Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260524_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)
=== ===== === ==== ==== === === === ====
Sun 05/24 18Z 0.30 1532 97 97 4 0.00
Sun 05/24 19Z 0.30 1535 95 97 5 0.04
Sun 05/24 20Z 0.27 1637 94 96 5 0.05
Sun 05/24 21Z 0.24 1638 93 92 6 0.02
Sun 05/24 22Z 0.22 1638 93 92 6 0.01
Sun 05/24 23Z 0.25 1639 93 86 7 0.02
Mon 05/25 00Z 0.28 1640 94 75 7 0.02
Mon 05/25 01Z 0.26 1641 95 70 7 0.02
Mon 05/25 02Z 0.17 1640 96 66 8 0.02
Mon 05/25 03Z 0.11 1736 96 60 8 0.02
Mon 05/25 04Z 0.04 1730 95 63 9 0.01
Mon 05/25 05Z 0.04 1826 95 71 10 0.01
Mon 05/25 06Z 0.06 1823 96 79 10 0.01
Mon 05/25 07Z 0.10 1921 96 84 11 0.01
Mon 05/25 08Z 0.12 1919 97 89 11 0.04
Mon 05/25 09Z 0.12 1918 99 94 10 0.09
Mon 05/25 10Z 0.10 1917 99 95 10 0.16
Mon 05/25 11Z 0.21 2118 99 96 10 0.22
Mon 05/25 12Z 0.30 2218 99 95 9 0.12
Mon 05/25 13Z 0.31 2415 98 94 9 0.07
Mon 05/25 14Z 0.30 2613 97 82 10 0.01
Mon 05/25 15Z 0.28 2812 96 77 10 0.01
Mon 05/25 16Z 0.28 2913 96 72 10 0.00
Mon 05/25 17Z 0.30 2914 96 72 10 0.00
Mon 05/25 18Z 0.35 2915 95 68 10 0.00
Mon 05/25 19Z 0.41 2916 95 27 10 0.00
Mon 05/25 20Z 0.53 2918 93 5 9 0.00
Mon 05/25 21Z 0.62 2918 94 6 9 0.00
Mon 05/25 22Z 0.62 2918 94 8 9 0.01
Mon 05/25 23Z 0.55 2917 91 15 9 0.01
Tue 05/26 00Z 0.53 2917 89 14 9 0.01
Tue 05/26 01Z 0.56 2918 87 19 9 0.01
Tue 05/26 02Z 0.42 3017 76 12 9 0.01
Tue 05/26 03Z 0.37 3017 68 6 10 0.01
Tue 05/26 04Z 0.38 3017 68 4 10 0.01
Tue 05/26 05Z 0.37 3117 64 5 10 0.01
Tue 05/26 06Z 0.36 3116 62 7 9 0.00
Tue 05/26 07Z 0.45 3116 64 12 9 0.00
Tue 05/26 08Z 0.81 3017 72 16 9 0.00
Tue 05/26 09Z 0.62 3017 73 19 9 0.00
Tue 05/26 10Z 0.58 2917 71 15 9 0.00
Tue 05/26 11Z 0.53 2817 66 11 10 0.00
Tue 05/26 12Z 0.51 2818 64 12 10 0.00
Tue 05/26 13Z 0.53 2818 63 20 10 0.00
Tue 05/26 14Z 0.56 2719 62 56 11 0.00
Tue 05/26 15Z 0.57 2718 57 75 12 0.00
Tue 05/26 16Z 0.61 2718 57 64 12 0.00
Tue 05/26 17Z 0.95 2618 70 66 12 0.00
Tue 05/26 18Z 1.36 2620 81 73 12 0.00
Tue 05/26 19Z 1.28 2522 82 70 12 0.00
Tue 05/26 20Z 1.55 2522 80 65 12 0.00
Tue 05/26 21Z 4.12 2524 76 70 13 0.00
Tue 05/26 22Z 5.60 2528 71 73 13 0.00
Tue 05/26 23Z 2.70 2530 71 74 13 0.00
Wed 05/27 00Z 2.37 2632 76 76 13 0.01
Wed 05/27 01Z 1.98 2635 77 73 13 0.04
Wed 05/27 02Z 1.75 2736 79 67 12 0.02
Wed 05/27 03Z 1.52 2835 83 58 12 0.05
Wed 05/27 04Z 1.33 2831 86 65 12 0.02
Wed 05/27 05Z 1.14 2828 88 55 11 0.01
Run total areal avg QPF for the greens is: 1.21 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