Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260827_0000
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 00Z 0.15 2204 84 36 12 0.00
Thu 08/27 01Z 0.16 2105 84 29 12 0.00
Thu 08/27 02Z 0.12 2005 85 20 12 0.00
Thu 08/27 03Z 0.12 2007 86 15 11 0.00
Thu 08/27 04Z 0.10 1909 86 15 11 0.00
Thu 08/27 05Z 0.13 1911 86 15 11 0.00
Thu 08/27 06Z 0.20 1913 86 16 11 0.00
Thu 08/27 07Z 0.24 2014 85 17 11 0.00
Thu 08/27 08Z 0.32 2016 83 18 11 0.00
Thu 08/27 09Z 0.41 2117 84 18 11 0.00
Thu 08/27 10Z 0.45 2118 85 17 11 0.00
Thu 08/27 11Z 0.47 2119 85 15 11 0.00
Thu 08/27 12Z 0.48 2120 86 16 11 0.00
Thu 08/27 13Z 0.46 2120 87 14 11 0.00
Thu 08/27 14Z 0.50 2120 90 19 11 0.00
Thu 08/27 15Z 0.54 2019 92 36 11 0.00
Thu 08/27 16Z 0.45 1919 92 69 12 0.00
Thu 08/27 17Z 0.24 1919 94 65 12 0.00
Thu 08/27 18Z 0.24 1921 95 60 13 0.01
Thu 08/27 19Z 0.33 1921 94 73 13 0.01
Thu 08/27 20Z 0.33 1922 93 71 13 0.00
Thu 08/27 21Z 0.37 2022 94 84 13 0.00
Thu 08/27 22Z 0.37 2022 93 81 14 0.00
Thu 08/27 23Z 0.34 2022 93 78 14 0.00
Fri 08/28 00Z 0.39 2025 93 77 14 0.00
Fri 08/28 01Z 0.43 2028 92 71 14 0.01
Fri 08/28 02Z 0.51 2129 93 64 14 0.09
Fri 08/28 03Z 0.63 2230 93 58 14 0.05
Fri 08/28 04Z 0.61 2227 92 62 14 0.01
Fri 08/28 05Z 0.64 2325 93 68 13 0.01
Fri 08/28 06Z 0.65 2422 90 62 13 0.01
Fri 08/28 07Z 0.65 2420 94 68 13 0.01
Fri 08/28 08Z 0.58 2518 94 65 13 0.01
Fri 08/28 09Z 0.52 2516 93 45 13 0.01
Fri 08/28 10Z 0.48 2615 92 50 12 0.01
Fri 08/28 11Z 0.43 2613 92 43 12 0.00
Fri 08/28 12Z 0.39 2612 92 33 12 0.01
Fri 08/28 13Z 0.33 2710 92 39 12 0.00
Fri 08/28 14Z 0.30 2709 92 47 12 0.00
Fri 08/28 15Z 0.27 2707 91 43 12 0.01
Fri 08/28 16Z 0.27 2707 92 45 11 0.01
Fri 08/28 17Z 0.37 2707 92 29 11 0.00
Fri 08/28 18Z 0.42 2707 93 29 11 0.00
Fri 08/28 19Z 0.39 2707 94 31 11 0.00
Fri 08/28 20Z 0.32 2807 93 20 11 0.00
Fri 08/28 21Z 0.31 2908 92 16 11 0.00
Fri 08/28 22Z 0.28 3009 92 17 11 0.00
Fri 08/28 23Z 0.27 3009 90 15 11 0.00
Sat 08/29 00Z 0.25 3009 88 14 11 0.00
Sat 08/29 01Z 0.25 3110 84 16 10 0.00
Sat 08/29 02Z 0.23 3110 80 19 10 0.00
Sat 08/29 03Z 0.21 3210 78 16 10 0.00
Sat 08/29 04Z 0.20 3210 76 17 10 0.00
Sat 08/29 05Z 0.20 3110 72 17 10 0.00
Sat 08/29 06Z 0.20 3110 69 20 10 0.00
Sat 08/29 07Z 0.21 3110 65 18 10 0.00
Sat 08/29 08Z 0.18 3210 61 18 9 0.00
Sat 08/29 09Z 0.18 3210 57 16 9 0.00
Sat 08/29 10Z 0.18 3210 54 16 9 0.00
Sat 08/29 11Z 0.16 3310 50 16 9 0.00
Run total areal avg QPF for the greens is: 0.27 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