Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260313_0000
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 03/13 00Z 6.41 3120  81  51 -12 0.00
Fri 03/13 01Z 6.19 3028  86  56 -13 0.00
Fri 03/13 02Z 3.44 3127  87  60 -13 0.00
Fri 03/13 03Z 3.31 3127  89  51 -14 0.00
Fri 03/13 04Z 2.74 3126  90  48 -14 0.00
Fri 03/13 05Z 2.50 3126  92  46 -14 0.00
Fri 03/13 06Z 1.93 3124  93  50 -14 0.00
Fri 03/13 07Z 1.38 3021  92  54 -14 0.00
Fri 03/13 08Z 0.86 3021  89  55 -14 0.00
Fri 03/13 09Z 0.66 3020  87  50 -13 0.00
Fri 03/13 10Z 0.58 2920  85  40 -13 0.00
Fri 03/13 11Z 0.60 2920  85  30 -13 0.00
Fri 03/13 12Z 0.53 2918  83  17 -13 0.00
Fri 03/13 13Z 0.53 2815  81  11 -13 0.00
Fri 03/13 14Z 0.56 2813  78   6 -13 0.00
Fri 03/13 15Z 0.74 2609  73   3 -12 0.00
Fri 03/13 16Z 1.36 2307  69   2 -12 0.00
Fri 03/13 17Z 1.56 2207  63   3 -11 0.00
Fri 03/13 18Z 1.82 2108  58   6 -11 0.00
Fri 03/13 19Z 1.16 2011  57  51 -10 0.00
Fri 03/13 20Z 0.24 1913  52  79  -9 0.00
Fri 03/13 21Z 0.71 1815  61  85 -10 0.00
Fri 03/13 22Z 0.17 1717  78  87 -10 0.00
Fri 03/13 23Z 0.17 1720  85  91  -9 0.01
Sat 03/14 00Z 0.14 1723  89  90  -9 0.01
Sat 03/14 01Z 0.18 1626  90  92  -9 0.02
Sat 03/14 02Z 0.19 1627  87  93  -9 0.02
Sat 03/14 03Z 0.20 1629  90  91  -9 0.01
Sat 03/14 04Z 0.15 1728  93  92  -8 0.06
Sat 03/14 05Z 0.13 1726  95  80  -8 0.04
Sat 03/14 06Z 0.18 1823  96  84  -7 0.02
Sat 03/14 07Z 0.24 1919  96  87  -7 0.02
Sat 03/14 08Z 0.30 2019  97  88  -6 0.02
Sat 03/14 09Z 0.46 2119  96  87  -6 0.03
Sat 03/14 10Z 0.60 2220  96  85  -7 0.03
Sat 03/14 11Z 0.75 2321  96  84  -7 0.04
Sat 03/14 12Z 0.93 2421  96  84  -8 0.03
Sat 03/14 13Z 1.07 2521  95  83  -8 0.02
Sat 03/14 14Z 1.16 2620  94  84  -8 0.02
Sat 03/14 15Z 1.73 2620  94  83  -8 0.02
Sat 03/14 16Z 1.17 2619  94  86  -7 0.02
Sat 03/14 17Z 1.21 2520  95  85  -7 0.02
Sat 03/14 18Z 2.63 2622  94  86  -7 0.02
Sat 03/14 19Z 2.78 2726  93  85  -7 0.03
Sat 03/14 20Z 3.71 2829  91  83  -8 0.03
Sat 03/14 21Z 2.92 2931  91  82  -9 0.03
Sat 03/14 22Z 2.52 2931  92  79 -10 0.02
Sat 03/14 23Z 1.75 2932  94  77 -10 0.02
Sun 03/15 00Z 1.54 2933  96  72 -10 0.02
Sun 03/15 01Z 1.35 2932  96  73 -10 0.02
Sun 03/15 02Z 1.33 2931  96  57 -10 0.02
Sun 03/15 03Z 1.37 2930  96  31 -11 0.02
Sun 03/15 04Z 1.28 3029  96  14 -12 0.02
Sun 03/15 05Z 1.13 3027  90  12 -12 0.01
Sun 03/15 06Z 0.78 3026  81  11 -12 0.00
Sun 03/15 07Z 0.59 3125  70  11 -11 0.00
Sun 03/15 08Z 0.50 3023  60   9 -11 0.00
Sun 03/15 09Z 0.41 3121  52   9 -10 0.00
Sun 03/15 10Z 0.33 3119  44  13 -10 0.00
Sun 03/15 11Z 0.24 3116  34  19  -9 0.00

Run total areal avg QPF for the greens is: 0.72 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