Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260208_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)
=== ===== === ==== ==== === === === ====
Sun 02/08 18Z 0.08 3421  76  36 -16 0.00
Sun 02/08 21Z 0.12 3426  66  56 -14 0.00
Mon 02/09 00Z 0.13 3430  62  52 -12 0.00
Mon 02/09 03Z 0.13 3428  70  37 -12 0.00
Mon 02/09 06Z 0.18 3328  70  34 -13 0.00
Mon 02/09 09Z 0.21 3331  67  27 -13 0.00
Mon 02/09 12Z 0.18 3432  61  15 -13 0.00
Mon 02/09 15Z 0.16 3431  56  31 -11 0.00
Mon 02/09 18Z 0.19 3327  53  74 -11 0.00
Mon 02/09 21Z 0.18 3323  40  50  -9 0.00
Tue 02/10 00Z 0.18 3323  31  43  -8 0.00
Tue 02/10 03Z 0.14 3319  31  52  -8 0.00
Tue 02/10 06Z 0.13 3315  32  79  -8 0.00
Tue 02/10 09Z 0.13 3211  40  78  -8 0.00
Tue 02/10 12Z 0.14 2907  49  74  -8 0.00
Tue 02/10 15Z 0.11 2206  60  66  -7 0.00
Tue 02/10 18Z 0.11 1913  55  64  -6 0.00
Tue 02/10 21Z 0.02 1819  44  89  -6 0.00
Wed 02/11 00Z 0.08 1726  91  90  -6 0.09
Wed 02/11 03Z 0.19 1924  91  90  -9 0.15
Wed 02/11 06Z 0.33 2315  92  87  -8 0.07
Wed 02/11 09Z 0.45 2814  91  89 -10 0.03
Wed 02/11 12Z 0.34 3113  92  89 -11 0.02
Wed 02/11 15Z 0.37 2712  92  88 -10 0.01
Wed 02/11 18Z 0.23 3413  93  85 -11 0.02
Wed 02/11 21Z 0.25 3418  96  81 -11 0.01
Thu 02/12 00Z 0.14 3422  93  82 -11 0.01
Thu 02/12 03Z 0.19 3425  93  57 -12 0.01
Thu 02/12 06Z 0.32 3423  94  14 -12 0.01

Run total areal avg QPF for the greens is: 0.43 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