Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260421_1200
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)
=== ===== === ==== ==== === === === ====
Tue 04/21 12Z 0.28 2506 75 9 -9 0.00
Tue 04/21 13Z 0.62 2506 75 8 -9 0.00
Tue 04/21 14Z 2.09 2406 70 8 -9 0.00
Tue 04/21 15Z 4.68 2408 63 8 -8 0.00
Tue 04/21 16Z 4.91 2407 58 14 -8 0.00
Tue 04/21 17Z 4.65 2307 53 30 -7 0.00
Tue 04/21 18Z 3.99 2307 50 66 -6 0.00
Tue 04/21 19Z 3.66 2208 48 84 -6 0.00
Tue 04/21 20Z 3.60 2208 46 67 -5 0.00
Tue 04/21 21Z 2.35 2207 44 60 -5 0.00
Tue 04/21 22Z 0.90 2108 42 64 -4 0.00
Tue 04/21 23Z 0.34 2109 41 66 -4 0.00
Wed 04/22 00Z 0.33 2111 41 77 -3 0.00
Wed 04/22 01Z 0.35 2112 42 88 -3 0.00
Wed 04/22 02Z 0.33 2113 46 89 -3 0.00
Wed 04/22 03Z 0.44 2116 49 89 -3 0.00
Wed 04/22 04Z 0.55 2120 52 87 -3 0.00
Wed 04/22 05Z 0.57 2222 56 85 -2 0.00
Wed 04/22 06Z 0.51 2222 61 89 -2 0.00
Wed 04/22 07Z 0.44 2220 69 91 -1 0.00
Wed 04/22 08Z 0.34 2218 74 96 -1 0.00
Wed 04/22 09Z 0.30 2217 86 98 -1 0.00
Wed 04/22 10Z 0.28 2117 93 99 -1 0.00
Wed 04/22 11Z 0.21 2017 95 99 -1 0.01
Wed 04/22 12Z 0.12 1917 96 97 0 0.01
Wed 04/22 13Z 0.11 1917 97 91 0 0.01
Wed 04/22 14Z 0.11 1917 97 83 0 0.01
Wed 04/22 15Z 0.15 2017 97 66 -1 0.01
Wed 04/22 16Z 0.17 2016 97 56 -1 0.01
Wed 04/22 17Z 0.20 2014 98 47 -1 0.01
Wed 04/22 18Z 0.19 2013 98 31 -1 0.01
Wed 04/22 19Z 0.21 2112 98 24 -1 0.01
Wed 04/22 20Z 0.22 2210 98 17 0 0.01
Wed 04/22 21Z 0.19 2308 98 15 0 0.01
Wed 04/22 22Z 0.18 2507 98 7 0 0.01
Wed 04/22 23Z 0.20 2707 98 8 0 0.01
Thu 04/23 00Z 0.21 3008 98 10 0 0.01
Thu 04/23 01Z 0.21 3109 98 12 -1 0.00
Thu 04/23 02Z 0.20 3210 97 16 -1 0.00
Thu 04/23 03Z 0.17 3311 96 20 -1 0.00
Thu 04/23 04Z 0.20 3211 96 28 -1 0.00
Thu 04/23 05Z 0.25 3211 95 37 -1 0.00
Thu 04/23 06Z 0.30 3112 94 48 -1 0.01
Thu 04/23 07Z 0.41 3014 95 68 -1 0.01
Thu 04/23 08Z 0.52 3017 95 74 -1 0.01
Thu 04/23 09Z 0.58 3019 94 77 -1 0.02
Thu 04/23 10Z 0.63 3022 94 80 -1 0.03
Thu 04/23 11Z 0.61 3123 95 83 -1 0.03
Thu 04/23 12Z 0.55 3223 97 80 -2 0.02
Thu 04/23 13Z 0.51 3224 96 48 -3 0.02
Thu 04/23 14Z 0.51 3322 93 34 -4 0.00
Thu 04/23 15Z 0.72 3321 92 32 -5 0.00
Thu 04/23 16Z 3.38 3321 88 35 -5 0.00
Thu 04/23 17Z 4.47 3321 85 38 -5 0.00
Thu 04/23 18Z 4.40 3321 84 40 -5 0.00
Thu 04/23 19Z 4.48 3220 85 41 -5 0.00
Thu 04/23 20Z 5.20 3220 83 40 -5 0.00
Thu 04/23 21Z 5.03 3321 78 33 -4 0.00
Thu 04/23 22Z 4.51 3321 75 25 -4 0.00
Thu 04/23 23Z 2.43 3323 74 19 -4 0.00
Run total areal avg QPF for the greens is: 0.29 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