NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260318_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)
=== ===== === ==== ==== === === === ====
Wed 03/18 06Z 1.79 2716  84  65 -19 0.00
Wed 03/18 07Z 5.03 2821  88  53 -19 0.00
Wed 03/18 08Z 4.80 2821  90  38 -19 0.00
Wed 03/18 09Z 2.76 2920  88  22 -20 0.00
Wed 03/18 10Z 1.29 2919  83  10 -20 0.00
Wed 03/18 11Z 0.85 2918  80   6 -20 0.00
Wed 03/18 12Z 0.69 3017  77   4 -20 0.00
Wed 03/18 13Z 0.67 3014  74   5 -20 0.00
Wed 03/18 14Z 0.93 2912  70   6 -19 0.00
Wed 03/18 15Z 1.32 2809  66   6 -19 0.00
Wed 03/18 16Z 1.81 2707  61   7 -18 0.00
Wed 03/18 17Z 1.52 2605  58   6 -17 0.00
Wed 03/18 18Z 1.13 2703  52   6 -16 0.00
Wed 03/18 19Z 1.05 2804  48   6 -15 0.00
Wed 03/18 20Z 0.86 2504  46   4 -14 0.00
Wed 03/18 21Z 0.54 2204  46   3 -14 0.00
Wed 03/18 22Z 0.19 2005  48   3 -14 0.00
Wed 03/18 23Z 0.08 1807  49   4 -13 0.00
Thu 03/19 00Z 0.07 1810  49   5 -13 0.00
Thu 03/19 01Z 0.05 1813  50   7 -13 0.00
Thu 03/19 02Z 0.13 1814  49   7 -13 0.00
Thu 03/19 03Z 0.26 1915  48   7 -12 0.00
Thu 03/19 04Z 0.32 2015  52  10 -12 0.00
Thu 03/19 05Z 0.41 2016  57  23 -12 0.00
Thu 03/19 06Z 0.50 2117  63  41 -12 0.00
Thu 03/19 07Z 0.48 2117  63  49 -12 0.00
Thu 03/19 08Z 0.46 2117  60  81 -11 0.00
Thu 03/19 09Z 0.42 2117  60  88 -11 0.00
Thu 03/19 10Z 0.37 2117  62  91 -11 0.00
Thu 03/19 11Z 0.32 2017  66  91 -11 0.00
Thu 03/19 12Z 0.30 2017  69  87 -11 0.00
Thu 03/19 13Z 0.26 2018  69  87 -11 0.00
Thu 03/19 14Z 0.27 1917  69  87 -10 0.00
Thu 03/19 15Z 0.49 1916  69  92 -10 0.00
Thu 03/19 16Z 0.69 1815  70  93 -10 0.00
Thu 03/19 17Z 0.99 1814  70  90  -9 0.00
Thu 03/19 18Z 0.95 1814  71  90  -9 0.00
Thu 03/19 19Z 0.86 1915  75  90  -8 0.00
Thu 03/19 20Z 0.44 1916  81  90  -8 0.00
Thu 03/19 21Z 0.29 1917  86  92  -7 0.00
Thu 03/19 22Z 0.31 2018  90  85  -7 0.01
Thu 03/19 23Z 0.41 2119  93  72  -7 0.00
Fri 03/20 00Z 0.48 2120  96  52  -7 0.01
Fri 03/20 01Z 0.54 2220  97  43  -7 0.01
Fri 03/20 02Z 0.60 2319  98  27  -7 0.01
Fri 03/20 03Z 0.72 2519  98  20  -7 0.01
Fri 03/20 04Z 0.73 2616  96  12  -7 0.01
Fri 03/20 05Z 0.70 2616  98   8  -7 0.01
Fri 03/20 06Z 0.67 2615  98   6  -7 0.01
Fri 03/20 07Z 0.73 2716  97   7  -8 0.01
Fri 03/20 08Z 0.68 2715  96  13  -8 0.01
Fri 03/20 09Z 0.63 2715  95  12  -8 0.00
Fri 03/20 10Z 0.61 2714  91  15  -8 0.00
Fri 03/20 11Z 0.39 2712  79  24  -6 0.00
Fri 03/20 12Z 0.32 2712  71  21  -5 0.00
Fri 03/20 13Z 0.24 2610  62  25  -4 0.00
Fri 03/20 14Z 0.18 2408  54  34  -3 0.00
Fri 03/20 15Z 0.13 2208  47  43  -3 0.00
Fri 03/20 16Z 0.11 2010  43  72  -2 0.00
Fri 03/20 17Z 0.13 2013  43  56  -2 0.00

Run total areal avg QPF for the greens is: 0.10 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