NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260402_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)
=== ===== === ==== ==== === === === ====
Thu 04/02 00Z 0.07 3511  86  18  -3 0.00
Thu 04/02 01Z 0.09 0110  82  16  -3 0.00
Thu 04/02 02Z 0.10 0211  76  18  -3 0.00
Thu 04/02 03Z 0.13 0310  69  20  -2 0.00
Thu 04/02 04Z 0.13 0508  58  22   0 0.00
Thu 04/02 05Z 0.12 0706  51  28   0 0.00
Thu 04/02 06Z 0.11 1005  50  49   0 0.00
Thu 04/02 07Z 0.09 1307  54  49   0 0.00
Thu 04/02 08Z 0.09 1407  63  48  -1 0.01
Thu 04/02 09Z 0.09 1407  68  49  -1 0.01
Thu 04/02 10Z 0.09 1408  72  39  -1 0.01
Thu 04/02 11Z 0.10 1408  74  37  -1 0.00
Thu 04/02 12Z 0.10 1509  77  40  -1 0.00
Thu 04/02 13Z 0.09 1509  81  46  -1 0.00
Thu 04/02 14Z 0.08 1507  84  42  -1 0.00
Thu 04/02 15Z 0.08 1407  86  43  -1 0.00
Thu 04/02 16Z 0.09 1407  86  50   0 0.00
Thu 04/02 17Z 0.11 1307  85  54   0 0.00
Thu 04/02 18Z 0.16 1308  85  56   0 0.00
Thu 04/02 19Z 0.17 1308  83  63   0 0.00
Thu 04/02 20Z 0.20 1410  82  72   0 0.00
Thu 04/02 21Z 0.18 1410  81  81   0 0.00
Thu 04/02 22Z 0.19 1413  82  88   0 0.00
Thu 04/02 23Z 0.19 1516  83  93   1 0.00
Fri 04/03 00Z 0.17 1519  83  80   1 0.00
Fri 04/03 01Z 0.14 1622  82  90   2 0.00
Fri 04/03 02Z 0.09 1624  85  92   3 0.00
Fri 04/03 03Z 0.05 1726  87  91   4 0.00
Fri 04/03 04Z 0.03 1728  90  84   4 0.01
Fri 04/03 05Z 0.04 1831  94  83   5 0.01
Fri 04/03 06Z 0.07 1833  95  83   6 0.01
Fri 04/03 07Z 0.13 1935  96  80   7 0.01
Fri 04/03 08Z 0.18 1936  96  83   8 0.01
Fri 04/03 09Z 0.24 2038  96  84   9 0.01
Fri 04/03 10Z 0.31 2140  96  81  10 0.02
Fri 04/03 11Z 0.36 2142  96  83  11 0.03
Fri 04/03 12Z 0.38 2240  92  84  11 0.02
Fri 04/03 13Z 0.42 2239  94  88  11 0.02
Fri 04/03 14Z 0.47 2338  95  85  11 0.04
Fri 04/03 15Z 0.54 2339  96  74  11 0.02
Fri 04/03 16Z 0.58 2439  95  75  11 0.01
Fri 04/03 17Z 0.62 2438  95  79  11 0.01
Fri 04/03 18Z 0.69 2439  93  85  11 0.01
Fri 04/03 19Z 0.77 2439  93  75  11 0.00
Fri 04/03 20Z 0.87 2439  91  56  10 0.00
Fri 04/03 21Z 1.02 2539  92  28  10 0.00
Fri 04/03 22Z 1.16 2540  93  14   9 0.00
Fri 04/03 23Z 1.25 2540  93  14   9 0.00
Sat 04/04 00Z 1.35 2542  96  17   8 0.01
Sat 04/04 01Z 1.41 2542  97  17   8 0.02
Sat 04/04 02Z 1.46 2640  98  15   7 0.04
Sat 04/04 03Z 1.38 2737  98  14   5 0.04
Sat 04/04 04Z 1.19 2835  96  17   3 0.03
Sat 04/04 05Z 0.94 2830  94  19   2 0.02
Sat 04/04 06Z 0.59 2926  90  19   2 0.01
Sat 04/04 07Z 0.41 3022  87  20   2 0.01
Sat 04/04 08Z 0.29 3019  88  21   3 0.01
Sat 04/04 09Z 0.18 3216  89  23   3 0.01
Sat 04/04 10Z 0.10 3413  89  23   3 0.01
Sat 04/04 11Z 0.07 0010  89  23   2 0.00

Run total areal avg QPF for the greens is: 0.48 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