Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260409_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)
=== ===== === ==== ==== === === === ====
Thu 04/09 18Z 0.28 1920 32 36 3 0.00
Thu 04/09 19Z 0.34 1924 34 30 3 0.00
Thu 04/09 20Z 0.34 1926 33 33 4 0.00
Thu 04/09 21Z 0.33 1928 31 43 4 0.00
Thu 04/09 22Z 0.32 2029 31 50 5 0.00
Fri 04/10 00Z 0.34 2131 34 54 7 0.00
Fri 04/10 01Z 0.44 2233 39 59 7 0.00
Fri 04/10 02Z 0.52 2334 45 76 7 0.00
Fri 04/10 03Z 0.58 2334 54 78 7 0.00
Fri 04/10 04Z 0.65 2435 64 64 6 0.00
Fri 04/10 05Z 0.69 2433 71 61 6 0.00
Fri 04/10 06Z 0.69 2532 73 43 6 0.00
Fri 04/10 07Z 0.67 2629 77 27 5 0.00
Fri 04/10 08Z 0.63 2626 79 9 5 0.00
Fri 04/10 09Z 0.60 2624 82 5 4 0.00
Fri 04/10 10Z 0.57 2523 87 4 4 0.00
Fri 04/10 11Z 0.56 2522 88 8 4 0.00
Fri 04/10 13Z 0.48 2420 88 15 5 0.00
Fri 04/10 14Z 0.45 2420 89 45 5 0.00
Fri 04/10 15Z 0.43 2321 87 68 6 0.00
Fri 04/10 16Z 0.42 2321 89 95 6 0.00
Fri 04/10 17Z 0.43 2222 88 85 6 0.00
Fri 04/10 18Z 0.46 2223 84 55 7 0.00
Fri 04/10 19Z 0.55 2226 79 41 8 0.01
Fri 04/10 20Z 0.64 2229 75 39 8 0.01
Fri 04/10 22Z 0.77 2231 76 73 8 0.00
Fri 04/10 23Z 0.84 2232 85 96 7 0.00
Sat 04/11 00Z 0.79 2430 96 94 6 0.01
Sat 04/11 01Z 0.71 2527 98 91 5 0.00
Sat 04/11 02Z 0.61 2623 98 90 4 0.03
Sat 04/11 03Z 0.51 2821 98 80 3 0.10
Sat 04/11 04Z 0.43 2920 98 83 1 0.10
Sat 04/11 05Z 0.48 3023 97 80 0 0.05
Sat 04/11 06Z 0.53 3025 96 46 -1 0.04
Sat 04/11 07Z 0.60 3027 92 23 -3 0.03
Sat 04/11 08Z 0.60 3028 86 8 -3 0.03
Sat 04/11 09Z 0.63 3029 86 4 -4 0.02
Sat 04/11 10Z 0.66 3029 86 5 -5 0.01
Sat 04/11 11Z 0.62 3029 81 5 -5 0.01
Sat 04/11 12Z 0.58 3128 81 7 -6 0.01
Sat 04/11 13Z 0.60 3127 81 9 -6 0.01
Sat 04/11 14Z 0.59 3126 80 10 -6 0.01
Sat 04/11 15Z 0.63 3125 82 10 -7 0.01
Sat 04/11 16Z 0.73 3124 84 12 -7 0.01
Sat 04/11 17Z 0.99 3123 84 13 -7 0.01
Sat 04/11 18Z 1.63 3122 84 13 -7 0.00
Sat 04/11 19Z 1.42 3121 83 12 -7 0.00
Sat 04/11 21Z 2.25 3120 80 10 -6 0.00
Sat 04/11 22Z 1.75 3120 78 9 -6 0.00
Sat 04/11 23Z 1.31 3121 76 9 -6 0.00
Sun 04/12 00Z 1.09 3122 75 9 -6 0.00
Sun 04/12 01Z 0.82 3122 74 7 -6 0.00
Sun 04/12 02Z 0.57 3222 69 8 -5 0.00
Sun 04/12 03Z 0.42 3221 63 8 -5 0.00
Sun 04/12 04Z 0.35 3220 59 8 -4 0.00
Sun 04/12 05Z 0.29 3218 55 8 -4 0.00
Run total areal avg QPF for the greens is: 0.51 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