NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260417_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)
=== ===== === ==== ==== === === === ====
Fri 04/17 06Z 0.25 3112  89  88  10 0.00
Fri 04/17 07Z 0.20 3313  91  83   9 0.00
Fri 04/17 08Z 0.16 3313  92  85   9 0.00
Fri 04/17 09Z 0.08 3514  90  81   9 0.00
Fri 04/17 10Z 0.05 0016  88  76   8 0.00
Fri 04/17 11Z 0.08 0116  89  75   8 0.00
Fri 04/17 12Z 0.11 0115  88  71   7 0.00
Fri 04/17 13Z 0.13 0115  85  61   8 0.00
Fri 04/17 14Z 0.15 0213  82  50   8 0.00
Fri 04/17 15Z 0.15 0212  79  47   7 0.00
Fri 04/17 16Z 0.15 0211  76  46   7 0.00
Fri 04/17 17Z 0.16 0210  72  43   7 0.00
Fri 04/17 18Z 0.16 0210  66  36   7 0.00
Fri 04/17 19Z 0.21 0211  61  24   8 0.00
Fri 04/17 20Z 0.23 0211  56  14   8 0.00
Fri 04/17 21Z 0.21 0211  52   9   8 0.00
Fri 04/17 22Z 0.19 0311  47   7   9 0.00
Fri 04/17 23Z 0.21 0311  45   6   9 0.00
Sat 04/18 00Z 0.25 0511  43   4   9 0.00
Sat 04/18 01Z 0.28 0611  43   4   9 0.00
Sat 04/18 02Z 0.29 0811  44   5   9 0.00
Sat 04/18 03Z 0.25 1009  49   5   9 0.00
Sat 04/18 04Z 0.24 1210  62   5   8 0.00
Sat 04/18 05Z 0.21 1311  69   5   8 0.00
Sat 04/18 06Z 0.23 1413  70   5   8 0.00
Sat 04/18 07Z 0.27 1417  65   4   8 0.00
Sat 04/18 08Z 0.25 1417  62   3   8 0.00
Sat 04/18 09Z 0.21 1517  58   3   8 0.00
Sat 04/18 10Z 0.16 1517  58   3   8 0.00
Sat 04/18 11Z 0.22 1521  52   2   8 0.00
Sat 04/18 12Z 0.18 1521  54   2   8 0.00
Sat 04/18 13Z 0.16 1620  57   2   7 0.00
Sat 04/18 14Z 0.20 1521  57   1   8 0.00
Sat 04/18 15Z 0.23 1522  60   1   8 0.00
Sat 04/18 16Z 0.27 1524  64   1   8 0.00
Sat 04/18 17Z 0.26 1526  66   3   8 0.00
Sat 04/18 18Z 0.23 1526  67   3   8 0.00
Sat 04/18 19Z 0.22 1527  70   4   8 0.00
Sat 04/18 20Z 0.12 1626  67   3   9 0.00
Sat 04/18 21Z 0.06 1725  61   3  10 0.00
Sat 04/18 22Z 0.03 1726  58   9  10 0.00
Sat 04/18 23Z 0.04 1729  57  32  11 0.00
Sun 04/19 00Z 0.03 1830  60  51  11 0.01
Sun 04/19 01Z 0.04 1830  59  40  12 0.01
Sun 04/19 02Z 0.09 1831  60  50  13 0.01
Sun 04/19 03Z 0.19 1933  68  82  12 0.00
Sun 04/19 04Z 0.35 2135  81  93  10 0.01
Sun 04/19 05Z 0.40 2131  94  93   9 0.03
Sun 04/19 06Z 0.38 2228  95  92   9 0.04
Sun 04/19 07Z 0.38 2225  96  93   8 0.03
Sun 04/19 08Z 0.39 2224  98  92   7 0.02
Sun 04/19 09Z 0.44 2322  98  91   6 0.03
Sun 04/19 10Z 0.60 2322  99  96   4 0.06
Sun 04/19 11Z 0.56 2718  98  99   1 0.07
Sun 04/19 12Z 0.58 2923  98  99   0 0.10
Sun 04/19 13Z 0.68 2926  97  99  -2 0.12
Sun 04/19 14Z 0.52 2924  96  99  -2 0.07
Sun 04/19 15Z 0.62 2923  97  98  -4 0.05
Sun 04/19 16Z 0.65 2922  95  96  -6 0.05
Sun 04/19 17Z 0.65 2922  74  91  -5 0.01

Run total areal avg QPF for the greens is: 0.72 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