Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260815_1200
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 08/15 12Z 0.16 3310 84 13 8 0.00
Sat 08/15 13Z 0.10 3409 85 10 8 0.00
Sat 08/15 14Z 0.18 3309 85 11 7 0.00
Sat 08/15 15Z 0.23 3310 84 13 7 0.00
Sat 08/15 16Z 0.32 3308 86 16 7 0.00
Sat 08/15 17Z 0.57 3208 85 15 7 0.00
Sat 08/15 18Z 0.75 3109 82 15 7 0.00
Sat 08/15 19Z 0.89 3211 79 17 8 0.00
Sat 08/15 20Z 0.76 3211 78 19 8 0.00
Sat 08/15 21Z 0.46 3311 79 16 8 0.00
Sat 08/15 22Z 0.21 3410 76 13 8 0.00
Sat 08/15 23Z 0.13 3410 74 11 8 0.00
Sun 08/16 00Z 0.06 3510 71 10 8 0.00
Sun 08/16 01Z 0.06 3510 68 10 8 0.00
Sun 08/16 02Z 0.02 0010 63 10 9 0.00
Sun 08/16 03Z 0.03 0008 63 10 9 0.00
Sun 08/16 04Z 0.04 0006 61 11 9 0.00
Sun 08/16 05Z 0.04 3505 60 11 9 0.00
Sun 08/16 06Z 0.04 0004 58 10 9 0.00
Sun 08/16 07Z 0.04 0003 56 10 9 0.00
Sun 08/16 08Z 0.05 0102 54 9 10 0.00
Sun 08/16 09Z 0.04 0202 54 8 10 0.00
Sun 08/16 10Z 0.04 0301 52 8 10 0.00
Sun 08/16 11Z 0.03 2000 51 8 10 0.00
Sun 08/16 12Z 0.06 2103 51 7 10 0.00
Sun 08/16 13Z 0.07 2005 52 7 10 0.00
Sun 08/16 14Z 0.08 2005 56 6 10 0.00
Sun 08/16 15Z 0.12 2006 61 6 10 0.00
Sun 08/16 16Z 0.20 2007 68 6 10 0.00
Sun 08/16 17Z 0.28 2007 71 7 10 0.00
Sun 08/16 18Z 0.28 1908 73 7 10 0.00
Sun 08/16 19Z 0.24 1909 74 7 11 0.00
Sun 08/16 20Z 0.14 1810 75 7 11 0.00
Sun 08/16 21Z 0.18 1912 81 10 11 0.00
Sun 08/16 22Z 0.20 1914 86 25 11 0.00
Sun 08/16 23Z 0.21 1916 91 40 11 0.00
Mon 08/17 00Z 0.21 1918 94 65 12 0.00
Mon 08/17 01Z 0.20 1922 96 81 12 0.01
Mon 08/17 02Z 0.23 1924 98 78 12 0.02
Mon 08/17 03Z 0.31 2027 97 86 13 0.02
Mon 08/17 04Z 0.31 2028 97 97 13 0.02
Mon 08/17 05Z 0.33 2029 96 98 13 0.02
Mon 08/17 06Z 0.35 2030 95 97 12 0.02
Mon 08/17 07Z 0.32 2029 98 96 12 0.05
Mon 08/17 08Z 0.31 2027 98 97 12 0.06
Mon 08/17 09Z 0.29 2024 99 96 12 0.09
Mon 08/17 10Z 0.27 2024 98 93 12 0.08
Mon 08/17 11Z 0.25 2023 98 95 12 0.08
Mon 08/17 12Z 0.25 2023 99 94 12 0.06
Mon 08/17 13Z 0.25 2022 99 95 12 0.04
Mon 08/17 14Z 0.24 2019 99 95 12 0.03
Mon 08/17 15Z 0.21 2016 99 94 12 0.02
Mon 08/17 16Z 0.19 2014 99 90 12 0.02
Mon 08/17 17Z 0.16 2110 99 84 13 0.01
Mon 08/17 18Z 0.12 2106 98 72 13 0.01
Mon 08/17 19Z 0.13 2405 97 63 14 0.00
Mon 08/17 20Z 0.13 2805 96 54 14 0.00
Mon 08/17 21Z 0.13 3105 96 50 14 0.00
Mon 08/17 22Z 0.12 3206 96 39 14 0.00
Mon 08/17 23Z 0.11 3206 96 41 14 0.00
Run total areal avg QPF for the greens is: 0.66 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