NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260303_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)
=== ===== === ==== ==== === === === ====
Tue 03/03 00Z 0.45 2515  45  36 -14 0.00
Tue 03/03 01Z 0.42 2515  41  42 -12 0.00
Tue 03/03 02Z 0.38 2516  36  51 -11 0.00
Tue 03/03 03Z 0.37 2417  29  54 -10 0.00
Tue 03/03 04Z 0.40 2419  24  52  -9 0.00
Tue 03/03 05Z 0.42 2421  22  49  -8 0.00
Tue 03/03 06Z 0.44 2422  21  48  -7 0.00
Tue 03/03 07Z 0.45 2423  21  53  -7 0.00
Tue 03/03 08Z 0.47 2423  24  58  -7 0.00
Tue 03/03 09Z 0.46 2423  28  61  -6 0.00
Tue 03/03 10Z 0.44 2422  31  62  -5 0.00
Tue 03/03 11Z 0.39 2420  32  60  -5 0.00
Tue 03/03 12Z 0.36 2418  31  56  -4 0.00
Tue 03/03 13Z 0.34 2318  29  50  -4 0.00
Tue 03/03 14Z 0.33 2319  34  45  -4 0.00
Tue 03/03 15Z 0.32 2220  49  41  -3 0.00
Tue 03/03 16Z 0.34 2221  64  37  -3 0.00
Tue 03/03 17Z 0.36 2221  73  33  -4 0.00
Tue 03/03 18Z 0.38 2221  82  29  -4 0.00
Tue 03/03 19Z 0.37 2222  87  33  -4 0.01
Tue 03/03 20Z 0.36 2221  88  45  -4 0.01
Tue 03/03 21Z 0.36 2222  93  65  -4 0.02
Tue 03/03 22Z 0.35 2124  96  73  -3 0.02
Tue 03/03 23Z 0.35 2125  97  80  -3 0.02
Wed 03/04 00Z 0.39 2127  98  91  -3 0.04
Wed 03/04 02Z 0.54 2227  98  96  -3 0.06
Wed 03/04 03Z 0.60 2426  98  95  -4 0.07
Wed 03/04 04Z 0.63 2523  98  91  -4 0.07
Wed 03/04 05Z 0.55 2721  97  60  -4 0.04
Wed 03/04 06Z 0.42 2820  88  35  -2 0.02
Wed 03/04 07Z 0.35 2920  74  42  -1 0.01
Wed 03/04 08Z 0.34 3020  64  47   0 0.01
Wed 03/04 09Z 0.37 3021  58  23   0 0.01
Wed 03/04 10Z 0.41 3022  51  15  -1 0.01
Wed 03/04 11Z 0.42 3022  47  13  -1 0.00
Wed 03/04 12Z 0.43 3023  48   7   0 0.00
Wed 03/04 13Z 0.43 3025  54   4   0 0.00
Wed 03/04 14Z 0.38 3023  57   5   0 0.00
Wed 03/04 15Z 0.33 3020  58   6   0 0.00
Wed 03/04 16Z 0.29 2916  59   7   0 0.00
Wed 03/04 17Z 0.30 2815  60   9   0 0.00
Wed 03/04 18Z 0.32 2714  59   8   0 0.00
Wed 03/04 19Z 0.35 2715  56   9   0 0.00
Wed 03/04 20Z 0.32 2713  54  10   0 0.00
Wed 03/04 21Z 0.29 2712  53  12   0 0.00
Wed 03/04 22Z 0.28 2612  51  17   1 0.00
Wed 03/04 23Z 0.28 2512  49  21   1 0.00
Thu 03/05 00Z 0.29 2513  48  25   1 0.00
Thu 03/05 01Z 0.30 2513  46  29   2 0.00
Thu 03/05 02Z 0.32 2414  42  31   2 0.00
Thu 03/05 03Z 0.33 2415  38  28   2 0.00
Thu 03/05 04Z 0.36 2416  36  27   3 0.00
Thu 03/05 05Z 0.38 2417  37  27   3 0.00
Thu 03/05 06Z 0.39 2517  37  25   3 0.00
Thu 03/05 07Z 0.38 2516  35  22   3 0.00
Thu 03/05 08Z 0.38 2515  35  20   2 0.00
Thu 03/05 09Z 0.34 2514  33  16   3 0.00
Thu 03/05 10Z 0.32 2513  33  14   3 0.00
Thu 03/05 11Z 0.31 2512  33  13   3 0.00

Run total areal avg QPF for the greens is: 0.42 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