Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260124_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)
=== ===== === ==== ==== === === === ====
Sat 01/24 18Z 0.36 3017  73  23 -25 0.00
Sat 01/24 19Z 0.43 3019  69  23 -24 0.00
Sat 01/24 20Z 0.48 2921  68  20 -24 0.00
Sat 01/24 21Z 0.53 2923  68  16 -24 0.00
Sat 01/24 22Z 0.53 2924  66  13 -23 0.00
Sat 01/24 23Z 0.47 3024  62  12 -22 0.00
Sun 01/25 00Z 0.42 3023  60  13 -22 0.00
Sun 01/25 01Z 0.37 3023  58  14 -21 0.00
Sun 01/25 02Z 0.32 3122  55  15 -20 0.00
Sun 01/25 03Z 0.29 3122  50  13 -20 0.00
Sun 01/25 04Z 0.26 3120  45  11 -19 0.00
Sun 01/25 05Z 0.23 3118  42   9 -19 0.00
Sun 01/25 06Z 0.21 3117  39   7 -19 0.00
Sun 01/25 07Z 0.17 3215  37   6 -18 0.00
Sun 01/25 08Z 0.14 3213  36   7 -18 0.00
Sun 01/25 09Z 0.13 3111  36   9 -17 0.00
Sun 01/25 10Z 0.10 3107  35  13 -17 0.00
Sun 01/25 11Z 0.07 3004  33  23 -16 0.00
Sun 01/25 12Z 0.05 2802  30  32 -16 0.00
Sun 01/25 13Z 0.05 2502  27  46 -15 0.00
Sun 01/25 14Z 0.02 1903  26  61 -15 0.00
Sun 01/25 15Z 0.02 1906  31  69 -16 0.00
Sun 01/25 16Z 0.04 1607  44  80 -16 0.00
Sun 01/25 17Z 0.07 1508  59  85 -16 0.00
Sun 01/25 18Z 0.09 1511  71  87 -16 0.01
Sun 01/25 19Z 0.10 1512  79  86 -15 0.02
Sun 01/25 20Z 0.17 1313  85  87 -14 0.02
Sun 01/25 21Z 0.20 1316  85  89 -15 0.04
Sun 01/25 22Z 0.24 1420  85  89 -15 0.06
Sun 01/25 23Z 0.28 1424  85  89 -15 0.07
Mon 01/26 00Z 0.34 1427  85  89 -15 0.07
Mon 01/26 01Z 0.39 1328  85  89 -15 0.06
Mon 01/26 02Z 0.39 1327  85  90 -13 0.05
Mon 01/26 03Z 0.34 1223  86  91 -12 0.04
Mon 01/26 04Z 0.30 1119  86  91 -12 0.04
Mon 01/26 05Z 0.33 1119  86  89 -12 0.05
Mon 01/26 06Z 0.39 1122  86  88 -12 0.04
Mon 01/26 07Z 0.40 1122  86  90 -12 0.03
Mon 01/26 08Z 0.39 1121  87  91 -12 0.01
Mon 01/26 09Z 0.42 1122  88  88 -13 0.01
Mon 01/26 10Z 0.38 1120  87  88 -13 0.02
Mon 01/26 11Z 0.36 1118  87  87 -13 0.02
Mon 01/26 12Z 0.35 1118  87  86 -12 0.02
Mon 01/26 13Z 0.34 1117  88  86 -13 0.02
Mon 01/26 14Z 0.32 1115  88  85 -13 0.01
Mon 01/26 15Z 0.28 1112  89  85 -12 0.01
Mon 01/26 16Z 0.28 1110  90  86 -12 0.01
Mon 01/26 17Z 0.28 1108  90  87 -12 0.01
Mon 01/26 18Z 0.28 1106  90  87 -12 0.01
Mon 01/26 19Z 0.34 1106  91  87 -13 0.01
Mon 01/26 20Z 0.35 1005  91  87 -13 0.01
Mon 01/26 21Z 0.22 0904  91  85 -13 0.01
Mon 01/26 22Z 0.13 0603  91  85 -13 0.02
Mon 01/26 23Z 0.07 0302  90  84 -13 0.02
Tue 01/27 00Z 0.05 0103  91  83 -13 0.02
Tue 01/27 01Z 0.05 3404  90  82 -13 0.02
Tue 01/27 02Z 0.10 3308  89  82 -14 0.02
Tue 01/27 03Z 0.19 3214  87  81 -14 0.02
Tue 01/27 04Z 0.30 3221  87  81 -16 0.01
Tue 01/27 05Z 0.45 3226  88  79 -17 0.01

Run total areal avg QPF for the greens is: 0.92 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