Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260926_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 09/26 18Z 4.79 0415 78 28 8 0.00
Sat 09/26 19Z 4.00 0416 77 30 8 0.00
Sat 09/26 20Z 2.01 0418 76 31 8 0.01
Sat 09/26 21Z 1.16 0421 74 34 8 0.00
Sat 09/26 22Z 0.86 0424 70 43 9 0.00
Sat 09/26 23Z 0.90 0428 69 49 9 0.00
Sun 09/27 00Z 1.25 0531 67 26 9 0.00
Sun 09/27 01Z 1.23 0532 57 22 9 0.00
Sun 09/27 02Z 1.10 0632 50 18 9 0.00
Sun 09/27 03Z 1.07 0632 49 25 9 0.00
Sun 09/27 04Z 1.06 0633 57 50 9 0.00
Sun 09/27 05Z 0.95 0633 56 68 9 0.00
Sun 09/27 06Z 0.89 0733 58 74 9 0.00
Sun 09/27 07Z 0.87 0734 64 68 9 0.01
Sun 09/27 08Z 0.82 0734 73 69 9 0.02
Sun 09/27 09Z 0.84 0736 80 81 8 0.02
Sun 09/27 10Z 0.81 0736 82 80 9 0.01
Sun 09/27 11Z 0.73 0734 90 84 9 0.01
Sun 09/27 12Z 0.67 0735 90 89 10 0.02
Sun 09/27 13Z 0.67 0736 90 88 11 0.02
Sun 09/27 14Z 0.67 0836 91 91 11 0.04
Sun 09/27 15Z 0.66 0736 92 92 11 0.03
Sun 09/27 16Z 0.64 0734 93 92 11 0.04
Sun 09/27 17Z 0.64 0833 94 84 11 0.06
Sun 09/27 18Z 0.62 0833 95 89 11 0.08
Sun 09/27 19Z 0.61 0832 95 82 12 0.07
Sun 09/27 20Z 0.61 0832 95 81 12 0.06
Sun 09/27 21Z 0.59 0931 95 86 12 0.04
Sun 09/27 22Z 0.56 0930 94 77 12 0.04
Sun 09/27 23Z 0.54 0930 94 68 12 0.03
Mon 09/28 00Z 0.54 0930 94 42 12 0.02
Mon 09/28 01Z 0.54 1030 93 59 12 0.01
Mon 09/28 02Z 0.55 0931 93 65 12 0.01
Mon 09/28 03Z 0.52 1029 91 64 13 0.01
Mon 09/28 04Z 0.49 0928 91 70 13 0.01
Mon 09/28 05Z 0.50 0928 91 67 12 0.01
Mon 09/28 06Z 0.49 0928 90 75 13 0.01
Mon 09/28 07Z 0.48 1027 91 73 12 0.02
Mon 09/28 08Z 0.47 1026 94 78 12 0.02
Mon 09/28 09Z 0.45 0925 93 79 12 0.01
Mon 09/28 10Z 0.42 0924 90 81 13 0.01
Mon 09/28 11Z 0.39 0922 89 78 13 0.01
Mon 09/28 12Z 0.35 1020 90 76 12 0.01
Mon 09/28 13Z 0.31 1018 90 79 12 0.01
Mon 09/28 14Z 0.29 1016 91 86 12 0.01
Mon 09/28 15Z 0.28 0915 91 87 11 0.01
Mon 09/28 16Z 0.28 0915 91 85 11 0.01
Mon 09/28 17Z 0.28 0914 92 85 11 0.01
Mon 09/28 18Z 0.28 0813 93 84 11 0.02
Mon 09/28 19Z 0.27 0812 94 85 11 0.02
Mon 09/28 20Z 0.29 0713 94 90 10 0.03
Mon 09/28 21Z 0.31 0714 95 92 10 0.05
Mon 09/28 22Z 0.30 0813 95 96 10 0.04
Mon 09/28 23Z 0.28 0813 96 96 10 0.05
Tue 09/29 00Z 0.27 0912 96 93 10 0.04
Tue 09/29 01Z 0.26 0912 97 93 10 0.03
Tue 09/29 02Z 0.26 0912 97 84 10 0.03
Tue 09/29 03Z 0.24 0911 97 78 10 0.02
Tue 09/29 04Z 0.22 0910 97 75 10 0.01
Tue 09/29 05Z 0.19 0908 97 70 9 0.01
Run total areal avg QPF for the greens is: 1.17 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