National Weather Service United States Department of Commerce
Subject: BTV4 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_BTV4_20221127_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)
=== ===== === ==== ==== === === === ====
Sun 11/27 18Z 0.26 2217  33  47   7 0.00
Sun 11/27 19Z 0.26 2118  38  69   7 0.00
Sun 11/27 20Z 0.23 2020  51  85   6 0.00
Sun 11/27 21Z 0.22 2023  73  96   4 0.03
Sun 11/27 22Z 0.16 1920  88  99   4 0.02
Sun 11/27 23Z 0.16 2019  95  99   4 0.04
Mon 11/28 00Z 0.17 2020  98  99   4 0.12
Mon 11/28 01Z 0.20 2021  99  98   3 0.11
Mon 11/28 02Z 0.30 2122  98  97   3 0.10
Mon 11/28 03Z 0.46 2323  98  96   3 0.14
Mon 11/28 04Z 0.50 2422  98  91   4 0.12
Mon 11/28 05Z 0.51 2421  97  83   4 0.04
Mon 11/28 06Z 0.56 2521  98  89   3 0.03
Mon 11/28 07Z 0.64 2622  98  92   2 0.04
Mon 11/28 08Z 0.64 2621  98  83   1 0.03
Mon 11/28 09Z 0.65 2721  99  76   0 0.03
Mon 11/28 10Z 0.56 2919  99  77   0 0.04
Mon 11/28 11Z 0.43 3223  98  78  -1 0.03
Mon 11/28 12Z 0.34 3326  98  70  -2 0.02
Mon 11/28 13Z 0.38 3328  97  67  -3 0.02
Mon 11/28 14Z 0.37 3327  97  60  -4 0.02
Mon 11/28 15Z 0.39 3326  97  47  -4 0.02
Mon 11/28 16Z 0.39 3325  97  36  -4 0.01
Mon 11/28 17Z 0.37 3324  96  28  -5 0.01
Mon 11/28 18Z 0.39 3323  94  21  -5 0.01
Mon 11/28 19Z 0.41 3323  91  15  -5 0.00
Mon 11/28 20Z 0.44 3222  91  12  -6 0.00
Mon 11/28 21Z 0.45 3222  92  11  -6 0.00
Mon 11/28 22Z 0.44 3221  92  10  -7 0.01
Mon 11/28 23Z 0.43 3221  91   9  -7 0.01
Tue 11/29 00Z 0.35 3220  88  10  -7 0.01
Tue 11/29 01Z 0.29 3220  84  10  -6 0.00
Tue 11/29 02Z 0.24 3218  81  11  -6 0.00
Tue 11/29 03Z 0.21 3217  77  12  -6 0.00
Tue 11/29 04Z 0.17 3215  72  13  -5 0.00
Tue 11/29 05Z 0.15 3213  68  15  -4 0.00
Tue 11/29 06Z 0.14 3211  64  15  -4 0.00
Tue 11/29 07Z 0.14 3110  60  15  -3 0.00
Tue 11/29 08Z 0.15 3010  57  15  -3 0.00
Tue 11/29 09Z 0.15 3010  55  14  -3 0.00
Tue 11/29 10Z 0.15 3009  53  14  -3 0.00
Tue 11/29 11Z 0.13 3008  51  14  -3 0.00
Tue 11/29 12Z 0.12 2907  50  14  -3 0.00
Tue 11/29 13Z 0.10 2805  48  14  -2 0.00
Tue 11/29 14Z 0.09 2605  49  13  -2 0.00
Tue 11/29 15Z 0.10 2406  50  12  -2 0.00
Tue 11/29 16Z 0.12 2307  52  12  -2 0.00
Tue 11/29 17Z 0.13 2208  55  13  -2 0.00
Tue 11/29 18Z 0.13 2108  58  13  -3 0.00
Tue 11/29 19Z 0.14 2109  60  14  -3 0.00
Tue 11/29 20Z 0.14 2110  61  15  -3 0.00
Tue 11/29 21Z 0.16 2112  61  16  -2 0.00
Tue 11/29 22Z 0.17 2113  60  18  -2 0.00
Tue 11/29 23Z 0.17 2114  59  21  -1 0.00
Wed 11/30 00Z 0.16 2014  58  23  -1 0.00
Wed 11/30 01Z 0.15 2015  58  26   0 0.00
Wed 11/30 02Z 0.12 1916  59  28   0 0.00
Wed 11/30 03Z 0.11 1917  60  32   0 0.00
Wed 11/30 04Z 0.10 1920  61  35   0 0.00
Wed 11/30 05Z 0.09 1920  63  35   0 0.00
Wed 11/30 06Z 0.10 1922  66  35   0 0.00
Wed 11/30 07Z 0.11 1923  68  35   1 0.00
Wed 11/30 08Z 0.11 1925  71  35   2 0.00
Wed 11/30 09Z 0.12 1927  72  34   3 0.00
Wed 11/30 10Z 0.13 1929  75  32   3 0.00
Wed 11/30 11Z 0.15 1932  79  32   3 0.00
Wed 11/30 12Z 0.16 1936  83  31   2 0.00
Wed 11/30 13Z 0.14 1938  86  33   2 0.00
Wed 11/30 14Z 0.15 1842  89  33   1 0.00
Wed 11/30 15Z 0.14 1844  88  37   2 0.00
Wed 11/30 16Z 0.15 1847  89  50   2 0.00
Wed 11/30 17Z 0.13 1848  91  53   2 0.01
Wed 11/30 18Z 0.12 1849  94  71   1 0.01
Wed 11/30 19Z 0.09 1850  96  94   2 0.02
Wed 11/30 20Z 0.06 1852  96  96   3 0.05
Wed 11/30 21Z 0.04 1753  96  95   4 0.07
Wed 11/30 22Z 0.04 1754  97  97   4 0.07
Wed 11/30 23Z 0.12 1852  98  99   4 0.13
Thu 12/01 00Z 0.29 1951  98  98   5 0.16
Thu 12/01 01Z 0.50 2047  98  99   5 0.11
Thu 12/01 02Z 1.15 2242  99  97   4 0.18
Thu 12/01 03Z 1.90 2637  94  89  -1 0.14
Thu 12/01 04Z 2.27 2640  95  76  -3 0.02
Thu 12/01 05Z 2.82 2741  91  40  -5 0.01

Run total areal avg QPF for the greens is: 2.04 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