NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260302_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)
=== ===== === ==== ==== === === === ====
Mon 03/02 18Z 0.71 2610  58  20 -18 0.00
Mon 03/02 19Z 0.67 2712  54  19 -17 0.00
Mon 03/02 20Z 0.53 2611  53  25 -16 0.00
Mon 03/02 21Z 0.49 2611  56  32 -16 0.00
Mon 03/02 22Z 0.45 2612  58  42 -15 0.00
Mon 03/02 23Z 0.46 2513  58  47 -15 0.00
Tue 03/03 00Z 0.45 2614  56  41 -14 0.00
Tue 03/03 01Z 0.43 2615  52  35 -13 0.00
Tue 03/03 02Z 0.40 2615  48  31 -12 0.00
Tue 03/03 03Z 0.39 2516  42  34 -11 0.00
Tue 03/03 04Z 0.38 2517  36  44 -10 0.00
Tue 03/03 05Z 0.38 2518  32  50  -9 0.00
Tue 03/03 06Z 0.37 2418  30  47  -8 0.00
Tue 03/03 07Z 0.40 2420  30  45  -7 0.00
Tue 03/03 08Z 0.43 2422  31  47  -7 0.00
Tue 03/03 09Z 0.43 2421  31  47  -6 0.00
Tue 03/03 10Z 0.40 2420  31  49  -6 0.00
Tue 03/03 11Z 0.37 2419  29  50  -5 0.00
Tue 03/03 12Z 0.35 2318  28  50  -5 0.00
Tue 03/03 13Z 0.32 2318  29  47  -4 0.00
Tue 03/03 14Z 0.31 2219  33  43  -4 0.00
Tue 03/03 15Z 0.29 2220  51  39  -4 0.00
Tue 03/03 16Z 0.29 2220  66  37  -4 0.00
Tue 03/03 17Z 0.31 2221  76  32  -4 0.00
Tue 03/03 18Z 0.35 2222  85  37  -4 0.00
Tue 03/03 19Z 0.33 2221  89  52  -4 0.01
Tue 03/03 20Z 0.33 2221  90  66  -4 0.01
Tue 03/03 21Z 0.33 2222  96  76  -3 0.02
Tue 03/03 22Z 0.33 2125  97  81  -3 0.02
Tue 03/03 23Z 0.33 2126  98  90  -3 0.02
Wed 03/04 00Z 0.36 2127  98  93  -3 0.04
Wed 03/04 01Z 0.41 2127  98  95  -3 0.06
Wed 03/04 02Z 0.47 2226  98  94  -3 0.06
Wed 03/04 03Z 0.50 2324  98  90  -3 0.04
Wed 03/04 04Z 0.53 2522  98  85  -3 0.03
Wed 03/04 05Z 0.53 2720  98  86  -3 0.04
Wed 03/04 06Z 0.47 2920  97  71  -3 0.02
Wed 03/04 07Z 0.41 3020  94  55  -3 0.02
Wed 03/04 08Z 0.31 3119  79  55  -1 0.01
Wed 03/04 09Z 0.26 3118  63  50   0 0.01
Wed 03/04 10Z 0.27 3017  57  24   0 0.00
Wed 03/04 11Z 0.29 3017  55  15   0 0.00
Wed 03/04 12Z 0.35 2919  56  10   0 0.00
Wed 03/04 13Z 0.40 2921  58   5   0 0.00
Wed 03/04 14Z 0.43 2922  55   3  -1 0.00
Wed 03/04 16Z 0.35 2917  49   5   0 0.00
Wed 03/04 17Z 0.32 2815  50   5   0 0.00
Wed 03/04 18Z 0.33 2714  50   6   0 0.00
Wed 03/04 19Z 0.35 2614  50   8   0 0.00
Wed 03/04 20Z 0.41 2616  47  10   0 0.00
Wed 03/04 21Z 0.45 2617  46  12   0 0.00
Wed 03/04 22Z 0.44 2617  43  14   1 0.00
Wed 03/04 23Z 0.42 2617  39  16   1 0.00
Thu 03/05 00Z 0.42 2618  38  18   2 0.00
Thu 03/05 01Z 0.43 2618  36  22   2 0.00
Thu 03/05 02Z 0.43 2618  34  23   2 0.00
Thu 03/05 03Z 0.46 2519  32  25   2 0.00
Thu 03/05 04Z 0.50 2620  31  24   2 0.00
Thu 03/05 05Z 0.54 2621  30  25   2 0.00

Run total areal avg QPF for the greens is: 0.41 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