NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260318_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)
=== ===== === ==== ==== === === === ====
Wed 03/18 18Z 0.60 2603  69  10 -15 0.00
Wed 03/18 19Z 0.75 2502  61   9 -14 0.00
Wed 03/18 20Z 0.52 2103  58  12 -13 0.00
Wed 03/18 21Z 0.39 2005  57  18 -13 0.00
Wed 03/18 22Z 0.11 1806  58  25 -13 0.00
Wed 03/18 23Z 0.07 1809  59  21 -12 0.00
Thu 03/19 00Z 0.10 1811  61  25 -12 0.00
Thu 03/19 01Z 0.18 1912  58  23 -12 0.00
Thu 03/19 02Z 0.29 2014  55  14 -12 0.00
Thu 03/19 03Z 0.38 2016  56  24 -12 0.00
Thu 03/19 04Z 0.44 2116  62  35 -12 0.00
Thu 03/19 05Z 0.43 2116  67  36 -11 0.00
Thu 03/19 06Z 0.39 2116  69  50 -11 0.00
Thu 03/19 07Z 0.39 2017  67  60 -11 0.00
Thu 03/19 08Z 0.43 2118  69  81 -11 0.00
Thu 03/19 09Z 0.43 2118  73  87 -11 0.00
Thu 03/19 10Z 0.38 2018  75  86 -11 0.00
Thu 03/19 11Z 0.33 2018  78  87 -11 0.00
Thu 03/19 12Z 0.27 2019  80  89 -10 0.00
Thu 03/19 13Z 0.23 2018  81  90 -10 0.00
Thu 03/19 14Z 0.23 1918  82  91 -10 0.00
Thu 03/19 15Z 0.28 1918  84  92  -9 0.00
Thu 03/19 16Z 0.38 1917  85  92  -9 0.00
Thu 03/19 17Z 0.48 1917  86  94  -9 0.00
Thu 03/19 18Z 0.47 1917  87  95  -8 0.00
Thu 03/19 19Z 0.31 1918  88  92  -7 0.00
Thu 03/19 20Z 0.34 2018  89  85  -7 0.00
Thu 03/19 21Z 0.43 2019  92  79  -7 0.01
Thu 03/19 22Z 0.54 2121  95  62  -7 0.01
Thu 03/19 23Z 0.62 2223  96  38  -7 0.01
Fri 03/20 00Z 0.74 2422  97  16  -7 0.02
Fri 03/20 01Z 0.81 2522  98   8  -7 0.01
Fri 03/20 02Z 0.92 2522  98   9  -7 0.02
Fri 03/20 03Z 1.10 2622  99   8  -8 0.02
Fri 03/20 04Z 1.09 2821  97   8  -8 0.01
Fri 03/20 05Z 0.81 2818  94   7  -8 0.01
Fri 03/20 06Z 0.68 2918  91   8  -8 0.01
Fri 03/20 07Z 0.53 2917  85  11  -8 0.01
Fri 03/20 08Z 0.56 2918  82  20  -8 0.01
Fri 03/20 09Z 0.49 2918  79  32  -8 0.01
Fri 03/20 10Z 0.35 2916  67  39  -6 0.00
Fri 03/20 11Z 0.25 2913  49  43  -4 0.00
Fri 03/20 12Z 0.22 2812  45  63  -4 0.00
Fri 03/20 13Z 0.20 2710  44  65  -3 0.00
Fri 03/20 14Z 0.17 2508  44  63  -3 0.00
Fri 03/20 15Z 0.16 2308  41  81  -3 0.00
Fri 03/20 16Z 0.15 2109  40  83  -3 0.00
Fri 03/20 17Z 0.17 2110  40  90  -3 0.00
Fri 03/20 18Z 0.16 2012  37  93  -3 0.00
Fri 03/20 19Z 0.26 1914  40  92  -3 0.00
Fri 03/20 20Z 0.13 1916  53  93  -3 0.01
Fri 03/20 21Z 0.09 1819  64  92  -3 0.02
Fri 03/20 22Z 0.10 1824  76  91  -2 0.01
Fri 03/20 23Z 0.12 1825  96  95  -2 0.05
Sat 03/21 00Z 0.17 1926  98  95  -1 0.13
Sat 03/21 01Z 0.19 1925  98  96   0 0.11
Sat 03/21 02Z 0.24 2022  98  95   0 0.13
Sat 03/21 03Z 0.29 2121  98  91   0 0.11
Sat 03/21 04Z 0.27 2118  98  85   0 0.08
Sat 03/21 05Z 0.24 2115  98  72   0 0.05

Run total areal avg QPF for the greens is: 0.86 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