Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260124_1800
Using the MB925 to MB850 layer.
Assumes Central_Greens are orthogonal to 260 degree winds.

925-850 mb Layer Average Wind/RH. Avg QPF over the Central_Greens.
Date   Time        Wind RH% 700 850T QPF
Day mm/dd hhZ Frd# DDFF (%) RH% (C) (in)
=== ===== === ==== ==== === === === ====
Sat 01/24 18Z 0.27 3115  71  20 -24 0.00
Sat 01/24 19Z 0.34 3018  66  19 -24 0.00
Sat 01/24 20Z 0.40 3019  67  17 -24 0.00
Sat 01/24 21Z 0.45 2921  67  13 -23 0.00
Sat 01/24 22Z 0.46 2922  66  11 -23 0.00
Sat 01/24 23Z 0.39 3022  62  10 -22 0.00
Sun 01/25 00Z 0.34 3022  60  10 -21 0.00
Sun 01/25 01Z 0.29 3122  57  11 -21 0.00
Sun 01/25 02Z 0.25 3122  53  12 -20 0.00
Sun 01/25 03Z 0.21 3121  49  11 -19 0.00
Sun 01/25 04Z 0.18 3119  44   9 -19 0.00
Sun 01/25 05Z 0.16 3117  40   8 -18 0.00
Sun 01/25 06Z 0.14 3216  36   7 -18 0.00
Sun 01/25 07Z 0.10 3214  35   7 -17 0.00
Sun 01/25 08Z 0.08 3212  33   7 -17 0.00
Sun 01/25 09Z 0.08 3210  32  10 -16 0.00
Sun 01/25 10Z 0.06 3106  33  17 -16 0.00
Sun 01/25 11Z 0.04 3003  31  32 -16 0.00
Sun 01/25 12Z 0.03 2601  29  47 -15 0.00
Sun 01/25 13Z 0.03 2102  27  66 -15 0.00
Sun 01/25 14Z 0.02 1704  27  84 -15 0.00
Sun 01/25 15Z 0.04 1808  34  87 -16 0.00
Sun 01/25 16Z 0.02 1608  53  88 -16 0.00
Sun 01/25 17Z 0.06 1509  74  87 -16 0.00
Sun 01/25 18Z 0.07 1512  85  87 -15 0.02
Sun 01/25 19Z 0.09 1412  85  86 -14 0.03
Sun 01/25 20Z 0.17 1214  86  88 -14 0.03
Sun 01/25 21Z 0.18 1319  85  90 -15 0.06
Sun 01/25 22Z 0.22 1423  85  90 -15 0.08
Sun 01/25 23Z 0.25 1426  85  89 -15 0.09
Mon 01/26 00Z 0.36 1330  86  89 -15 0.08
Mon 01/26 01Z 0.41 1232  85  90 -14 0.07
Mon 01/26 02Z 0.39 1230  86  91 -12 0.05
Mon 01/26 03Z 0.35 1126  86  92 -11 0.05
Mon 01/26 04Z 0.32 1123  86  93 -11 0.04
Mon 01/26 05Z 0.37 1023  86  90 -12 0.05
Mon 01/26 06Z 0.43 1026  86  89 -12 0.04
Mon 01/26 07Z 0.45 1026  85  89 -12 0.03
Mon 01/26 08Z 0.44 1126  86  92 -12 0.01
Mon 01/26 09Z 0.47 1026  88  89 -12 0.01
Mon 01/26 10Z 0.43 1023  87  88 -12 0.02
Mon 01/26 11Z 0.40 1021  87  88 -12 0.02
Mon 01/26 12Z 0.40 1020  88  87 -12 0.02
Mon 01/26 13Z 0.40 1020  87  86 -12 0.02
Mon 01/26 14Z 0.38 1018  88  86 -12 0.02
Mon 01/26 15Z 0.36 1016  89  85 -12 0.01
Mon 01/26 16Z 0.37 1014  90  87 -12 0.00
Mon 01/26 17Z 0.37 1112  90  88 -12 0.00
Mon 01/26 18Z 0.36 1110  90  88 -12 0.01
Mon 01/26 19Z 0.45 1109  91  87 -12 0.01
Mon 01/26 20Z 0.47 1008  91  87 -12 0.01
Mon 01/26 21Z 0.29 1006  92  85 -12 0.02
Mon 01/26 22Z 0.17 0804  92  85 -12 0.02
Mon 01/26 23Z 0.09 0602  91  84 -12 0.03
Tue 01/27 00Z 0.07 0402  91  83 -12 0.02
Tue 01/27 01Z 0.05 3503  91  82 -13 0.02
Tue 01/27 02Z 0.05 3307  89  81 -13 0.03
Tue 01/27 03Z 0.11 3213  88  81 -14 0.02
Tue 01/27 04Z 0.19 3221  87  81 -15 0.01
Tue 01/27 05Z 0.30 3226  87  80 -17 0.01

Run total areal avg QPF for the Central_Greens is: 1.06 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