NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260511_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 05/11 18Z 3.82 2808  69  80   0 0.00
Mon 05/11 19Z 5.44 2808  65  77   0 0.00
Mon 05/11 20Z 5.53 2808  67  89   0 0.00
Mon 05/11 21Z 3.09 2909  71  79   0 0.00
Mon 05/11 22Z 2.50 2909  73  78   0 0.00
Mon 05/11 23Z 0.71 3012  81  82   0 0.01
Tue 05/12 00Z 0.51 3213  78  65  -1 0.01
Tue 05/12 01Z 0.38 3315  83  73  -1 0.01
Tue 05/12 02Z 0.37 3316  80  76  -2 0.00
Tue 05/12 03Z 0.29 3315  80  72  -2 0.00
Tue 05/12 04Z 0.21 3415  81  70  -2 0.00
Tue 05/12 05Z 0.20 3415  80  60  -2 0.00
Tue 05/12 06Z 0.22 3416  77  47  -2 0.00
Tue 05/12 07Z 0.19 3416  74  43  -2 0.00
Tue 05/12 08Z 0.15 3416  71  43  -3 0.00
Tue 05/12 09Z 0.14 3516  68  33  -3 0.00
Tue 05/12 10Z 0.19 3516  66  31  -3 0.00
Tue 05/12 11Z 0.22 3415  66  34  -3 0.00
Tue 05/12 12Z 0.33 3413  67  36  -4 0.00
Tue 05/12 13Z 1.99 3312  70  37  -4 0.00
Tue 05/12 14Z 3.04 3211  69  37  -4 0.00
Tue 05/12 15Z 4.52 3111  69  39  -3 0.00
Tue 05/12 16Z 5.31 3111  67  41  -3 0.00
Tue 05/12 17Z 7.08 3012  68  40  -3 0.00
Tue 05/12 18Z 7.46 2912  70  37  -2 0.00
Tue 05/12 19Z 7.16 3012  69  34  -2 0.00
Tue 05/12 20Z 5.93 3011  67  29  -1 0.00
Tue 05/12 21Z 4.68 3110  65  25   0 0.00
Tue 05/12 22Z 2.46 3108  64  23   0 0.00
Tue 05/12 23Z 1.18 3008  63  25   0 0.00
Wed 05/13 00Z 0.66 3008  63  25   0 0.00
Wed 05/13 01Z 0.47 2907  63  22   0 0.00
Wed 05/13 02Z 0.40 2706  63  22   0 0.00
Wed 05/13 03Z 0.39 2506  63  23   0 0.00
Wed 05/13 04Z 0.35 2307  63  22   0 0.00
Wed 05/13 05Z 0.36 2308  62  18   0 0.00
Wed 05/13 06Z 0.35 2209  62  15   0 0.00
Wed 05/13 07Z 0.34 2111  62  12   0 0.00
Wed 05/13 08Z 0.36 2112  62  27   0 0.00
Wed 05/13 09Z 0.38 2114  63  59   0 0.00
Wed 05/13 10Z 0.40 2114  64  78   1 0.00
Wed 05/13 11Z 0.28 2013  66  93   1 0.00
Wed 05/13 12Z 0.14 1914  73  97   1 0.00
Wed 05/13 13Z 0.07 1718  81  98   1 0.01
Wed 05/13 14Z 0.13 1724  86  98   2 0.01
Wed 05/13 15Z 0.16 1828  93  97   3 0.04
Wed 05/13 16Z 0.13 1828  95  97   4 0.05
Wed 05/13 17Z 0.13 1828  96  96   4 0.06
Wed 05/13 18Z 0.11 1829  94  89   5 0.05
Wed 05/13 19Z 0.14 1829  95  96   4 0.04
Wed 05/13 20Z 0.14 1828  96  98   4 0.05
Wed 05/13 21Z 0.19 1929  97  96   4 0.06
Wed 05/13 22Z 0.26 1930  97  96   4 0.06
Wed 05/13 23Z 0.30 2030  97  94   5 0.04
Thu 05/14 00Z 0.30 2030  96  90   5 0.02
Thu 05/14 01Z 0.29 2030  95  88   5 0.04
Thu 05/14 02Z 0.29 2031  94  88   5 0.01
Thu 05/14 03Z 0.29 2032  90  83   5 0.01
Thu 05/14 04Z 0.28 2031  86  86   5 0.01
Thu 05/14 05Z 0.25 1930  85  86   6 0.00

Run total areal avg QPF for the greens is: 0.59 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