Subject: GFS EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_GFS_20260111_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)
=== ===== === ==== ==== === === === ====
Sun 01/11 18Z 1.02 2616 83 73 -8 0.00
Sun 01/11 21Z 1.40 2522 88 91 -8 0.01
Mon 01/12 00Z 1.23 2624 87 90 -10 0.01
Mon 01/12 03Z 1.36 2830 95 94 -13 0.03
Mon 01/12 06Z 1.30 2934 94 85 -15 0.02
Mon 01/12 09Z 0.83 3033 91 32 -14 0.01
Mon 01/12 12Z 0.60 2929 65 31 -12 0.00
Mon 01/12 15Z 0.61 2826 86 78 -10 0.00
Mon 01/12 18Z 0.86 2626 95 81 -10 0.00
Mon 01/12 21Z 1.17 2529 97 89 -9 0.02
Tue 01/13 00Z 1.17 2529 94 86 -9 0.02
Tue 01/13 03Z 0.98 2627 95 64 -9 0.01
Tue 01/13 06Z 0.85 2625 94 63 -8 0.01
Tue 01/13 09Z 0.67 2721 95 66 -7 0.01
Tue 01/13 12Z 0.55 2619 94 34 -7 0.01
Tue 01/13 15Z 0.39 2515 91 22 -6 0.00
Tue 01/13 18Z 0.30 2314 87 16 -4 0.00
Tue 01/13 21Z 0.26 2118 78 51 -2 0.00
Wed 01/14 00Z 0.27 2025 61 87 0 0.00
Wed 01/14 03Z 0.44 2134 62 92 -1 0.00
Wed 01/14 06Z 0.68 2132 86 94 -3 0.02
Wed 01/14 09Z 0.61 2227 96 94 -3 0.03
Wed 01/14 12Z 0.53 2322 95 86 -3 0.02
Wed 01/14 15Z 0.48 2319 98 80 -2 0.00
Run total areal avg QPF for the greens is: 0.23 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