Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20250203_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)
=== ===== === ==== ==== === === === ====
Mon 02/03 18Z 0.33 2319  83  62   0 0.00
Mon 02/03 19Z 0.37 2223  84  55   0 0.00
Mon 02/03 20Z 0.49 2227  89  58  -1 0.01
Mon 02/03 21Z 0.55 2327  93  94  -1 0.01
Mon 02/03 22Z 0.57 2427  95  94  -1 0.02
Mon 02/03 23Z 0.64 2428  97  94  -1 0.05
Tue 02/04 00Z 0.65 2428  96  94  -1 0.06
Tue 02/04 01Z 0.74 2528  97  94  -1 0.09
Tue 02/04 02Z 0.76 2725  97  95  -3 0.08
Tue 02/04 03Z 0.65 2921  97  96  -4 0.08
Tue 02/04 04Z 0.57 3020  97  95  -5 0.05
Tue 02/04 05Z 0.54 3119  96  94  -6 0.04
Tue 02/04 06Z 0.49 3118  96  95  -6 0.04
Tue 02/04 07Z 0.47 3118  95  91  -7 0.02
Tue 02/04 08Z 0.51 3121  94  81  -7 0.01
Tue 02/04 09Z 0.57 3122  93  72  -8 0.01
Tue 02/04 10Z 0.60 3023  90  71  -9 0.01
Tue 02/04 11Z 0.59 3024  84  70 -10 0.01
Tue 02/04 12Z 0.55 3025  81  78 -11 0.00
Tue 02/04 13Z 0.65 3027  81  79 -12 0.00
Tue 02/04 14Z 0.79 3028  84  76 -14 0.00
Tue 02/04 15Z 1.11 3026  91  71 -15 0.01
Tue 02/04 16Z 2.00 3026  95  65 -16 0.01
Tue 02/04 17Z 4.40 3026  94  66 -16 0.01
Tue 02/04 18Z 7.40 3027  92  60 -17 0.00
Tue 02/04 19Z 6.83 3027  92  44 -17 0.00
Tue 02/04 20Z 5.95 3027  93  30 -18 0.00
Tue 02/04 21Z 4.01 3128  93  24 -18 0.00
Tue 02/04 22Z 2.24 3129  91  25 -19 0.00
Tue 02/04 23Z 1.28 3131  90  24 -19 0.00
Wed 02/05 00Z 0.90 3131  88  23 -18 0.00
Wed 02/05 01Z 0.65 3231  84  23 -18 0.00
Wed 02/05 02Z 0.58 3130  81  23 -17 0.00
Wed 02/05 03Z 0.56 3130  81  23 -17 0.00
Wed 02/05 04Z 0.52 3130  78  19 -17 0.00
Wed 02/05 05Z 0.47 3129  76  18 -17 0.00
Wed 02/05 06Z 0.44 3128  75  18 -17 0.00
Wed 02/05 07Z 0.43 3127  75  19 -18 0.00
Wed 02/05 08Z 0.42 3126  76  20 -18 0.00
Wed 02/05 09Z 0.39 3125  74  21 -18 0.00
Wed 02/05 10Z 0.38 3124  69  22 -17 0.00
Wed 02/05 11Z 0.37 3024  65  20 -17 0.00
Wed 02/05 12Z 0.37 3024  63  18 -17 0.00
Wed 02/05 13Z 0.37 3023  61  17 -17 0.00
Wed 02/05 14Z 0.36 3022  58  16 -17 0.00
Wed 02/05 15Z 0.36 3021  57  14 -17 0.00
Wed 02/05 16Z 0.37 3020  53  13 -16 0.00
Wed 02/05 17Z 0.38 3020  48  11 -16 0.00
Wed 02/05 18Z 0.38 2919  44  10 -16 0.00
Wed 02/05 19Z 0.36 2918  42   8 -15 0.00
Wed 02/05 20Z 0.37 2917  42   6 -14 0.00
Wed 02/05 21Z 0.36 2816  40   6 -14 0.00
Wed 02/05 22Z 0.42 2718  43   8 -14 0.00
Wed 02/05 23Z 0.43 2718  43  10 -13 0.00
Thu 02/06 00Z 0.40 2717  41   9 -12 0.00
Thu 02/06 01Z 0.36 2716  39   9 -12 0.00
Thu 02/06 02Z 0.32 2615  34   8 -11 0.00
Thu 02/06 03Z 0.31 2515  30   7 -11 0.00
Thu 02/06 04Z 0.28 2514  27   6 -10 0.00
Thu 02/06 05Z 0.25 2413  25   4 -10 0.00

Run total areal avg QPF for the greens is: 0.62 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