NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260412_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)
=== ===== === ==== ==== === === === ====
Sun 04/12 12Z 0.05 3505  66  20  -4 0.00
Sun 04/12 13Z 0.06 3101  64  22  -4 0.00
Sun 04/12 14Z 0.06 2101  61  24  -3 0.00
Sun 04/12 15Z 0.07 1903  60  25  -2 0.00
Sun 04/12 16Z 0.14 1905  60  27  -2 0.00
Sun 04/12 17Z 0.35 1906  61  29  -2 0.00
Sun 04/12 18Z 0.59 2009  59  73  -2 0.00
Sun 04/12 19Z 0.21 1911  54  89  -1 0.00
Sun 04/12 20Z 0.21 1913  52  97  -1 0.00
Sun 04/12 21Z 0.08 1813  68  95  -2 0.00
Sun 04/12 22Z 0.08 1816  72  91  -1 0.02
Sun 04/12 23Z 0.18 1921  74  98  -1 0.01
Mon 04/13 00Z 0.21 1924  88  97  -1 0.01
Mon 04/13 01Z 0.26 2026  88  99   0 0.01
Mon 04/13 02Z 0.32 2028  90  99   1 0.02
Mon 04/13 03Z 0.35 2129  93  99   3 0.03
Mon 04/13 04Z 0.35 2130  95  97   5 0.03
Mon 04/13 05Z 0.39 2232  94  97   6 0.08
Mon 04/13 06Z 0.39 2232  92  97   8 0.09
Mon 04/13 07Z 0.43 2234  90  88   9 0.05
Mon 04/13 08Z 0.47 2336  87  39  10 0.02
Mon 04/13 09Z 0.56 2337  89  61   9 0.01
Mon 04/13 10Z 0.62 2340  92  87   9 0.02
Mon 04/13 11Z 0.67 2439  90  80  10 0.02
Mon 04/13 12Z 0.71 2440  92  72   9 0.01
Mon 04/13 13Z 0.76 2440  87  80   9 0.01
Mon 04/13 14Z 0.72 2437  92  90   9 0.04
Mon 04/13 15Z 0.77 2341  89  95   9 0.05
Mon 04/13 16Z 0.84 2246  97  97   8 0.12
Mon 04/13 17Z 0.99 2345  98  96   8 0.12
Mon 04/13 18Z 1.11 2445  98  96   8 0.12
Mon 04/13 19Z 1.12 2641  98  71   8 0.05
Mon 04/13 20Z 1.07 2638  97  62   9 0.02
Mon 04/13 21Z 1.12 2637  98  62   9 0.02
Mon 04/13 22Z 1.22 2737  98  58   9 0.05
Mon 04/13 23Z 1.36 2737  98  60   9 0.05
Tue 04/14 00Z 1.41 2738  98  42   9 0.03
Tue 04/14 01Z 1.45 2839  99  39   8 0.06
Tue 04/14 02Z 1.27 2935  98  26   7 0.05
Tue 04/14 03Z 1.05 2932  98  32   7 0.04
Tue 04/14 04Z 0.87 3028  97  35   6 0.03
Tue 04/14 05Z 0.65 3025  94  38   6 0.02
Tue 04/14 07Z 0.38 3020  83  44   8 0.02
Tue 04/14 08Z 0.31 3018  76  42   9 0.01
Tue 04/14 09Z 0.27 2915  70  42   9 0.01
Tue 04/14 10Z 0.25 2813  67  34   9 0.01
Tue 04/14 11Z 0.25 2712  64  36  10 0.01
Tue 04/14 12Z 0.24 2612  61  38  10 0.00
Tue 04/14 13Z 0.24 2513  59  47  10 0.00
Tue 04/14 14Z 0.22 2313  58  45  10 0.00
Tue 04/14 15Z 0.30 2218  61  54  10 0.00
Tue 04/14 16Z 0.40 2321  76  58  10 0.00
Tue 04/14 17Z 0.46 2222  85  56  11 0.00
Tue 04/14 18Z 0.56 2324  89  84  11 0.00
Tue 04/14 19Z 0.62 2229  86  78  11 0.02
Tue 04/14 20Z 0.68 2232  82  73  12 0.01
Tue 04/14 21Z 0.69 2329  76  61  13 0.01
Tue 04/14 22Z 0.66 2326  79  70  12 0.01
Tue 04/14 23Z 0.69 2228  88  73  12 0.00

Run total areal avg QPF for the greens is: 1.42 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