NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260107_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)
=== ===== === ==== ==== === === === ====
Wed 01/07 18Z 0.27 2710  99  86  -3 0.00
Wed 01/07 19Z 0.34 2813  98  81  -3 0.01
Wed 01/07 20Z 0.41 2917  98  79  -3 0.01
Wed 01/07 21Z 0.48 3020  98  78  -4 0.02
Wed 01/07 22Z 0.41 3020  97  77  -3 0.02
Wed 01/07 23Z 0.35 3120  97  76  -3 0.01
Thu 01/08 00Z 0.31 3221  97  73  -3 0.01
Thu 01/08 01Z 0.29 3222  96  62  -3 0.01
Thu 01/08 02Z 0.28 3222  96  50  -3 0.01
Thu 01/08 03Z 0.29 3222  95  44  -3 0.01
Thu 01/08 04Z 0.33 3123  95  42  -3 0.01
Thu 01/08 05Z 0.31 3122  94  45  -2 0.01
Thu 01/08 06Z 0.30 3121  95  53  -2 0.01
Thu 01/08 07Z 0.33 3121  95  60  -2 0.01
Thu 01/08 08Z 0.39 3022  95  68  -2 0.01
Thu 01/08 09Z 0.43 3022  95  65  -3 0.01
Thu 01/08 10Z 0.46 3023  95  56  -3 0.01
Thu 01/08 11Z 0.47 3023  95  44  -3 0.01
Thu 01/08 12Z 0.47 3123  95  25  -3 0.01
Thu 01/08 13Z 0.46 3123  94  12  -3 0.01
Thu 01/08 14Z 0.45 3123  95  14  -4 0.01
Thu 01/08 15Z 0.43 3122  94  24  -4 0.01
Thu 01/08 16Z 0.38 3119  92  27  -4 0.01
Thu 01/08 17Z 0.37 3117  91  19  -3 0.01
Thu 01/08 18Z 0.36 3016  88  15  -3 0.01
Thu 01/08 19Z 0.35 2915  85  13  -2 0.01
Thu 01/08 20Z 0.30 2914  79  13  -1 0.00
Thu 01/08 21Z 0.27 2914  73  17   0 0.00
Thu 01/08 22Z 0.21 3012  65  15   0 0.00
Thu 01/08 23Z 0.14 3009  57  14   0 0.00
Fri 01/09 00Z 0.16 2909  54  21   1 0.00
Fri 01/09 01Z 0.15 2807  49  45   1 0.00
Fri 01/09 02Z 0.13 2706  44  55   2 0.00
Fri 01/09 03Z 0.13 2506  41  52   2 0.00
Fri 01/09 04Z 0.16 2408  41  50   2 0.00
Fri 01/09 05Z 0.15 2308  39  46   3 0.00
Fri 01/09 06Z 0.18 2310  38  46   3 0.00
Fri 01/09 07Z 0.19 2211  35  43   4 0.00
Fri 01/09 08Z 0.19 2113  35  38   4 0.00
Fri 01/09 09Z 0.17 2014  35  36   4 0.00
Fri 01/09 10Z 0.13 1917  36  35   5 0.00
Fri 01/09 11Z 0.19 2021  36  33   5 0.00
Fri 01/09 12Z 0.21 2025  36  37   5 0.00
Fri 01/09 13Z 0.26 2029  35  39   5 0.00
Fri 01/09 14Z 0.33 2034  33  62   5 0.00
Fri 01/09 15Z 0.37 2039  34  63   5 0.00
Fri 01/09 16Z 0.40 2043  35  51   6 0.00
Fri 01/09 17Z 0.45 2146  34  33   6 0.00
Fri 01/09 18Z 0.48 2147  44  50   6 0.00
Fri 01/09 19Z 0.50 2146  68  51   5 0.00
Fri 01/09 20Z 0.50 2147  80  73   6 0.01
Fri 01/09 21Z 0.50 2147  89  86   7 0.02
Fri 01/09 22Z 0.49 2146  95  93   7 0.06
Fri 01/09 23Z 0.55 2247  97  89   7 0.08
Sat 01/10 00Z 0.62 2247  97  76   7 0.06
Sat 01/10 01Z 0.69 2248  96  68   6 0.05
Sat 01/10 02Z 0.82 2349  97  68   5 0.04
Sat 01/10 03Z 1.05 2449  97  43   3 0.03
Sat 01/10 04Z 1.39 2546  96  12   0 0.03
Sat 01/10 05Z 1.42 2641  95   2   0 0.04

Run total areal avg QPF for the greens is: 0.69 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