Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260206_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 02/06 06Z 0.04 1906  61  23 -10 0.00
Fri 02/06 07Z 0.04 2006  58  20 -10 0.00
Fri 02/06 08Z 0.05 2005  58  21 -10 0.00
Fri 02/06 09Z 0.07 2008  60  28 -10 0.00
Fri 02/06 10Z 0.05 1909  62  44 -11 0.00
Fri 02/06 11Z 0.05 1911  63  61 -11 0.00
Fri 02/06 12Z 0.06 1910  66  67 -11 0.00
Fri 02/06 13Z 0.04 1811  69  71 -11 0.00
Fri 02/06 14Z 0.04 1814  71  78 -11 0.00
Fri 02/06 15Z 0.08 1915  73  81 -11 0.00
Fri 02/06 16Z 0.12 2013  75  85 -11 0.00
Fri 02/06 17Z 0.12 2011  75  87 -11 0.00
Fri 02/06 18Z 0.11 1910  76  87 -11 0.00
Fri 02/06 19Z 0.11 1909  79  86 -11 0.00
Fri 02/06 21Z 0.07 1908  85  89 -11 0.00
Fri 02/06 22Z 0.05 1810  86  90 -11 0.00
Sat 02/07 01Z 0.11 1913  89  88 -11 0.00
Sat 02/07 02Z 0.16 2014  91  87 -11 0.00
Sat 02/07 03Z 0.18 2014  92  86 -11 0.00
Sat 02/07 04Z 0.15 1913  91  86 -11 0.00
Sat 02/07 05Z 0.11 1913  91  86 -11 0.01
Sat 02/07 08Z 0.07 1812  91  85 -11 0.01
Sat 02/07 09Z 0.07 1910  92  85 -11 0.01
Sat 02/07 10Z 0.08 1908  92  85 -12 0.01
Sat 02/07 11Z 0.10 2005  93  84 -12 0.01
Sat 02/07 12Z 0.12 2802  92  83 -12 0.02
Sat 02/07 13Z 0.08 3307  91  82 -13 0.01
Sat 02/07 15Z 0.09 3413  87  81 -14 0.01
Sat 02/07 16Z 0.10 3314  86  82 -14 0.01
Sat 02/07 18Z 0.14 3316  84  83 -15 0.01
Sat 02/07 19Z 0.16 3318  84  83 -16 0.02
Sat 02/07 20Z 0.19 3320  84  83 -17 0.02
Sat 02/07 21Z 0.21 3322  85  82 -18 0.02
Sat 02/07 22Z 0.21 3324  85  80 -19 0.01
Sat 02/07 23Z 0.21 3325  85  78 -19 0.01
Sun 02/08 00Z 0.21 3326  85  77 -19 0.01
Sun 02/08 01Z 0.21 3326  85  76 -20 0.01
Sun 02/08 02Z 0.20 3326  85  77 -20 0.01
Sun 02/08 03Z 0.20 3325  84  78 -20 0.01
Sun 02/08 04Z 0.18 3324  84  80 -20 0.01
Sun 02/08 05Z 0.17 3323  83  79 -19 0.01
Sun 02/08 06Z 0.16 3322  83  76 -19 0.00
Sun 02/08 07Z 0.16 3322  82  75 -19 0.00
Sun 02/08 08Z 0.19 3224  82  72 -19 0.00
Sun 02/08 09Z 0.21 3225  81  70 -20 0.00
Sun 02/08 10Z 0.21 3226  80  69 -20 0.00
Sun 02/08 11Z 0.21 3226  76  65 -20 0.00
Sun 02/08 12Z 0.21 3227  71  57 -20 0.00
Sun 02/08 13Z 0.20 3227  67  52 -20 0.00
Sun 02/08 14Z 0.20 3226  65  46 -21 0.00
Sun 02/08 15Z 0.20 3224  66  38 -21 0.00
Sun 02/08 16Z 0.20 3223  66  36 -22 0.00
Sun 02/08 17Z 0.23 3222  67  40 -22 0.00

Run total areal avg QPF for the greens is: 0.25 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