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

Model run: BTV_GRID_D2D_BTV4_20190219_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)
=== ===== === ==== ==== === === === ====
Tue 02/19 18Z 0.40 3018  62  50 -17 0.00
Tue 02/19 19Z 0.50 3022  65  43 -17 0.00
Tue 02/19 20Z 0.60 2923  71  43 -17 0.00
Tue 02/19 21Z 0.69 2924  76  42 -17 0.00
Tue 02/19 22Z 0.69 2925  75  42 -17 0.00
Tue 02/19 23Z 0.65 2927  71  40 -16 0.00
Wed 02/20 00Z 0.61 2929  67  38 -16 0.00
Wed 02/20 01Z 0.61 3029  65  38 -16 0.00
Wed 02/20 02Z 0.61 3030  65  39 -16 0.00
Wed 02/20 03Z 0.62 3030  65  38 -15 0.00
Wed 02/20 04Z 0.59 3030  66  37 -15 0.00
Wed 02/20 05Z 0.55 3029  68  34 -14 0.00
Wed 02/20 06Z 0.52 3029  68  32 -14 0.00
Wed 02/20 07Z 0.50 3028  67  29 -14 0.00
Wed 02/20 08Z 0.47 3128  65  26 -14 0.00
Wed 02/20 09Z 0.41 3126  62  24 -13 0.00
Wed 02/20 10Z 0.35 3124  60  22 -13 0.00
Wed 02/20 11Z 0.31 3122  58  20 -12 0.00
Wed 02/20 12Z 0.28 3119  55  17 -12 0.00
Wed 02/20 13Z 0.25 3016  53  15 -11 0.00
Wed 02/20 14Z 0.22 3014  52  14 -11 0.00
Wed 02/20 15Z 0.20 2911  52  13 -10 0.00
Wed 02/20 16Z 0.19 2710  54  13 -10 0.00
Wed 02/20 17Z 0.19 2510  57  13 -10 0.00
Wed 02/20 18Z 0.20 2411  58  14  -9 0.00
Wed 02/20 19Z 0.22 2412  55  16  -8 0.00
Wed 02/20 20Z 0.25 2313  53  21  -8 0.00
Wed 02/20 21Z 0.27 2315  53  29  -8 0.00
Wed 02/20 22Z 0.26 2215  53  38  -7 0.00
Wed 02/20 23Z 0.23 2215  57  54  -7 0.00
Thu 02/21 00Z 0.16 2114  63  78  -5 0.00
Thu 02/21 01Z 0.11 1915  75  87  -5 0.00
Thu 02/21 02Z 0.06 1818  82  93  -5 0.00
Thu 02/21 03Z 0.04 1819  88  96  -4 0.00
Thu 02/21 04Z 0.08 1824  94  95  -4 0.01
Thu 02/21 05Z 0.10 1928  95  97  -3 0.02
Thu 02/21 06Z 0.10 1831  94  97  -3 0.03
Thu 02/21 07Z 0.10 1833  95  99  -2 0.03
Thu 02/21 08Z 0.16 1937  95  98  -2 0.05
Thu 02/21 09Z 0.23 2036  96  95  -2 0.04
Thu 02/21 10Z 0.30 2033  96  79  -3 0.02
Thu 02/21 11Z 0.36 2130  96  63  -3 0.02
Thu 02/21 12Z 0.46 2327  96  38  -3 0.01
Thu 02/21 13Z 0.55 2427  93  28  -4 0.00
Thu 02/21 14Z 0.62 2527  89  26  -4 0.00
Thu 02/21 15Z 0.64 2627  86  23  -3 0.00
Thu 02/21 16Z 0.68 2727  87  23  -4 0.00
Thu 02/21 17Z 0.78 2727  91  23  -5 0.00
Thu 02/21 18Z 0.96 2629  93  23  -6 0.01
Thu 02/21 19Z 1.23 2730  95  24  -6 0.01
Thu 02/21 20Z 1.69 2730  95  24  -7 0.02
Thu 02/21 21Z 1.73 2731  94  25  -8 0.01
Thu 02/21 22Z 1.72 2832  94  28  -8 0.01
Thu 02/21 23Z 1.68 2833  93  31  -9 0.01
Fri 02/22 00Z 1.60 2835  92  33  -9 0.01
Fri 02/22 01Z 1.52 2836  93  36  -9 0.01
Fri 02/22 02Z 1.42 2935  94  37  -9 0.01
Fri 02/22 03Z 1.31 2935  95  36 -10 0.01
Fri 02/22 04Z 1.22 3034  96  33 -10 0.01
Fri 02/22 05Z 1.07 3033  95  30 -10 0.01
Fri 02/22 06Z 0.93 3131  95  29 -10 0.01
Fri 02/22 07Z 0.78 3130  94  29 -10 0.01
Fri 02/22 08Z 0.65 3127  93  29 -10 0.01
Fri 02/22 09Z 0.52 3126  91  30 -10 0.01
Fri 02/22 10Z 0.44 3125  89  32 -10 0.00
Fri 02/22 11Z 0.39 3124  88  32  -9 0.00
Fri 02/22 12Z 0.37 3123  85  33  -8 0.00
Fri 02/22 13Z 0.32 3122  82  32  -8 0.00
Fri 02/22 14Z 0.31 3122  80  32  -8 0.00
Fri 02/22 15Z 0.32 3121  81  31  -8 0.00
Fri 02/22 16Z 0.33 3120  82  31  -8 0.00
Fri 02/22 17Z 0.35 3120  84  30  -8 0.00
Fri 02/22 18Z 0.35 3020  85  29  -8 0.00
Fri 02/22 19Z 0.36 3020  84  29  -8 0.00
Fri 02/22 20Z 0.36 3020  83  27  -8 0.00
Fri 02/22 21Z 0.36 3020  81  25  -8 0.00
Fri 02/22 22Z 0.36 3020  79  23  -8 0.00
Fri 02/22 23Z 0.35 3120  78  22  -7 0.00
Sat 02/23 00Z 0.33 3120  77  21  -7 0.00
Sat 02/23 01Z 0.31 3119  76  20  -7 0.00
Sat 02/23 02Z 0.28 3117  74  20  -7 0.00
Sat 02/23 03Z 0.25 3116  72  21  -7 0.00
Sat 02/23 04Z 0.23 3014  70  22  -6 0.00
Sat 02/23 05Z 0.21 3013  68  22  -6 0.00

Run total areal avg QPF for the greens is: 0.40 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