NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260408_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)
=== ===== === ==== ==== === === === ====
Wed 04/08 12Z 0.19 3211  78  16 -10 0.00
Wed 04/08 13Z 0.16 3108  73  14  -9 0.00
Wed 04/08 14Z 0.14 3006  69  13  -9 0.00
Wed 04/08 15Z 0.13 2904  64   8  -9 0.00
Wed 04/08 16Z 0.16 2703  60   6  -8 0.00
Wed 04/08 17Z 0.28 2604  54   4  -7 0.00
Wed 04/08 18Z 0.38 2505  46   3  -6 0.00
Wed 04/08 19Z 0.34 2305  39   3  -5 0.00
Wed 04/08 20Z 0.34 2305  34   3  -5 0.00
Wed 04/08 21Z 0.27 2406  30   4  -4 0.00
Wed 04/08 22Z 0.22 2308  28   5  -3 0.00
Wed 04/08 23Z 0.24 2309  28   6  -3 0.00
Thu 04/09 00Z 0.25 2310  27   8  -2 0.00
Thu 04/09 01Z 0.23 2212  24  13  -1 0.00
Thu 04/09 02Z 0.25 2214  21  20  -1 0.00
Thu 04/09 03Z 0.30 2216  20  26   0 0.00
Thu 04/09 04Z 0.31 2217  21  32   0 0.00
Thu 04/09 05Z 0.30 2218  20  35   0 0.00
Thu 04/09 06Z 0.26 2218  19  38   1 0.00
Thu 04/09 07Z 0.25 2119  20  38   2 0.00
Thu 04/09 08Z 0.23 2119  19  36   2 0.00
Thu 04/09 09Z 0.24 2121  17  38   2 0.00
Thu 04/09 10Z 0.27 2123  16  36   2 0.00
Thu 04/09 11Z 0.30 2124  14  37   3 0.00
Thu 04/09 12Z 0.30 2124  12  46   3 0.00
Thu 04/09 13Z 0.29 2124  10  58   3 0.00
Thu 04/09 14Z 0.28 2125  10  62   4 0.00
Thu 04/09 15Z 0.27 2125  11  52   4 0.00
Thu 04/09 16Z 0.27 2026  13  38   4 0.00
Thu 04/09 17Z 0.30 2026  15  32   3 0.00
Thu 04/09 18Z 0.36 2027  17  33   3 0.00
Thu 04/09 19Z 0.40 2028  19  32   3 0.00
Thu 04/09 20Z 0.38 2028  20  32   4 0.00
Thu 04/09 21Z 0.37 2028  21  34   5 0.00
Thu 04/09 22Z 0.34 2028  24  36   5 0.00
Thu 04/09 23Z 0.32 2029  33  41   6 0.00
Fri 04/10 00Z 0.35 2130  40  44   6 0.00
Fri 04/10 01Z 0.41 2131  46  45   6 0.00
Fri 04/10 02Z 0.49 2232  53  51   6 0.00
Fri 04/10 03Z 0.54 2332  59  56   6 0.00
Fri 04/10 04Z 0.57 2332  61  76   6 0.00
Fri 04/10 05Z 0.61 2432  65  77   6 0.00
Fri 04/10 06Z 0.66 2434  69  76   5 0.00
Fri 04/10 07Z 0.67 2532  76  70   5 0.00
Fri 04/10 08Z 0.64 2529  79  64   4 0.00
Fri 04/10 09Z 0.59 2526  81  43   4 0.00
Fri 04/10 10Z 0.54 2623  81  32   3 0.00
Fri 04/10 11Z 0.53 2622  80  22   3 0.00
Fri 04/10 12Z 0.49 2621  76  20   3 0.00
Fri 04/10 13Z 0.49 2520  78  21   3 0.00
Fri 04/10 14Z 0.51 2520  80  17   3 0.00
Fri 04/10 15Z 0.54 2519  79  67   3 0.00
Fri 04/10 16Z 0.57 2517  77  94   2 0.00
Fri 04/10 17Z 0.51 2415  74  90   2 0.01
Fri 04/10 18Z 0.49 2315  76  89   2 0.01
Fri 04/10 19Z 0.46 2316  82  86   3 0.01
Fri 04/10 20Z 0.38 2217  85  52   4 0.01
Fri 04/10 21Z 0.44 2219  88  73   5 0.00
Fri 04/10 22Z 0.43 2219  91  96   5 0.03
Fri 04/10 23Z 0.32 2118  91  90   5 0.04

Run total areal avg QPF for the greens is: 0.11 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