Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260827_0600
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)
=== ===== === ==== ==== === === === ====
Thu 08/27 06Z 0.15 1913 80 20 11 0.00
Thu 08/27 07Z 0.23 1914 78 20 11 0.00
Thu 08/27 08Z 0.32 2017 76 22 11 0.00
Thu 08/27 09Z 0.39 2019 74 21 12 0.00
Thu 08/27 10Z 0.37 2021 72 23 12 0.00
Thu 08/27 11Z 0.37 2022 65 30 12 0.00
Thu 08/27 12Z 0.44 2023 61 31 12 0.00
Thu 08/27 13Z 0.46 2024 61 36 13 0.00
Thu 08/27 14Z 0.48 2124 67 41 13 0.00
Thu 08/27 15Z 0.50 2023 71 43 13 0.00
Thu 08/27 16Z 1.22 2022 78 38 13 0.00
Thu 08/27 17Z 1.32 2021 83 58 13 0.00
Thu 08/27 18Z 0.68 1921 90 84 13 0.01
Thu 08/27 19Z 0.43 2022 91 90 13 0.01
Thu 08/27 20Z 0.34 1921 92 89 14 0.00
Thu 08/27 21Z 0.33 1922 94 85 14 0.03
Thu 08/27 22Z 0.23 1924 95 79 14 0.04
Thu 08/27 23Z 0.39 2025 91 81 14 0.28
Fri 08/28 00Z 0.46 2127 90 79 13 0.19
Fri 08/28 01Z 0.51 2127 92 74 14 0.02
Fri 08/28 02Z 0.55 2226 83 69 14 0.03
Fri 08/28 03Z 0.61 2325 84 71 14 0.02
Fri 08/28 04Z 0.64 2325 88 63 13 0.01
Fri 08/28 05Z 0.64 2424 88 61 14 0.01
Fri 08/28 06Z 0.64 2522 91 56 13 0.01
Fri 08/28 07Z 0.57 2519 92 51 13 0.01
Fri 08/28 08Z 0.48 2616 91 54 13 0.01
Fri 08/28 09Z 0.40 2614 90 46 12 0.01
Fri 08/28 10Z 0.36 2612 90 52 12 0.01
Fri 08/28 11Z 0.32 2611 90 46 12 0.00
Fri 08/28 12Z 0.32 2611 90 41 12 0.00
Fri 08/28 13Z 0.33 2611 91 50 11 0.00
Fri 08/28 14Z 0.31 2610 91 45 11 0.00
Fri 08/28 15Z 0.30 2609 93 40 11 0.00
Fri 08/28 16Z 0.32 2609 93 27 11 0.00
Fri 08/28 17Z 0.35 2609 93 17 11 0.00
Fri 08/28 18Z 0.38 2709 92 8 11 0.00
Fri 08/28 19Z 0.40 2709 94 9 11 0.00
Fri 08/28 20Z 0.37 2810 94 9 11 0.01
Fri 08/28 21Z 0.33 2810 92 10 11 0.00
Fri 08/28 22Z 0.33 2810 93 11 11 0.01
Fri 08/28 23Z 0.31 2910 91 13 10 0.01
Sat 08/29 00Z 0.32 2911 89 15 10 0.00
Sat 08/29 01Z 0.30 3012 86 18 10 0.00
Sat 08/29 02Z 0.27 3012 83 19 10 0.00
Sat 08/29 03Z 0.25 3012 80 19 9 0.00
Sat 08/29 04Z 0.24 3012 74 19 9 0.00
Sat 08/29 05Z 0.23 3012 68 19 9 0.00
Sat 08/29 06Z 0.24 3012 63 15 9 0.00
Sat 08/29 07Z 0.22 3112 56 14 9 0.00
Sat 08/29 08Z 0.22 3112 51 11 9 0.00
Sat 08/29 09Z 0.19 3212 52 12 9 0.00
Sat 08/29 10Z 0.18 3212 52 10 8 0.00
Sat 08/29 11Z 0.16 3211 49 9 8 0.00
Sat 08/29 12Z 0.15 3210 50 7 8 0.00
Sat 08/29 13Z 0.15 3210 52 6 8 0.00
Sat 08/29 14Z 0.17 3109 55 6 8 0.00
Sat 08/29 15Z 0.19 3108 58 6 8 0.00
Sat 08/29 16Z 0.20 3006 60 6 8 0.00
Sat 08/29 17Z 0.30 2906 61 5 8 0.00
Run total areal avg QPF for the greens is: 0.73 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