Subject: BTV4 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_BTV4_20190420_0600
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 04/20 06Z 0.18 1934  99  97  12 0.00
Sat 04/20 07Z 0.26 1937  98  98  12 0.03
Sat 04/20 08Z 0.41 2037  98  96  12 0.06
Sat 04/20 09Z 0.49 2136  96  89  12 0.03
Sat 04/20 10Z 0.51 2136  96  80  12 0.02
Sat 04/20 11Z 0.45 2132  97  75  11 0.01
Sat 04/20 12Z 0.41 2129  97  75  11 0.01
Sat 04/20 13Z 0.38 2126  97  76  11 0.01
Sat 04/20 14Z 0.34 2023  97  71  11 0.01
Sat 04/20 15Z 0.30 2019  96  64  11 0.01
Sat 04/20 16Z 0.12 1917  96  72  11 0.01
Sat 04/20 17Z 0.06 1815  96  77  11 0.01
Sat 04/20 18Z 0.06 1814  95  80  12 0.01
Sat 04/20 19Z 0.13 1817  95  78  12 0.02
Sat 04/20 20Z 0.13 1821  96  67  12 0.03
Sat 04/20 21Z 0.12 1822  97  55  11 0.04
Sat 04/20 22Z 0.12 1824  96  42  11 0.03
Sat 04/20 23Z 0.13 1827  94  45  12 0.00
Sun 04/21 00Z 0.09 1829  92  60  12 0.00
Sun 04/21 01Z 0.05 1832  90  69  13 0.00
Sun 04/21 02Z 0.03 1735  92  73  12 0.00
Sun 04/21 03Z 0.05 1837  93  70  11 0.01
Sun 04/21 04Z 0.14 1836  89  63  10 0.00
Sun 04/21 05Z 0.21 1934  79  61  11 0.00
Sun 04/21 06Z 0.23 1933  69  66  11 0.00
Sun 04/21 07Z 0.23 1931  63  73  10 0.00
Sun 04/21 08Z 0.20 1928  63  84  10 0.00
Sun 04/21 09Z 0.28 2025  69  92   9 0.00
Sun 04/21 10Z 0.39 2126  74  95   8 0.03
Sun 04/21 11Z 0.27 2021  63  86   9 0.00
Sun 04/21 12Z 0.22 2019  64  66   9 0.00
Sun 04/21 13Z 0.23 2019  68  52   8 0.00
Sun 04/21 14Z 0.27 2018  69  44   8 0.00
Sun 04/21 15Z 0.41 2016  70  42   7 0.00
Sun 04/21 16Z 1.07 2015  68  41   7 0.00
Sun 04/21 17Z 1.99 1915  65  43   8 0.00
Sun 04/21 18Z 2.30 1915  64  48   8 0.00
Sun 04/21 19Z 2.41 1915  66  52   8 0.00
Sun 04/21 20Z 2.45 1914  68  54   8 0.00
Sun 04/21 21Z 1.84 1913  70  51   8 0.00
Sun 04/21 22Z 0.51 1911  73  48   8 0.00
Sun 04/21 23Z 0.26 1909  76  51   7 0.00
Mon 04/22 00Z 0.24 2009  79  56   7 0.00
Mon 04/22 01Z 0.22 2008  81  54   7 0.01
Mon 04/22 02Z 0.24 2107  83  52   7 0.01
Mon 04/22 03Z 0.25 2206  85  46   6 0.00
Mon 04/22 04Z 0.23 2405  87  39   6 0.00
Mon 04/22 05Z 0.20 2504  89  36   6 0.00
Mon 04/22 06Z 0.13 2703  91  36   6 0.00
Mon 04/22 07Z 0.06 3302  91  36   6 0.00
Mon 04/22 08Z 0.06 0103  90  38   6 0.00
Mon 04/22 09Z 0.09 0205  90  41   6 0.00
Mon 04/22 10Z 0.16 0307  89  42   6 0.00
Mon 04/22 11Z 0.24 0309  89  43   6 0.00
Mon 04/22 12Z 0.29 0411  89  46   6 0.00
Mon 04/22 13Z 0.29 0509  88  49   6 0.00
Mon 04/22 14Z 0.26 0408  88  50   6 0.00
Mon 04/22 15Z 0.28 0209  90  51   6 0.00
Mon 04/22 16Z 0.69 0111  91  57   6 0.00
Mon 04/22 17Z 0.44 0112  91  66   6 0.00
Mon 04/22 18Z 0.14 0013  90  75   7 0.00
Mon 04/22 19Z 0.09 0015  90  85   7 0.03
Mon 04/22 20Z 0.12 0017  89  91   7 0.08
Mon 04/22 21Z 0.18 0020  93  90   7 0.08
Mon 04/22 22Z 0.22 0121  96  88   6 0.08
Mon 04/22 23Z 0.16 0122  98  85   6 0.06
Tue 04/23 00Z 0.13 0023  97  90   6 0.02
Tue 04/23 01Z 0.11 0022  96  88   6 0.01
Tue 04/23 02Z 0.09 0021  95  82   6 0.01
Tue 04/23 03Z 0.10 0121  94  73   5 0.01
Tue 04/23 04Z 0.09 0119  94  63   5 0.00
Tue 04/23 05Z 0.05 0017  91  57   5 0.00
Tue 04/23 06Z 0.04 0017  86  58   6 0.00
Tue 04/23 07Z 0.07 3515  86  69   5 0.00
Tue 04/23 08Z 0.11 3415  85  76   5 0.00
Tue 04/23 09Z 0.13 3416  85  83   5 0.00
Tue 04/23 10Z 0.13 3416  84  87   5 0.00
Tue 04/23 11Z 0.13 3315  83  88   5 0.00
Tue 04/23 12Z 0.13 3313  84  89   4 0.00
Tue 04/23 13Z 0.14 3312  84  90   4 0.00
Tue 04/23 14Z 0.15 3210  82  90   4 0.00
Tue 04/23 15Z 0.19 3107  81  92   4 0.00
Tue 04/23 16Z 0.23 2906  80  94   5 0.00
Tue 04/23 17Z 0.33 2605  77  94   5 0.00

Run total areal avg QPF for the greens is: 0.78 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