Subject: GFS EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_GFS_20260720_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)
=== ===== === ==== ==== === === === ====
Mon 07/20 06Z 0.71 3020 67 33 8 0.00
Mon 07/20 09Z 0.61 3019 68 53 8 0.00
Mon 07/20 12Z 0.43 3015 66 59 9 0.00
Mon 07/20 15Z 0.82 2810 63 62 9 0.00
Mon 07/20 18Z 4.17 2510 54 58 11 0.00
Mon 07/20 21Z 1.41 2210 53 49 13 0.00
Tue 07/21 00Z 0.61 2216 49 31 13 0.00
Tue 07/21 03Z 0.53 2115 50 19 13 0.00
Tue 07/21 06Z 0.38 2021 48 19 13 0.00
Tue 07/21 09Z 0.41 2125 57 26 14 0.00
Tue 07/21 12Z 0.25 2022 63 27 14 0.00
Tue 07/21 15Z 0.04 1720 73 85 13 0.00
Tue 07/21 18Z 0.19 1727 79 73 13 0.01
Tue 07/21 21Z 0.31 1630 90 89 13 0.09
Wed 07/22 00Z 0.21 1626 98 96 14 0.49
Wed 07/22 03Z 0.13 1914 94 89 14 0.88
Wed 07/22 06Z 0.46 2219 94 73 13 0.06
Wed 07/22 09Z 0.60 2323 95 68 13 0.00
Wed 07/22 12Z 0.54 2318 96 79 12 0.01
Wed 07/22 15Z 0.65 2320 93 83 11 0.03
Wed 07/22 18Z 1.50 2424 84 44 11 0.05
Wed 07/22 21Z 2.03 2521 72 29 11 0.01
Thu 07/23 00Z 1.03 2719 84 32 8 0.00
Thu 07/23 03Z 0.60 2816 89 37 6 0.01
Run total areal avg QPF for the greens is: 1.64 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