Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20251205_1200
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 12Z 0.12 2606  41  53 -16 0.00
Fri 12/05 15Z 0.18 2116  24  33 -14 0.00
Fri 12/05 18Z 0.29 2122  18  30 -11 0.00
Fri 12/05 21Z 0.39 2224  18  22 -10 0.00
Sat 12/06 00Z 0.37 2420  42  30  -9 0.00
Sat 12/06 03Z 0.29 2415  45  51  -8 0.00
Sat 12/06 06Z 0.24 2314  51  42  -8 0.00
Sat 12/06 09Z 0.32 2318  64  28  -9 0.00
Sat 12/06 12Z 0.42 2321  73  65  -9 0.00
Sat 12/06 15Z 0.47 2322  75  79  -7 0.00
Sat 12/06 18Z 0.40 2318  70  82  -6 0.00
Sat 12/06 21Z 0.54 2420  73  93  -6 0.00
Sun 12/07 00Z 0.81 2624  85  54  -7 0.00
Sun 12/07 03Z 1.01 2723  92  40  -8 0.00
Sun 12/07 06Z 1.02 2821  94  51  -9 0.00
Sun 12/07 09Z 0.73 2917  94  51 -10 0.00
Sun 12/07 12Z 0.35 3013  88  49 -10 0.00
Sun 12/07 15Z 0.25 2809  82  53 -10 0.00
Sun 12/07 18Z 0.08 2003  87  33 -10 0.00
Sun 12/07 21Z 0.10 1610  92  67 -10 0.00
Mon 12/08 00Z 0.08 1717  94  87  -8 0.02
Mon 12/08 03Z 0.32 2020  94  90  -9 0.04
Mon 12/08 06Z 0.53 2223  94  88  -7 0.04
Mon 12/08 09Z 0.28 2019  94  88  -8 0.07
Mon 12/08 12Z 0.66 3131  91  51 -16 0.05
Mon 12/08 15Z 1.59 3131  91  26 -20 0.01
Mon 12/08 18Z 5.50 3230  89  17 -21 0.01
Mon 12/08 21Z 0.85 3229  83   9 -20 0.00
Tue 12/09 00Z 0.31 3328  69   9 -19 0.00

Run total areal avg QPF for the greens is: 0.24 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