NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260417_1800
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 18Z 0.09 0107  81  29   8 0.00
Fri 04/17 19Z 0.08 0109  76  23   8 0.00
Fri 04/17 20Z 0.08 0010  71  19   8 0.00
Fri 04/17 21Z 0.10 0111  66  14   9 0.00
Fri 04/17 22Z 0.16 0212  62  10   9 0.00
Fri 04/17 23Z 0.21 0313  55   7   9 0.00
Sat 04/18 00Z 0.25 0413  50   5   9 0.00
Sat 04/18 01Z 0.28 0612  45   6  10 0.00
Sat 04/18 02Z 0.31 0812  45   6  10 0.00
Sat 04/18 03Z 0.36 1014  52   3   9 0.00
Sat 04/18 04Z 0.40 1217  66   4   9 0.00
Sat 04/18 05Z 0.34 1317  75   4   9 0.00
Sat 04/18 06Z 0.24 1517  80   5   8 0.00
Sat 04/18 07Z 0.23 1520  84   4   8 0.00
Sat 04/18 08Z 0.19 1623  90   4   8 0.00
Sat 04/18 09Z 0.16 1621  86   4   7 0.00
Sat 04/18 10Z 0.15 1620  77   5   8 0.00
Sat 04/18 11Z 0.14 1620  68   6   8 0.00
Sat 04/18 12Z 0.19 1521  61   8   8 0.00
Sat 04/18 13Z 0.17 1521  60  12   8 0.01
Sat 04/18 14Z 0.21 1523  58  12   8 0.00
Sat 04/18 15Z 0.23 1524  63  14   8 0.00
Sat 04/18 16Z 0.28 1527  64  13   8 0.00
Sat 04/18 17Z 0.29 1527  65  13   9 0.00
Sat 04/18 18Z 0.22 1527  69  14   8 0.00
Sat 04/18 19Z 0.22 1529  71  15   9 0.00
Sat 04/18 20Z 0.19 1629  71  18   9 0.00
Sat 04/18 21Z 0.18 1630  71   9   9 0.00
Sat 04/18 22Z 0.13 1630  69   5   9 0.00
Sat 04/18 23Z 0.06 1730  69   3  10 0.00
Sun 04/19 00Z 0.03 1731  67   5  10 0.00
Sun 04/19 01Z 0.06 1832  64   9  11 0.00
Sun 04/19 02Z 0.09 1834  64  31  12 0.00
Sun 04/19 03Z 0.22 2035  73  40  11 0.00
Sun 04/19 04Z 0.35 2135  89  88   9 0.02
Sun 04/19 05Z 0.33 2133  95  96   9 0.06
Sun 04/19 06Z 0.39 2130  97  91   8 0.07
Sun 04/19 07Z 0.39 2227  97  84   8 0.01
Sun 04/19 08Z 0.51 2326  98  70   6 0.02
Sun 04/19 09Z 0.54 2423  98  76   5 0.02
Sun 04/19 10Z 0.54 2519  96  88   4 0.03
Sun 04/19 11Z 0.53 2619  91  94   2 0.03
Sun 04/19 12Z 0.61 2823  79  99   0 0.01
Sun 04/19 13Z 0.70 2928  81  99  -2 0.03
Sun 04/19 14Z 0.52 2926  96  98  -2 0.09
Sun 04/19 15Z 0.53 2926  96  98  -3 0.17
Sun 04/19 16Z 0.58 3026  92  96  -5 0.10
Sun 04/19 17Z 0.58 3025  86  96  -5 0.04
Sun 04/19 18Z 0.59 3022  77  94  -6 0.02
Sun 04/19 19Z 0.56 3017  67  68  -6 0.02
Sun 04/19 20Z 0.64 3017  64  17  -6 0.00
Sun 04/19 21Z 0.98 2919  68  13  -6 0.00
Sun 04/19 22Z 0.94 2919  66  19  -6 0.00
Sun 04/19 23Z 1.61 2919  68  25  -5 0.00
Mon 04/20 00Z 1.18 2921  71  34  -6 0.00
Mon 04/20 01Z 1.57 2922  71  37  -6 0.00
Mon 04/20 02Z 1.60 2922  72  42  -7 0.00
Mon 04/20 03Z 1.71 2922  77  62  -7 0.00
Mon 04/20 04Z 1.22 2921  81  51  -8 0.00
Mon 04/20 05Z 1.08 2919  80  44  -8 0.00

Run total areal avg QPF for the greens is: 0.75 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