NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260411_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)
=== ===== === ==== ==== === === === ====
Sat 04/11 12Z 1.24 3123  95  11  -6 0.00
Sat 04/11 13Z 1.73 3025  95   5  -6 0.00
Sat 04/11 14Z 1.85 3026  96   6  -7 0.00
Sat 04/11 15Z 1.61 3026  97   9  -7 0.01
Sat 04/11 16Z 1.95 3024  93  11  -7 0.00
Sat 04/11 17Z 3.69 3022  89  12  -7 0.00
Sat 04/11 18Z 5.11 3021  86  12  -6 0.00
Sat 04/11 19Z 7.68 3020  85  13  -6 0.00
Sat 04/11 20Z 8.64 3020  81  14  -6 0.00
Sat 04/11 21Z 8.38 3121  78  15  -6 0.00
Sat 04/11 22Z 6.37 3121  76  14  -5 0.00
Sat 04/11 23Z 3.91 3122  76  13  -6 0.00
Sun 04/12 00Z 2.72 3123  77  11  -6 0.00
Sun 04/12 01Z 2.30 3124  78  10  -6 0.00
Sun 04/12 02Z 1.88 3225  77  11  -6 0.00
Sun 04/12 03Z 1.31 3225  75  11  -6 0.00
Sun 04/12 04Z 0.85 3225  72  12  -6 0.00
Sun 04/12 05Z 0.63 3324  69  11  -6 0.00
Sun 04/12 06Z 0.48 3323  65  12  -6 0.00
Sun 04/12 07Z 0.37 3321  64  13  -6 0.00
Sun 04/12 08Z 0.31 3319  66  15  -6 0.00
Sun 04/12 09Z 0.27 3317  66  17  -5 0.00
Sun 04/12 10Z 0.19 3315  64  18  -4 0.00
Sun 04/12 11Z 0.13 3311  59  21  -4 0.00
Sun 04/12 12Z 0.11 3209  54  23  -2 0.00
Sun 04/12 13Z 0.11 3106  49  24  -2 0.00
Sun 04/12 14Z 0.10 2804  46  25  -1 0.00
Sun 04/12 15Z 0.09 2403  45  26  -1 0.00
Sun 04/12 16Z 0.13 2304  47  27  -1 0.00
Sun 04/12 17Z 0.28 2206  47  32  -2 0.00
Sun 04/12 18Z 0.33 2108  45  64  -1 0.00
Sun 04/12 19Z 0.09 1909  39  75   0 0.00
Sun 04/12 20Z 0.16 1912  51  93  -1 0.00
Sun 04/12 21Z 0.13 1816  59  94  -1 0.00
Sun 04/12 22Z 0.09 1821  69  97  -1 0.02
Sun 04/12 23Z 0.17 1826  76  98  -1 0.03
Mon 04/13 00Z 0.17 1929  86  99  -1 0.05
Mon 04/13 01Z 0.21 1931  90  98   0 0.08
Mon 04/13 02Z 0.26 2032  91  99   1 0.10
Mon 04/13 03Z 0.32 2034  93  98   3 0.10
Mon 04/13 04Z 0.39 2135  94  97   5 0.09
Mon 04/13 05Z 0.42 2237  94  96   7 0.09
Mon 04/13 06Z 0.54 2243  95  93   8 0.10
Mon 04/13 07Z 0.62 2344  96  92   9 0.06
Mon 04/13 08Z 0.68 2445  94  96  10 0.09
Mon 04/13 09Z 0.73 2446  94  92  10 0.09
Mon 04/13 10Z 0.79 2548  94  92  10 0.14
Mon 04/13 11Z 0.82 2547  95  92  10 0.13
Mon 04/13 12Z 0.80 2544  95  93  10 0.11
Mon 04/13 13Z 0.80 2643  95  93  11 0.08
Mon 04/13 14Z 0.81 2641  96  84  10 0.06
Mon 04/13 15Z 0.86 2642  96  82  10 0.10
Mon 04/13 16Z 0.87 2639  96  81  10 0.06
Mon 04/13 17Z 0.97 2640  94  91  10 0.07
Mon 04/13 18Z 1.08 2638  94  96   9 0.08
Mon 04/13 19Z 1.07 2534  97  97   8 0.09
Mon 04/13 20Z 0.97 2532  97  92   8 0.10
Mon 04/13 21Z 1.00 2533  98  92   8 0.09
Mon 04/13 22Z 0.95 2631  97  85   8 0.06
Mon 04/13 23Z 0.91 2730  96  77   9 0.04

Run total areal avg QPF for the greens is: 2.12 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