Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260812_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)
=== ===== === ==== ==== === === === ====
Wed 08/12 00Z 0.64 3011 75 67 12 0.00
Wed 08/12 01Z 0.73 3013 75 71 12 0.00
Wed 08/12 02Z 0.74 3115 77 70 12 0.00
Wed 08/12 03Z 0.60 3114 75 68 12 0.00
Wed 08/12 04Z 0.55 3113 74 68 12 0.00
Wed 08/12 05Z 0.53 3113 75 65 12 0.00
Wed 08/12 06Z 0.50 3113 75 63 12 0.00
Wed 08/12 07Z 0.41 3112 75 58 12 0.00
Wed 08/12 08Z 0.35 3111 74 53 11 0.00
Wed 08/12 09Z 0.35 3110 73 55 11 0.00
Wed 08/12 10Z 0.34 3110 71 55 11 0.00
Wed 08/12 11Z 0.34 3009 69 50 11 0.00
Wed 08/12 12Z 0.37 3009 68 51 11 0.00
Wed 08/12 13Z 0.39 3008 68 54 11 0.00
Wed 08/12 14Z 0.58 2808 70 61 11 0.00
Wed 08/12 15Z 1.54 2809 71 61 11 0.00
Wed 08/12 16Z 3.08 2808 70 65 12 0.00
Wed 08/12 17Z 3.81 2807 68 73 12 0.00
Wed 08/12 18Z 3.74 2707 69 84 12 0.00
Wed 08/12 19Z 2.12 2608 72 85 12 0.01
Wed 08/12 20Z 0.80 2610 76 87 12 0.02
Wed 08/12 21Z 0.60 2510 78 83 12 0.01
Wed 08/12 22Z 0.47 2610 77 68 12 0.00
Wed 08/12 23Z 0.41 2608 75 72 12 0.00
Thu 08/13 00Z 0.39 2707 74 79 12 0.00
Thu 08/13 01Z 0.48 2608 78 85 12 0.00
Thu 08/13 02Z 0.53 2510 79 91 12 0.06
Thu 08/13 03Z 0.50 2510 78 92 12 0.01
Thu 08/13 04Z 0.43 2410 78 84 12 0.00
Thu 08/13 05Z 0.41 2410 79 85 12 0.01
Thu 08/13 06Z 0.34 2310 80 89 13 0.01
Thu 08/13 07Z 0.38 2212 83 92 12 0.01
Thu 08/13 08Z 0.30 2212 86 89 12 0.01
Thu 08/13 09Z 0.26 2111 88 90 12 0.01
Thu 08/13 10Z 0.21 2110 87 90 12 0.01
Thu 08/13 11Z 0.19 2108 90 83 12 0.01
Thu 08/13 12Z 0.22 2407 94 74 12 0.01
Thu 08/13 13Z 0.27 2707 96 71 12 0.00
Thu 08/13 14Z 0.34 2810 96 73 12 0.01
Thu 08/13 15Z 0.34 2910 97 72 11 0.01
Thu 08/13 16Z 0.35 3112 98 66 11 0.02
Thu 08/13 17Z 0.32 3213 96 53 11 0.01
Thu 08/13 18Z 0.37 3214 94 45 11 0.00
Thu 08/13 19Z 0.44 3214 92 42 11 0.01
Thu 08/13 20Z 0.47 3217 89 38 12 0.02
Thu 08/13 21Z 0.40 3217 84 39 12 0.03
Thu 08/13 22Z 0.49 3218 81 34 12 0.00
Thu 08/13 23Z 0.90 3121 85 30 11 0.00
Fri 08/14 00Z 0.97 3123 83 34 11 0.00
Fri 08/14 01Z 0.92 3125 82 34 10 0.00
Fri 08/14 02Z 0.99 3225 80 34 10 0.00
Fri 08/14 03Z 0.81 3223 79 34 9 0.00
Fri 08/14 04Z 0.58 3223 78 31 9 0.00
Fri 08/14 05Z 0.49 3322 75 32 9 0.00
Fri 08/14 06Z 0.47 3221 74 34 8 0.00
Fri 08/14 07Z 0.46 3220 74 40 8 0.00
Fri 08/14 08Z 0.47 3220 76 41 8 0.00
Fri 08/14 09Z 0.46 3120 72 43 8 0.00
Fri 08/14 10Z 0.45 3121 70 54 9 0.00
Fri 08/14 11Z 0.43 3121 73 56 9 0.00
Run total areal avg QPF for the greens is: 0.30 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