NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260409_0000
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 04/09 00Z 0.22 2109  46   8  -2 0.00
Thu 04/09 01Z 0.23 2111  40  16  -2 0.00
Thu 04/09 02Z 0.26 2114  35  25  -1 0.00
Thu 04/09 03Z 0.28 2116  30  32   0 0.00
Thu 04/09 04Z 0.31 2118  27  39   0 0.00
Thu 04/09 05Z 0.34 2120  25  46   0 0.00
Thu 04/09 06Z 0.34 2221  23  51   1 0.00
Thu 04/09 07Z 0.32 2221  22  51   2 0.00
Thu 04/09 08Z 0.31 2221  23  50   2 0.00
Thu 04/09 09Z 0.29 2221  20  50   3 0.00
Thu 04/09 10Z 0.27 2122  18  51   3 0.00
Thu 04/09 11Z 0.27 2122  16  52   3 0.00
Thu 04/09 12Z 0.26 2122  14  55   3 0.00
Thu 04/09 13Z 0.26 2123  13  60   3 0.00
Thu 04/09 14Z 0.27 2125  14  62   3 0.00
Thu 04/09 15Z 0.26 2025  16  66   3 0.00
Thu 04/09 16Z 0.27 2026  18  69   3 0.00
Thu 04/09 17Z 0.30 2026  19  67   3 0.00
Thu 04/09 18Z 0.33 2027  20  55   3 0.00
Thu 04/09 19Z 0.40 2028  22  21   3 0.00
Thu 04/09 20Z 0.36 2029  22  48   4 0.00
Thu 04/09 21Z 0.37 2029  24  48   5 0.00
Thu 04/09 22Z 0.35 2030  28  52   6 0.00
Thu 04/09 23Z 0.37 2131  33  53   7 0.00
Fri 04/10 00Z 0.44 2132  37  58   6 0.00
Fri 04/10 01Z 0.50 2232  44  69   6 0.00
Fri 04/10 02Z 0.55 2332  50  78   6 0.00
Fri 04/10 03Z 0.59 2332  55  74   6 0.00
Fri 04/10 04Z 0.65 2433  60  54   5 0.00
Fri 04/10 05Z 0.71 2432  66  52   5 0.00
Fri 04/10 06Z 0.69 2530  70  37   4 0.00
Fri 04/10 07Z 0.66 2528  71  54   4 0.00
Fri 04/10 08Z 0.62 2525  77  54   4 0.00
Fri 04/10 09Z 0.59 2525  85  52   4 0.00
Fri 04/10 10Z 0.56 2524  85  43   5 0.00
Fri 04/10 11Z 0.55 2523  80  24   5 0.00
Fri 04/10 12Z 0.51 2522  81  28   5 0.00
Fri 04/10 13Z 0.50 2422  85  65   5 0.00
Fri 04/10 14Z 0.47 2322  86  90   6 0.00
Fri 04/10 15Z 0.52 2325  91  95   5 0.01
Fri 04/10 16Z 0.55 2326  89  86   6 0.01
Fri 04/10 17Z 0.61 2326  87  40   7 0.00
Fri 04/10 18Z 0.74 2326  82  32   7 0.00
Fri 04/10 19Z 0.96 2328  74  54   7 0.00
Fri 04/10 20Z 1.20 2228  70  51   7 0.00
Fri 04/10 21Z 1.27 2228  73  60   7 0.01
Fri 04/10 22Z 0.91 2229  76  75   7 0.02
Fri 04/10 23Z 0.78 2228  85  96   7 0.05
Sat 04/11 00Z 0.81 2427  92  97   6 0.13
Sat 04/11 01Z 0.56 2622  98  96   5 0.08
Sat 04/11 02Z 0.47 2720  98  90   4 0.08
Sat 04/11 03Z 0.39 2818  98  89   2 0.04
Sat 04/11 04Z 0.34 3017  98  81   1 0.03
Sat 04/11 05Z 0.35 3019  97  70   0 0.02
Sat 04/11 06Z 0.40 3121  96  49  -2 0.01
Sat 04/11 07Z 0.49 3123  95  35  -3 0.01
Sat 04/11 08Z 0.58 3125  95  12  -4 0.02
Sat 04/11 09Z 0.60 3125  95  13  -5 0.02
Sat 04/11 10Z 0.59 3125  92  14  -6 0.01
Sat 04/11 11Z 0.55 3126  88  12  -6 0.01

Run total areal avg QPF for the greens is: 0.56 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