NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260512_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)
=== ===== === ==== ==== === === === ====
Tue 05/12 12Z 0.48 3210  81  49  -3 0.00
Tue 05/12 13Z 1.70 3111  80  38  -3 0.00
Tue 05/12 14Z 4.88 3112  79  35  -3 0.00
Tue 05/12 15Z 6.42 3012  74  35  -3 0.00
Tue 05/12 16Z 6.48 3011  72  38  -3 0.00
Tue 05/12 17Z 6.78 2912  76  37  -2 0.00
Tue 05/12 18Z 6.55 3011  76  34  -2 0.00
Tue 05/12 19Z 6.53 2911  74  31  -1 0.00
Tue 05/12 20Z 5.59 3010  71  28  -1 0.00
Tue 05/12 21Z 4.62 3009  69  26   0 0.00
Tue 05/12 22Z 3.07 3008  68  26   0 0.00
Tue 05/12 23Z 1.11 2908  67  27   0 0.00
Wed 05/13 00Z 0.68 2909  67  27   0 0.00
Wed 05/13 01Z 0.60 2808  68  26   0 0.00
Wed 05/13 02Z 0.52 2707  67  23   0 0.00
Wed 05/13 03Z 0.46 2607  67  19   0 0.00
Wed 05/13 04Z 0.45 2407  67  17   0 0.00
Wed 05/13 05Z 0.38 2208  66  15   0 0.00
Wed 05/13 06Z 0.33 2208  64  12   0 0.00
Wed 05/13 07Z 0.34 2110  62  19   0 0.00
Wed 05/13 08Z 0.35 2112  62  43   0 0.00
Wed 05/13 09Z 0.42 2113  66  85   0 0.00
Wed 05/13 10Z 0.27 2013  73  96   0 0.00
Wed 05/13 11Z 0.15 1914  77  98   1 0.00
Wed 05/13 12Z 0.11 1916  83  98   2 0.00
Wed 05/13 13Z 0.10 1819  89  97   2 0.02
Wed 05/13 14Z 0.13 1821  94  95   3 0.04
Wed 05/13 15Z 0.14 1823  93  97   3 0.06
Wed 05/13 16Z 0.08 1827  93  98   4 0.06
Wed 05/13 17Z 0.14 1830  94  98   4 0.05
Wed 05/13 18Z 0.22 1931  95  96   5 0.05
Wed 05/13 19Z 0.27 2029  94  96   5 0.04
Wed 05/13 20Z 0.23 1927  90  93   6 0.02
Wed 05/13 21Z 0.21 1927  92  96   6 0.02
Wed 05/13 22Z 0.20 1929  95  97   6 0.05
Wed 05/13 23Z 0.21 1930  94  94   6 0.04
Thu 05/14 00Z 0.25 1932  92  86   6 0.02
Thu 05/14 01Z 0.30 1933  91  77   6 0.00
Thu 05/14 02Z 0.34 2033  89  84   6 0.00
Thu 05/14 03Z 0.37 2033  89  81   6 0.01
Thu 05/14 04Z 0.37 2033  88  76   6 0.01
Thu 05/14 05Z 0.36 2031  85  77   6 0.00
Thu 05/14 06Z 0.32 2030  81  79   6 0.00
Thu 05/14 07Z 0.26 1928  85  86   5 0.01
Thu 05/14 08Z 0.16 1925  91  91   5 0.01
Thu 05/14 09Z 0.12 1923  93  97   5 0.05
Thu 05/14 10Z 0.06 1822  96  97   5 0.06
Thu 05/14 11Z 0.05 1821  97  94   5 0.06
Thu 05/14 12Z 0.08 1719  98  95   5 0.05
Thu 05/14 13Z 0.12 1619  98  94   5 0.04
Thu 05/14 14Z 0.23 1519  98  95   5 0.03
Thu 05/14 15Z 0.34 1520  98  93   5 0.03
Thu 05/14 16Z 0.37 1419  97  85   5 0.05
Thu 05/14 17Z 0.45 1316  97  79   5 0.04
Thu 05/14 18Z 0.60 1215  98  74   5 0.03
Thu 05/14 19Z 0.68 1015  97  80   5 0.04
Thu 05/14 20Z 0.77 0816  97  84   5 0.04
Thu 05/14 21Z 0.89 0817  94  87   5 0.06
Thu 05/14 22Z 0.81 0719  89  87   5 0.04
Thu 05/14 23Z 0.66 0619  90  88   5 0.04

Run total areal avg QPF for the greens is: 1.17 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