NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260420_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)
=== ===== === ==== ==== === === === ====
Mon 04/20 12Z 0.86 2916  94  67  -9 0.00
Mon 04/20 13Z 1.09 3017  93  60 -10 0.01
Mon 04/20 14Z 1.38 3015  94  54 -10 0.00
Mon 04/20 15Z 3.29 3113  90  54 -10 0.00
Mon 04/20 16Z 4.70 3112  85  60 -10 0.00
Mon 04/20 17Z 5.35 3113  80  53 -10 0.01
Mon 04/20 18Z 5.91 3012  73  45 -10 0.00
Mon 04/20 19Z 6.36 3011  69  39  -9 0.00
Mon 04/20 20Z 6.16 3012  67  33  -9 0.00
Mon 04/20 21Z 5.40 3011  64  31  -9 0.00
Mon 04/20 22Z 4.59 3111  61  30  -9 0.00
Mon 04/20 23Z 2.64 3111  58  29  -9 0.00
Tue 04/21 00Z 1.47 3112  55  25  -9 0.00
Tue 04/21 01Z 1.31 3213  54  22  -9 0.00
Tue 04/21 02Z 1.53 3214  52  19 -10 0.00
Tue 04/21 03Z 1.39 3214  49  15 -10 0.00
Tue 04/21 04Z 1.03 3114  48  14 -10 0.00
Tue 04/21 05Z 0.86 3114  47  14 -11 0.00
Tue 04/21 06Z 0.70 3113  46  14 -11 0.00
Tue 04/21 07Z 0.64 3112  46  12 -11 0.00
Tue 04/21 08Z 0.56 3111  47  11 -11 0.00
Tue 04/21 09Z 0.49 3110  47  10 -11 0.00
Tue 04/21 10Z 0.51 3009  47  10 -11 0.00
Tue 04/21 11Z 0.39 3007  48  10 -11 0.00
Tue 04/21 12Z 0.36 2804  48   9 -10 0.00
Tue 04/21 13Z 1.49 2504  47   9 -10 0.00
Tue 04/21 14Z 2.50 2305  46   9 -10 0.00
Tue 04/21 15Z 3.44 2306  45  10  -9 0.00
Tue 04/21 16Z 2.79 2205  43  14  -8 0.00
Tue 04/21 17Z 2.53 2206  40  33  -7 0.00
Tue 04/21 18Z 2.67 2206  38  72  -6 0.00
Tue 04/21 19Z 2.65 2207  38  86  -6 0.00
Tue 04/21 20Z 2.27 2207  37  83  -5 0.00
Tue 04/21 21Z 1.65 2207  37  81  -5 0.00
Tue 04/21 22Z 0.54 2108  37  84  -4 0.00
Tue 04/21 23Z 0.24 2009  38  86  -4 0.00
Wed 04/22 00Z 0.33 2112  41  88  -4 0.00
Wed 04/22 01Z 0.44 2113  43  91  -4 0.00
Wed 04/22 02Z 0.48 2115  46  89  -4 0.00
Wed 04/22 03Z 0.55 2217  51  84  -4 0.00
Wed 04/22 04Z 0.62 2219  52  79  -3 0.00
Wed 04/22 05Z 0.64 2220  55  86  -4 0.00
Wed 04/22 06Z 0.51 2220  66  93  -4 0.00
Wed 04/22 07Z 0.31 2117  79  96  -3 0.00
Wed 04/22 08Z 0.20 2014  88  92  -3 0.00
Wed 04/22 09Z 0.15 2013  93  85  -3 0.01
Wed 04/22 10Z 0.16 2015  96  60  -3 0.01
Wed 04/22 11Z 0.16 2016  97  33  -3 0.01
Wed 04/22 12Z 0.19 2016  98  20  -3 0.01
Wed 04/22 13Z 0.20 2014  98  11  -3 0.01
Wed 04/22 14Z 0.18 2012  98   6  -3 0.01
Wed 04/22 15Z 0.15 2010  98   3  -3 0.01
Wed 04/22 16Z 0.12 2008  98   4  -3 0.01
Wed 04/22 17Z 0.11 2006  98   4  -3 0.00
Wed 04/22 18Z 0.12 2105  98   5  -3 0.00
Wed 04/22 19Z 0.12 2204  98   5  -3 0.00
Wed 04/22 20Z 0.15 2403  98   7  -3 0.00
Wed 04/22 21Z 0.17 2703  98   9  -3 0.00
Wed 04/22 22Z 0.22 2804  97  10  -3 0.00
Wed 04/22 23Z 0.24 3005  96  11  -3 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