
Wed 04/08 06Z 0.41 3121  47  24 -12 0.00
Wed 04/08 07Z 0.37 3119  47  22 -12 0.00
Wed 04/08 08Z 0.34 3119  46  20 -12 0.00
Wed 04/08 09Z 0.31 3118  44  22 -11 0.00
Wed 04/08 10Z 0.27 3117  42  20 -10 0.00
Wed 04/08 11Z 0.25 3115  40  16 -10 0.00
Wed 04/08 12Z 0.23 3113  38  14  -9 0.00
Wed 04/08 13Z 0.21 3011  37  11  -8 0.00
Wed 04/08 14Z 0.21 2909  35   8  -8 0.00
Wed 04/08 15Z 0.23 2809  33   5  -7 0.00
Wed 04/08 16Z 0.23 2808  29   4  -7 0.00
Wed 04/08 17Z 0.24 2707  26   4  -6 0.00
Wed 04/08 18Z 0.28 2707  24   4  -5 0.00
Wed 04/08 19Z 0.35 2506  22   5  -5 0.00
Wed 04/08 20Z 0.37 2405  22   6  -5 0.00
Wed 04/08 21Z 0.36 2306  23   6  -4 0.00
Wed 04/08 22Z 0.26 2208  22   7  -4 0.00
Wed 04/08 23Z 0.23 2110  21   8  -3 0.00
Thu 04/09 00Z 0.24 2113  20   7  -2 0.00
Thu 04/09 01Z 0.27 2116  17   7  -1 0.00
Thu 04/09 02Z 0.30 2118  16   8   0 0.00
Thu 04/09 03Z 0.32 2120  18   9   0 0.00
Thu 04/09 04Z 0.32 2221  21  20   0 0.00
Thu 04/09 05Z 0.31 2122  22  39   1 0.00
Thu 04/09 06Z 0.31 2123  24  53   1 0.00
Thu 04/09 07Z 0.31 2124  20  62   2 0.00
Thu 04/09 08Z 0.33 2125  15  66   2 0.00
Thu 04/09 09Z 0.34 2126  12  59   3 0.00
Thu 04/09 10Z 0.36 2128   9  53   3 0.00
Thu 04/09 11Z 0.39 2129   7  50   3 0.00
Thu 04/09 12Z 0.39 2131   6  50   3 0.00
Thu 04/09 13Z 0.39 2131   6  55   4 0.00
Thu 04/09 14Z 0.38 2131   6  56   4 0.00
Thu 04/09 15Z 0.38 2131   6  43   4 0.00
Thu 04/09 16Z 0.39 2130   7  38   4 0.00
Thu 04/09 17Z 0.41 2129  10  37   4 0.00

Run total areal avg QPF for the greens is: 0.31 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