Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20251205_1800
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 12/05 18Z 0.23 2116  29  32 -12 0.00
Fri 12/05 19Z 0.24 2117  27  30 -11 0.00
Fri 12/05 20Z 0.26 2119  28  27 -11 0.00
Fri 12/05 21Z 0.28 2120  29  23 -10 0.00
Fri 12/05 22Z 0.31 2220  32  20  -9 0.00
Fri 12/05 23Z 0.34 2320  44  21  -9 0.00
Sat 12/06 00Z 0.34 2419  53  36  -9 0.00
Sat 12/06 01Z 0.32 2417  54  47  -8 0.00
Sat 12/06 02Z 0.28 2415  54  54  -8 0.00
Sat 12/06 03Z 0.27 2315  55  56  -8 0.00
Sat 12/06 04Z 0.23 2314  54  57  -8 0.00
Sat 12/06 05Z 0.22 2314  55  53  -8 0.00
Sat 12/06 06Z 0.22 2215  57  47  -7 0.00
Sat 12/06 07Z 0.20 2215  60  40  -7 0.00
Sat 12/06 08Z 0.25 2217  67  32  -7 0.00
Sat 12/06 09Z 0.29 2218  71  26  -7 0.00
Sat 12/06 10Z 0.33 2219  75  29  -8 0.00
Sat 12/06 11Z 0.34 2219  77  40  -8 0.00
Sat 12/06 12Z 0.40 2320  78  73  -8 0.00
Sat 12/06 13Z 0.40 2419  71  83  -7 0.00
Sat 12/06 14Z 0.38 2418  68  84  -7 0.00
Sat 12/06 15Z 0.35 2417  69  82  -7 0.00
Sat 12/06 16Z 0.32 2415  70  83  -6 0.00
Sat 12/06 17Z 0.33 2315  73  81  -6 0.00
Sat 12/06 18Z 0.35 2315  75  86  -6 0.00
Sat 12/06 19Z 0.37 2316  83  85  -6 0.00
Sat 12/06 20Z 0.41 2317  83  86  -6 0.00
Sat 12/06 21Z 0.49 2418  85  87  -6 0.00
Sat 12/06 22Z 0.56 2519  83  83  -7 0.00
Sat 12/06 23Z 0.64 2520  83  79  -7 0.00
Sun 12/07 00Z 0.75 2622  88  66  -7 0.00
Sun 12/07 01Z 0.78 2623  90  46  -7 0.00
Sun 12/07 02Z 0.78 2723  91  30  -7 0.00
Sun 12/07 03Z 0.82 2723  93  19  -8 0.00
Sun 12/07 04Z 0.86 2722  95  19  -8 0.00
Sun 12/07 05Z 0.93 2722  95  36  -8 0.01
Sun 12/07 06Z 0.99 2822  96  45  -9 0.01
Sun 12/07 07Z 1.04 2922  95  34  -9 0.01
Sun 12/07 08Z 0.75 2922  93  37 -10 0.01
Sun 12/07 09Z 0.63 2920  91  40 -10 0.01
Sun 12/07 10Z 0.45 3018  83  49 -10 0.00
Sun 12/07 11Z 0.39 3018  78  59 -10 0.00
Sun 12/07 12Z 0.32 3017  67  57  -9 0.00
Sun 12/07 13Z 0.27 3015  56  61  -9 0.00
Sun 12/07 14Z 0.22 2912  48  59  -8 0.00
Sun 12/07 15Z 0.19 2909  44  55  -8 0.00
Sun 12/07 16Z 0.16 2707  43  57  -8 0.00
Sun 12/07 17Z 0.12 2505  45  56  -8 0.00
Sun 12/07 18Z 0.10 2305  51  55  -9 0.00
Sun 12/07 19Z 0.09 2105  55  56  -9 0.00
Sun 12/07 20Z 0.09 2105  60  61  -9 0.00
Sun 12/07 21Z 0.09 2106  66  55  -9 0.00
Sun 12/07 22Z 0.07 1907  72  80  -9 0.00
Sun 12/07 23Z 0.07 1909  80  87 -10 0.00
Mon 12/08 00Z 0.05 1809  89  89 -10 0.00
Mon 12/08 01Z 0.05 1810  92  89  -9 0.02
Mon 12/08 02Z 0.06 1810  93  90  -9 0.02
Mon 12/08 03Z 0.08 1910  93  88  -9 0.02
Mon 12/08 04Z 0.12 1911  93  87  -9 0.03
Mon 12/08 05Z 0.16 2011  93  87  -8 0.03

Run total areal avg QPF for the greens is: 0.17 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