Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260717_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)
=== ===== === ==== ==== === === === ====
Fri 07/17 18Z 3.96 3010 64 15 9 0.00
Fri 07/17 19Z 2.79 2911 62 17 10 0.00
Fri 07/17 20Z 2.27 2911 59 18 10 0.00
Fri 07/17 21Z 1.54 2910 57 19 11 0.00
Fri 07/17 22Z 0.87 2810 54 17 11 0.00
Fri 07/17 23Z 0.53 2809 52 13 11 0.00
Sat 07/18 00Z 0.56 2710 51 11 12 0.00
Sat 07/18 01Z 0.60 2610 53 11 11 0.00
Sat 07/18 02Z 0.55 2610 56 11 11 0.00
Sat 07/18 03Z 0.59 2511 57 9 11 0.00
Sat 07/18 04Z 0.55 2511 57 9 11 0.00
Sat 07/18 05Z 0.55 2511 56 9 11 0.00
Sat 07/18 06Z 0.48 2412 53 19 12 0.00
Sat 07/18 07Z 0.43 2412 50 51 12 0.00
Sat 07/18 08Z 0.44 2413 47 73 12 0.00
Sat 07/18 09Z 0.47 2415 45 87 12 0.00
Sat 07/18 10Z 0.56 2414 49 91 11 0.00
Sat 07/18 11Z 0.38 2215 56 95 12 0.00
Sat 07/18 12Z 0.43 2120 62 88 12 0.00
Sat 07/18 13Z 0.57 2126 70 83 13 0.00
Sat 07/18 14Z 0.67 2128 77 63 14 0.00
Sat 07/18 15Z 0.73 2129 81 63 15 0.00
Sat 07/18 16Z 0.66 2128 82 57 16 0.00
Sat 07/18 17Z 0.73 2129 87 62 15 0.01
Sat 07/18 18Z 0.71 2130 91 81 15 0.02
Sat 07/18 19Z 0.52 2031 95 94 15 0.04
Sat 07/18 20Z 0.57 2131 97 48 16 0.04
Sat 07/18 21Z 0.66 2230 97 52 16 0.02
Sat 07/18 22Z 0.62 2231 94 61 17 0.02
Sat 07/18 23Z 0.68 2232 94 73 17 0.04
Sun 07/19 00Z 0.82 2333 93 64 16 0.06
Sun 07/19 01Z 0.77 2331 95 56 16 0.12
Sun 07/19 02Z 0.77 2427 95 44 15 0.14
Sun 07/19 03Z 0.77 2624 95 60 15 0.06
Sun 07/19 04Z 0.71 2821 98 68 14 0.10
Sun 07/19 05Z 0.58 3023 98 73 12 0.08
Sun 07/19 06Z 0.44 3226 96 60 11 0.03
Sun 07/19 07Z 0.41 3227 96 44 9 0.01
Sun 07/19 08Z 0.39 3328 94 33 9 0.01
Sun 07/19 09Z 0.34 3328 91 29 8 0.01
Sun 07/19 10Z 0.32 3328 90 27 8 0.01
Sun 07/19 11Z 0.34 3326 91 22 8 0.02
Sun 07/19 12Z 0.38 3224 93 21 7 0.02
Sun 07/19 13Z 0.44 3224 95 15 7 0.02
Sun 07/19 14Z 0.48 3223 94 9 7 0.01
Sun 07/19 15Z 0.51 3222 92 8 7 0.01
Sun 07/19 16Z 0.83 3219 89 13 7 0.00
Sun 07/19 17Z 5.02 3118 85 18 7 0.00
Sun 07/19 18Z 8.43 3118 76 22 8 0.00
Sun 07/19 19Z 10.18 3018 72 27 8 0.00
Sun 07/19 20Z 10.52 3019 69 18 8 0.00
Sun 07/19 21Z 10.02 3019 68 8 9 0.00
Sun 07/19 22Z 9.20 3020 67 4 9 0.00
Sun 07/19 23Z 3.51 3020 67 7 9 0.00
Mon 07/20 00Z 2.09 3022 67 10 9 0.00
Mon 07/20 01Z 1.73 3022 66 11 8 0.00
Mon 07/20 02Z 1.45 3122 66 15 8 0.00
Mon 07/20 03Z 1.21 3123 65 14 8 0.00
Mon 07/20 04Z 0.89 3222 64 14 8 0.00
Mon 07/20 05Z 0.75 3221 63 17 8 0.00
Run total areal avg QPF for the greens is: 0.90 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