Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260410_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)
=== ===== === ==== ==== === === === ====
Fri 04/10 00Z 0.31 2028 33 53 7 0.00
Fri 04/10 01Z 0.41 2129 35 70 7 0.00
Fri 04/10 02Z 0.50 2231 39 77 7 0.00
Fri 04/10 03Z 0.60 2333 49 83 6 0.00
Fri 04/10 04Z 0.63 2432 56 79 6 0.00
Fri 04/10 05Z 0.68 2432 64 69 6 0.00
Fri 04/10 06Z 0.70 2432 70 60 6 0.00
Fri 04/10 07Z 0.72 2431 74 44 6 0.00
Fri 04/10 08Z 0.71 2530 75 32 6 0.00
Fri 04/10 09Z 0.71 2529 78 27 5 0.00
Fri 04/10 10Z 0.68 2527 82 19 5 0.00
Fri 04/10 11Z 0.64 2525 82 21 5 0.00
Fri 04/10 12Z 0.58 2522 78 29 5 0.00
Fri 04/10 13Z 0.49 2421 77 18 6 0.00
Fri 04/10 14Z 0.43 2419 78 47 6 0.00
Fri 04/10 15Z 0.39 2320 78 89 6 0.00
Fri 04/10 16Z 0.40 2222 75 87 7 0.00
Fri 04/10 17Z 0.45 2225 71 84 8 0.00
Fri 04/10 18Z 0.56 2224 67 74 8 0.00
Fri 04/10 19Z 0.61 2322 66 52 8 0.00
Fri 04/10 20Z 0.69 2323 65 39 8 0.00
Fri 04/10 21Z 0.73 2224 64 38 8 0.00
Fri 04/10 22Z 0.59 2224 70 52 8 0.00
Fri 04/10 23Z 0.54 2224 80 78 7 0.02
Sat 04/11 00Z 0.66 2423 92 96 6 0.10
Sat 04/11 01Z 0.51 2420 96 96 6 0.05
Sat 04/11 02Z 0.41 2318 97 98 5 0.07
Sat 04/11 03Z 0.39 2418 98 98 5 0.13
Sat 04/11 04Z 0.41 2718 98 96 3 0.11
Sat 04/11 05Z 0.41 3021 98 96 1 0.07
Sat 04/11 06Z 0.44 3126 97 83 0 0.04
Sat 04/11 07Z 0.54 3128 95 54 -2 0.03
Sat 04/11 08Z 0.67 3131 94 29 -3 0.02
Sat 04/11 09Z 0.72 3131 93 12 -4 0.01
Sat 04/11 10Z 0.71 3131 91 6 -5 0.01
Sat 04/11 11Z 0.71 3130 93 4 -5 0.01
Sat 04/11 12Z 0.77 3129 94 5 -6 0.01
Sat 04/11 13Z 0.77 3128 93 4 -6 0.01
Sat 04/11 14Z 0.78 3128 90 6 -7 0.01
Sat 04/11 15Z 0.76 3127 86 8 -7 0.00
Sat 04/11 16Z 0.77 3126 84 9 -7 0.00
Sat 04/11 18Z 1.02 3122 85 11 -7 0.00
Sat 04/11 19Z 1.53 3121 85 11 -7 0.00
Sat 04/11 20Z 2.76 3120 85 11 -6 0.00
Sat 04/11 21Z 3.01 3120 84 11 -6 0.00
Sat 04/11 22Z 2.51 3021 83 12 -6 0.00
Sat 04/11 23Z 1.90 3123 81 11 -6 0.00
Sun 04/12 00Z 1.77 3125 81 13 -6 0.00
Sun 04/12 01Z 1.31 3125 78 13 -6 0.00
Sun 04/12 02Z 0.92 3126 74 12 -6 0.00
Sun 04/12 03Z 0.65 3225 68 12 -6 0.00
Sun 04/12 04Z 0.52 3225 64 11 -5 0.00
Sun 04/12 05Z 0.40 3223 60 9 -5 0.00
Sun 04/12 06Z 0.33 3221 57 12 -5 0.00
Sun 04/12 07Z 0.25 3318 54 21 -4 0.00
Sun 04/12 08Z 0.23 3215 53 28 -4 0.00
Sun 04/12 09Z 0.23 3112 54 24 -4 0.00
Sun 04/12 10Z 0.18 3110 49 23 -3 0.00
Sun 04/12 11Z 0.12 3006 41 33 -2 0.00
Run total areal avg QPF for the greens is: 0.70 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