Subject: BTV4 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_BTV4_20220521_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)
=== ===== === ==== ==== === === === ====
Sat 05/21 12Z 0.48 2519  91  68  15 0.00
Sat 05/21 13Z 0.49 2518  90  66  16 0.00
Sat 05/21 14Z 0.50 2616  88  63  16 0.00
Sat 05/21 15Z 0.47 2514  85  53  17 0.00
Sat 05/21 16Z 0.50 2413  82  47  17 0.00
Sat 05/21 17Z 0.60 2313  80  44  17 0.00
Sat 05/21 18Z 0.72 2315  80  42  17 0.00
Sat 05/21 19Z 0.98 2317  81  42  17 0.00
Sat 05/21 20Z 1.12 2318  83  48  17 0.00
Sat 05/21 21Z 1.07 2320  85  50  17 0.01
Sat 05/21 22Z 1.14 2423  82  61  17 0.01
Sat 05/21 23Z 0.62 2318  77  59  18 0.05
Sun 05/22 00Z 0.60 2321  79  79  17 0.03
Sun 05/22 01Z 0.59 2419  88  78  17 0.26
Sun 05/22 02Z 0.81 2622  90  72  16 0.15
Sun 05/22 03Z 0.58 2516  88  79  16 0.08
Sun 05/22 04Z 0.78 2418  91  76  16 0.09
Sun 05/22 05Z 0.86 2821  92  82  14 0.43
Sun 05/22 06Z 0.35 2612  75  80  16 0.10
Sun 05/22 07Z 0.39 2513  73  75  15 0.00
Sun 05/22 08Z 0.35 2412  76  72  15 0.00
Sun 05/22 09Z 0.36 2413  77  65  15 0.00
Sun 05/22 10Z 0.38 2415  74  60  16 0.00
Sun 05/22 11Z 0.41 2416  71  61  16 0.00
Sun 05/22 12Z 0.43 2316  69  63  15 0.00
Sun 05/22 13Z 0.48 2318  72  69  15 0.00
Sun 05/22 14Z 0.56 2320  74  67  15 0.00
Sun 05/22 15Z 0.66 2222  73  63  16 0.00
Sun 05/22 16Z 1.05 2222  71  60  16 0.00
Sun 05/22 17Z 2.10 2222  73  56  16 0.00
Sun 05/22 18Z 2.70 2223  76  58  16 0.00
Sun 05/22 19Z 4.19 2223  78  62  16 0.00
Sun 05/22 20Z 5.86 2321  80  78  16 0.03
Sun 05/22 21Z 3.68 2321  74  73  16 0.04
Sun 05/22 22Z 2.31 2619  81  79  16 0.04
Sun 05/22 23Z 3.15 2835  94  90  12 0.36
Mon 05/23 00Z 0.63 2928  92  87  12 0.25
Mon 05/23 01Z 0.55 3026  92  85  10 0.04
Mon 05/23 02Z 0.47 3124  92  62   8 0.03
Mon 05/23 03Z 0.45 3123  94  42   7 0.02
Mon 05/23 04Z 0.41 3223  93  28   5 0.01
Mon 05/23 05Z 0.38 3223  92  20   5 0.01
Mon 05/23 06Z 0.34 3223  89  16   4 0.01
Mon 05/23 07Z 0.30 3223  85  13   4 0.01
Mon 05/23 08Z 0.26 3322  79  12   4 0.00
Mon 05/23 09Z 0.23 3322  74  12   3 0.00
Mon 05/23 10Z 0.19 3322  69  12   3 0.00
Mon 05/23 11Z 0.15 3421  64  11   3 0.00
Mon 05/23 12Z 0.11 3421  61  11   3 0.00
Mon 05/23 13Z 0.08 3520  60  12   3 0.00
Mon 05/23 14Z 0.06 3520  58  13   2 0.00
Mon 05/23 15Z 0.05 3518  60  14   2 0.00
Mon 05/23 16Z 0.11 3517  63  15   1 0.00
Mon 05/23 17Z 0.40 3516  65  16   1 0.00
Mon 05/23 18Z 0.64 3515  65  16   1 0.00
Mon 05/23 19Z 0.74 3515  65  16   2 0.00
Mon 05/23 20Z 0.92 3515  65  15   2 0.00
Mon 05/23 21Z 0.81 3514  66  15   2 0.00
Mon 05/23 22Z 0.61 3514  68  15   2 0.00
Mon 05/23 23Z 0.24 3514  68  16   1 0.00
Tue 05/24 00Z 0.15 3515  67  16   1 0.00
Tue 05/24 01Z 0.12 0016  65  16   1 0.00
Tue 05/24 02Z 0.09 0015  64  16   1 0.00
Tue 05/24 03Z 0.10 0014  64  15   1 0.00
Tue 05/24 04Z 0.14 0112  64  15   1 0.00
Tue 05/24 05Z 0.19 0111  63  14   1 0.00
Tue 05/24 06Z 0.24 0211  61  15   1 0.00
Tue 05/24 07Z 0.31 0310  60  15   1 0.00
Tue 05/24 08Z 0.37 0410  61  15   1 0.00
Tue 05/24 09Z 0.38 0410  59  15   1 0.00
Tue 05/24 10Z 0.37 0510  55  15   1 0.00
Tue 05/24 11Z 0.35 0609  52  16   1 0.00
Tue 05/24 12Z 0.32 0708  51  16   1 0.00
Tue 05/24 13Z 0.35 0807  53  16   1 0.00
Tue 05/24 14Z 0.65 1006  57  16   1 0.00
Tue 05/24 15Z 1.92 1105  59  17   1 0.00
Tue 05/24 16Z 2.69 1306  60  17   2 0.00
Tue 05/24 17Z 2.22 1506  60  19   2 0.00
Tue 05/24 18Z 2.10 1508  60  20   3 0.00
Tue 05/24 19Z 1.69 1609  60  22   4 0.00
Tue 05/24 20Z 0.89 1710  61  24   4 0.00
Tue 05/24 21Z 0.70 1711  62  26   4 0.00
Tue 05/24 22Z 0.67 1712  64  29   4 0.00
Tue 05/24 23Z 0.39 1715  66  32   4 0.00

Run total areal avg QPF for the greens is: 2.06 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