Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260409_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 04/09 00Z 0.22 2109 46 8 -2 0.00
Thu 04/09 01Z 0.23 2111 40 16 -2 0.00
Thu 04/09 02Z 0.26 2114 35 25 -1 0.00
Thu 04/09 03Z 0.28 2116 30 32 0 0.00
Thu 04/09 04Z 0.31 2118 27 39 0 0.00
Thu 04/09 05Z 0.34 2120 25 46 0 0.00
Thu 04/09 06Z 0.34 2221 23 51 1 0.00
Thu 04/09 07Z 0.32 2221 22 51 2 0.00
Thu 04/09 08Z 0.31 2221 23 50 2 0.00
Thu 04/09 09Z 0.29 2221 20 50 3 0.00
Thu 04/09 10Z 0.27 2122 18 51 3 0.00
Thu 04/09 11Z 0.27 2122 16 52 3 0.00
Thu 04/09 12Z 0.26 2122 14 55 3 0.00
Thu 04/09 13Z 0.26 2123 13 60 3 0.00
Thu 04/09 14Z 0.27 2125 14 62 3 0.00
Thu 04/09 15Z 0.26 2025 16 66 3 0.00
Thu 04/09 16Z 0.27 2026 18 69 3 0.00
Thu 04/09 17Z 0.30 2026 19 67 3 0.00
Thu 04/09 18Z 0.33 2027 20 55 3 0.00
Thu 04/09 19Z 0.40 2028 22 21 3 0.00
Thu 04/09 20Z 0.36 2029 22 48 4 0.00
Thu 04/09 21Z 0.37 2029 24 48 5 0.00
Thu 04/09 22Z 0.35 2030 28 52 6 0.00
Thu 04/09 23Z 0.37 2131 33 53 7 0.00
Fri 04/10 00Z 0.44 2132 37 58 6 0.00
Fri 04/10 01Z 0.50 2232 44 69 6 0.00
Fri 04/10 02Z 0.55 2332 50 78 6 0.00
Fri 04/10 03Z 0.59 2332 55 74 6 0.00
Fri 04/10 04Z 0.65 2433 60 54 5 0.00
Fri 04/10 05Z 0.71 2432 66 52 5 0.00
Fri 04/10 06Z 0.69 2530 70 37 4 0.00
Fri 04/10 07Z 0.66 2528 71 54 4 0.00
Fri 04/10 08Z 0.62 2525 77 54 4 0.00
Fri 04/10 09Z 0.59 2525 85 52 4 0.00
Fri 04/10 10Z 0.56 2524 85 43 5 0.00
Fri 04/10 11Z 0.55 2523 80 24 5 0.00
Fri 04/10 12Z 0.51 2522 81 28 5 0.00
Fri 04/10 13Z 0.50 2422 85 65 5 0.00
Fri 04/10 14Z 0.47 2322 86 90 6 0.00
Fri 04/10 15Z 0.52 2325 91 95 5 0.01
Fri 04/10 16Z 0.55 2326 89 86 6 0.01
Fri 04/10 17Z 0.61 2326 87 40 7 0.00
Fri 04/10 18Z 0.74 2326 82 32 7 0.00
Fri 04/10 19Z 0.96 2328 74 54 7 0.00
Fri 04/10 20Z 1.20 2228 70 51 7 0.00
Fri 04/10 21Z 1.27 2228 73 60 7 0.01
Fri 04/10 22Z 0.91 2229 76 75 7 0.02
Fri 04/10 23Z 0.78 2228 85 96 7 0.05
Sat 04/11 00Z 0.81 2427 92 97 6 0.13
Sat 04/11 01Z 0.56 2622 98 96 5 0.08
Sat 04/11 02Z 0.47 2720 98 90 4 0.08
Sat 04/11 03Z 0.39 2818 98 89 2 0.04
Sat 04/11 04Z 0.34 3017 98 81 1 0.03
Sat 04/11 05Z 0.35 3019 97 70 0 0.02
Sat 04/11 06Z 0.40 3121 96 49 -2 0.01
Sat 04/11 07Z 0.49 3123 95 35 -3 0.01
Sat 04/11 08Z 0.58 3125 95 12 -4 0.02
Sat 04/11 09Z 0.60 3125 95 13 -5 0.02
Sat 04/11 10Z 0.59 3125 92 14 -6 0.01
Sat 04/11 11Z 0.55 3126 88 12 -6 0.01
Run total areal avg QPF for the greens is: 0.56 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