Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260320_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)
=== ===== === ==== ==== === === === ====
Fri 03/20 06Z 0.62 2617  90   8  -7 0.00
Fri 03/20 07Z 0.73 2719  93   8  -7 0.00
Fri 03/20 08Z 0.68 2719  94  11  -7 0.00
Fri 03/20 09Z 0.59 2718  90  14  -7 0.00
Fri 03/20 10Z 0.57 2819  87  22  -7 0.00
Fri 03/20 11Z 0.41 2815  82  34  -6 0.00
Fri 03/20 12Z 0.30 2712  75  27  -5 0.00
Fri 03/20 13Z 0.22 2609  68  23  -4 0.00
Fri 03/20 14Z 0.22 2409  71  31  -4 0.00
Fri 03/20 15Z 0.16 2111  72  48  -4 0.00
Fri 03/20 16Z 0.20 2015  74  84  -4 0.00
Fri 03/20 17Z 0.19 1918  83  93  -3 0.00
Fri 03/20 18Z 0.12 1824  94  96  -1 0.01
Fri 03/20 19Z 0.13 1826  94  92   0 0.05
Fri 03/20 20Z 0.26 1928  98  96   0 0.17
Fri 03/20 21Z 0.39 2030  98  95   0 0.17
Fri 03/20 22Z 0.63 2232  98  97   0 0.14
Fri 03/20 23Z 0.78 2431  98  93   0 0.11
Sat 03/21 00Z 0.86 2530  98  64   0 0.08
Sat 03/21 01Z 0.81 2626  98  15   0 0.06
Sat 03/21 02Z 0.77 2724  97  11  -1 0.04
Sat 03/21 03Z 0.65 2821  96   9  -1 0.02
Sat 03/21 04Z 0.56 2920  96  17  -2 0.02
Sat 03/21 05Z 0.38 2916  94  30  -2 0.01
Sat 03/21 06Z 0.34 3017  93  48  -3 0.01
Sat 03/21 07Z 0.32 3117  93  56  -4 0.01
Sat 03/21 08Z 0.27 3117  92  59  -4 0.01
Sat 03/21 09Z 0.21 3216  90  66  -5 0.00
Sat 03/21 10Z 0.21 3317  90  70  -6 0.00
Sat 03/21 11Z 0.24 3219  91  66  -7 0.00
Sat 03/21 12Z 0.25 3320  90  50  -7 0.00
Sat 03/21 13Z 0.23 3319  88  27  -8 0.00
Sat 03/21 14Z 0.23 3318  87  21  -8 0.00
Sat 03/21 15Z 0.32 3316  87  20  -8 0.00
Sat 03/21 16Z 0.43 3215  87  18  -8 0.00
Sat 03/21 17Z 0.46 3314  84  16  -8 0.00
Sat 03/21 18Z 0.55 3313  80  17  -8 0.00
Sat 03/21 19Z 0.72 3212  78  21  -7 0.00
Sat 03/21 20Z 0.53 3311  73  23  -7 0.00
Sat 03/21 21Z 0.26 3311  67  26  -7 0.00
Sat 03/21 22Z 0.17 3309  60  27  -7 0.00
Sat 03/21 23Z 0.13 3308  52  28  -7 0.00
Sun 03/22 00Z 0.12 3308  47  29  -7 0.00
Sun 03/22 01Z 0.12 3206  45  36  -7 0.00
Sun 03/22 02Z 0.13 3004  48  53  -6 0.00
Sun 03/22 03Z 0.15 2704  54  75  -6 0.00
Sun 03/22 04Z 0.10 2204  58  87  -6 0.00
Sun 03/22 05Z 0.08 2006  63  95  -6 0.00
Sun 03/22 06Z 0.12 2008  68  94  -6 0.00
Sun 03/22 07Z 0.14 2011  75  95  -6 0.00
Sun 03/22 08Z 0.21 2014  89  93  -7 0.01
Sun 03/22 09Z 0.18 2016  95  92  -6 0.05
Sun 03/22 10Z 0.13 1918  95  92  -5 0.07
Sun 03/22 11Z 0.13 1922  96  93  -5 0.06
Sun 03/22 12Z 0.10 1825  96  95  -3 0.06
Sun 03/22 13Z 0.13 1928  97  96  -3 0.08
Sun 03/22 14Z 0.10 1827  97  98  -2 0.10
Sun 03/22 15Z 0.14 1925  97  98  -2 0.10
Sun 03/22 16Z 0.18 1923  98  98  -1 0.08
Sun 03/22 17Z 0.22 2019  98  99  -1 0.08

Run total areal avg QPF for the greens is: 1.60 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