Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260830_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)
=== ===== === ==== ==== === === === ====
Sun 08/30 00Z 0.59 2512 78 17 10 0.00
Sun 08/30 01Z 0.72 2514 79 16 10 0.00
Sun 08/30 02Z 0.83 2517 79 19 10 0.00
Sun 08/30 03Z 0.93 2619 73 30 10 0.00
Sun 08/30 04Z 1.02 2621 70 39 10 0.00
Sun 08/30 05Z 0.96 2622 69 51 11 0.00
Sun 08/30 06Z 1.04 2727 75 61 11 0.00
Sun 08/30 07Z 1.11 2730 82 70 11 0.00
Sun 08/30 08Z 1.12 2730 84 66 11 0.00
Sun 08/30 09Z 1.05 2829 84 52 11 0.00
Sun 08/30 10Z 0.88 2826 84 48 11 0.02
Sun 08/30 11Z 0.83 2824 85 54 11 0.01
Sun 08/30 12Z 0.76 2823 86 62 11 0.02
Sun 08/30 13Z 0.71 2821 86 66 11 0.01
Sun 08/30 14Z 0.67 2719 87 68 11 0.01
Sun 08/30 15Z 0.68 2618 86 60 11 0.00
Sun 08/30 16Z 0.78 2618 86 65 11 0.01
Sun 08/30 17Z 0.94 2618 86 66 11 0.02
Sun 08/30 18Z 1.02 2620 85 70 11 0.02
Sun 08/30 19Z 1.25 2620 83 85 11 0.02
Sun 08/30 20Z 1.03 2520 84 90 11 0.02
Sun 08/30 21Z 1.31 2623 90 92 11 0.10
Sun 08/30 22Z 1.09 2724 92 91 10 0.08
Sun 08/30 23Z 0.94 2824 89 88 10 0.02
Mon 08/31 00Z 0.81 2923 89 84 10 0.01
Mon 08/31 01Z 0.65 3021 94 74 10 0.01
Mon 08/31 02Z 0.50 3120 93 70 10 0.01
Mon 08/31 03Z 0.40 3218 92 61 10 0.01
Mon 08/31 04Z 0.33 3216 91 50 10 0.01
Mon 08/31 05Z 0.29 3215 89 39 10 0.02
Mon 08/31 06Z 0.21 3315 85 32 10 0.02
Mon 08/31 07Z 0.20 3314 84 20 10 0.01
Mon 08/31 08Z 0.16 3312 79 8 10 0.01
Mon 08/31 09Z 0.16 3312 80 8 10 0.00
Mon 08/31 10Z 0.13 3310 76 10 10 0.00
Mon 08/31 11Z 0.13 3309 75 10 10 0.00
Mon 08/31 12Z 0.11 3308 72 6 10 0.00
Mon 08/31 13Z 0.12 3207 70 7 10 0.00
Mon 08/31 14Z 0.12 3206 71 9 10 0.00
Mon 08/31 15Z 0.11 3104 71 9 10 0.00
Mon 08/31 16Z 0.11 3003 72 8 10 0.00
Mon 08/31 17Z 0.18 2703 75 7 10 0.00
Mon 08/31 18Z 0.29 2705 74 7 10 0.00
Mon 08/31 19Z 0.31 2605 71 8 11 0.00
Mon 08/31 20Z 0.26 2604 69 9 11 0.00
Mon 08/31 21Z 0.24 2603 69 8 11 0.00
Mon 08/31 22Z 0.22 2504 70 8 11 0.00
Mon 08/31 23Z 0.20 2404 70 9 11 0.00
Tue 09/01 00Z 0.17 2504 71 12 11 0.00
Tue 09/01 01Z 0.11 2503 72 16 11 0.00
Tue 09/01 02Z 0.12 2403 73 20 11 0.00
Tue 09/01 03Z 0.10 2303 74 22 11 0.00
Tue 09/01 04Z 0.09 2303 76 20 11 0.00
Tue 09/01 05Z 0.07 2103 78 26 11 0.00
Tue 09/01 06Z 0.06 2103 80 23 11 0.00
Tue 09/01 07Z 0.04 2002 81 23 11 0.00
Tue 09/01 08Z 0.03 1903 82 24 12 0.00
Tue 09/01 09Z 0.02 1805 84 24 12 0.00
Tue 09/01 10Z 0.04 1905 86 26 12 0.00
Tue 09/01 11Z 0.03 1707 87 33 12 0.00
Run total areal avg QPF for the greens is: 0.47 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