Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20241222_0000
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 12/22 00Z 0.63 3123  86  62 -20 0.00
Sun 12/22 01Z 0.59 3125  88  57 -20 0.00
Sun 12/22 02Z 0.51 3124  89  53 -20 0.00
Sun 12/22 03Z 0.44 3124  87  48 -20 0.00
Sun 12/22 04Z 0.39 3124  83  38 -20 0.00
Sun 12/22 05Z 0.37 3124  81  27 -20 0.00
Sun 12/22 06Z 0.34 3123  80  17 -20 0.00
Sun 12/22 07Z 0.33 3123  79  12 -20 0.00
Sun 12/22 08Z 0.32 3223  78  13 -20 0.00
Sun 12/22 09Z 0.31 3224  77  15 -20 0.00
Sun 12/22 10Z 0.29 3224  76  13 -20 0.00
Sun 12/22 11Z 0.26 3224  75  12 -20 0.00
Sun 12/22 12Z 0.23 3324  75  14 -20 0.00
Sun 12/22 13Z 0.19 3323  74  14 -20 0.00
Sun 12/22 14Z 0.16 3322  73  15 -19 0.00
Sun 12/22 15Z 0.14 3421  73  12 -19 0.00
Sun 12/22 16Z 0.14 3420  71   9 -19 0.00
Sun 12/22 17Z 0.13 3419  69  11 -19 0.00
Sun 12/22 18Z 0.13 3418  68  13 -19 0.00
Sun 12/22 19Z 0.12 3418  63  14 -18 0.00
Sun 12/22 20Z 0.10 3417  59  14 -18 0.00
Sun 12/22 21Z 0.10 3417  57  13 -18 0.00
Sun 12/22 22Z 0.09 3417  55  11 -17 0.00
Sun 12/22 23Z 0.07 3416  51   9 -17 0.00
Mon 12/23 00Z 0.04 3514  47   7 -16 0.00
Mon 12/23 01Z 0.04 3512  45   7 -16 0.00
Mon 12/23 02Z 0.04 3411  42   7 -15 0.00
Mon 12/23 03Z 0.04 3409  40   7 -15 0.00
Mon 12/23 04Z 0.05 3408  40   7 -15 0.00
Mon 12/23 05Z 0.08 3207  38   6 -15 0.00
Mon 12/23 06Z 0.08 3206  35   5 -14 0.00
Mon 12/23 07Z 0.09 3005  33   6 -14 0.00
Mon 12/23 08Z 0.10 2905  31   7 -14 0.00
Mon 12/23 09Z 0.10 2805  28   6 -13 0.00
Mon 12/23 10Z 0.14 2706  27   7 -13 0.00
Mon 12/23 11Z 0.15 2507  26   8 -13 0.00
Mon 12/23 12Z 0.16 2508  24  10 -12 0.00
Mon 12/23 13Z 0.17 2408  22  22 -12 0.00
Mon 12/23 14Z 0.17 2409  20  57 -12 0.00
Mon 12/23 15Z 0.19 2311  19  89 -11 0.00
Mon 12/23 16Z 0.20 2312  19  87 -11 0.00
Mon 12/23 17Z 0.21 2312  19  73 -10 0.00
Mon 12/23 18Z 0.23 2313  19  72 -10 0.00
Mon 12/23 19Z 0.22 2313  18  72  -9 0.00
Mon 12/23 20Z 0.21 2214  17  82  -8 0.00
Mon 12/23 21Z 0.23 2215  18  80  -8 0.00
Mon 12/23 22Z 0.25 2217  19  90  -8 0.00
Mon 12/23 23Z 0.28 2219  24  95  -8 0.00
Tue 12/24 00Z 0.31 2221  28  93  -8 0.00
Tue 12/24 01Z 0.33 2223  38  94  -8 0.00
Tue 12/24 02Z 0.29 2125  73  93  -7 0.00
Tue 12/24 03Z 0.27 2125  92  93  -6 0.01
Tue 12/24 04Z 0.36 2227  92  92  -7 0.05
Tue 12/24 05Z 0.40 2229  68  91  -7 0.04
Tue 12/24 06Z 0.43 2228  72  91  -7 0.03
Tue 12/24 07Z 0.47 2327  73  93  -9 0.03
Tue 12/24 08Z 0.46 2325  86  93  -9 0.03
Tue 12/24 09Z 0.44 2422  91  90  -9 0.04
Tue 12/24 10Z 0.40 2618  91  91  -9 0.04
Tue 12/24 11Z 0.37 2716  92  94  -9 0.03

Run total areal avg QPF for the greens is: 0.30 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