Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260205_1200
Using the MB925 to MB850 layer.
Assumes Central_Greens are orthogonal to 260 degree winds.

925-850 mb Layer Average Wind/RH. Avg QPF over the Central_Greens.
Date   Time        Wind RH% 700 850T QPF
Day mm/dd hhZ Frd# DDFF (%) RH% (C) (in)
=== ===== === ==== ==== === === === ====
Thu 02/05 12Z 0.23 3013  79   8 -13 0.00
Thu 02/05 13Z 0.22 3116  82   4 -14 0.00
Thu 02/05 14Z 0.20 3116  82  19 -13 0.00
Thu 02/05 15Z 0.17 3215  78  27 -13 0.00
Thu 02/05 16Z 0.15 3112  68  17 -12 0.00
Thu 02/05 17Z 0.16 3112  60   8 -12 0.00
Thu 02/05 18Z 0.17 3112  53   7 -11 0.00
Thu 02/05 19Z 0.17 3112  47   7 -11 0.00
Thu 02/05 20Z 0.15 3111  44   8 -11 0.00
Thu 02/05 21Z 0.13 3111  42   8 -11 0.00
Thu 02/05 22Z 0.09 3210  41   7 -10 0.00
Thu 02/05 23Z 0.06 3209  41   7  -9 0.00
Fri 02/06 00Z 0.04 3308  43   8  -9 0.00
Fri 02/06 01Z 0.02 3407  46   9 -10 0.00
Fri 02/06 02Z 0.01 3404  46  10 -10 0.00
Fri 02/06 03Z 0.01 0400  47  11 -10 0.00
Fri 02/06 04Z 0.03 0601  43  11 -10 0.00
Fri 02/06 05Z 0.03 1302  40  11 -10 0.00
Fri 02/06 06Z 0.02 1604  35   9  -9 0.00
Fri 02/06 07Z 0.02 1706  32   8  -9 0.00
Fri 02/06 08Z 0.04 1806  33  12  -9 0.00
Fri 02/06 09Z 0.05 1906  33  25  -9 0.00
Fri 02/06 10Z 0.06 1907  35  41 -10 0.00
Fri 02/06 11Z 0.06 1908  36  69 -10 0.00
Fri 02/06 12Z 0.05 1809  39  83 -10 0.00
Fri 02/06 13Z 0.04 1809  42  80 -10 0.00
Fri 02/06 14Z 0.07 1810  48  81 -11 0.00
Fri 02/06 15Z 0.09 1910  53  67 -11 0.00
Fri 02/06 16Z 0.09 1909  58  42 -11 0.00
Fri 02/06 17Z 0.11 1908  61  34 -11 0.00
Fri 02/06 18Z 0.16 2007  63  53 -11 0.00
Fri 02/06 19Z 0.21 2006  67  69 -11 0.00
Fri 02/06 20Z 0.16 2005  68  76 -11 0.00
Fri 02/06 21Z 0.11 1906  72  87 -11 0.00
Fri 02/06 22Z 0.11 1907  76  92 -11 0.00
Fri 02/06 23Z 0.13 1908  80  89 -11 0.00
Sat 02/07 00Z 0.17 1910  86  91 -12 0.00
Sat 02/07 01Z 0.17 1910  91  87 -12 0.00
Sat 02/07 02Z 0.21 2011  92  86 -12 0.01
Sat 02/07 03Z 0.25 2011  93  88 -12 0.01
Sat 02/07 04Z 0.31 2112  92  87 -12 0.01
Sat 02/07 05Z 0.28 2112  92  88 -12 0.01
Sat 02/07 06Z 0.23 2111  92  87 -11 0.01
Sat 02/07 07Z 0.20 2109  91  87 -11 0.01
Sat 02/07 08Z 0.16 2008  91  87 -11 0.02
Sat 02/07 09Z 0.11 1907  91  86 -11 0.02
Sat 02/07 10Z 0.05 1805  91  85 -11 0.01
Sat 02/07 11Z 0.02 1704  91  85 -12 0.01
Sat 02/07 12Z 0.02 1603  92  85 -12 0.01
Sat 02/07 13Z 0.05 2700  92  84 -12 0.02
Sat 02/07 14Z 0.04 3405  91  82 -13 0.02
Sat 02/07 15Z 0.09 3408  90  81 -13 0.02
Sat 02/07 16Z 0.07 3411  89  81 -13 0.01
Sat 02/07 17Z 0.04 3413  86  82 -14 0.01
Sat 02/07 18Z 0.06 3315  85  83 -15 0.01
Sat 02/07 19Z 0.07 3317  84  83 -15 0.01
Sat 02/07 20Z 0.08 3319  83  83 -16 0.01
Sat 02/07 21Z 0.09 3321  83  83 -17 0.01
Sat 02/07 22Z 0.10 3323  83  82 -18 0.01
Sat 02/07 23Z 0.10 3325  83  79 -19 0.01

Run total areal avg QPF for the Central_Greens is: 0.27 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