Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260430_0600
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 04/30 06Z 0.04 1726 82 95 6 0.00
Thu 04/30 07Z 0.05 1725 87 95 6 0.02
Thu 04/30 08Z 0.07 1824 95 98 5 0.07
Thu 04/30 09Z 0.14 1922 96 97 4 0.06
Thu 04/30 10Z 0.11 1918 97 99 4 0.03
Thu 04/30 11Z 0.09 1919 98 99 4 0.09
Thu 04/30 12Z 0.11 1919 98 96 3 0.07
Thu 04/30 13Z 0.14 2016 98 92 3 0.05
Thu 04/30 14Z 0.17 2013 97 87 3 0.02
Thu 04/30 15Z 0.23 2212 98 90 3 0.01
Thu 04/30 16Z 0.30 2310 98 91 2 0.01
Thu 04/30 17Z 0.36 2609 99 92 2 0.02
Thu 04/30 18Z 0.35 2909 99 91 1 0.02
Thu 04/30 19Z 0.35 2910 99 91 1 0.02
Thu 04/30 20Z 0.30 3011 98 91 1 0.02
Thu 04/30 21Z 0.31 3011 97 87 1 0.01
Thu 04/30 22Z 0.36 3013 95 78 0 0.01
Thu 04/30 23Z 0.47 3014 96 69 0 0.00
Fri 05/01 00Z 0.56 3015 96 64 0 0.01
Fri 05/01 01Z 0.60 3017 97 63 -1 0.01
Fri 05/01 02Z 0.53 3017 96 63 -1 0.01
Fri 05/01 03Z 0.48 3117 95 63 -1 0.01
Fri 05/01 04Z 0.44 3117 94 69 -1 0.01
Fri 05/01 05Z 0.41 3117 93 75 -2 0.00
Fri 05/01 06Z 0.38 3117 91 72 -2 0.00
Fri 05/01 07Z 0.37 3118 89 67 -2 0.00
Fri 05/01 08Z 0.37 3119 84 67 -2 0.00
Fri 05/01 09Z 0.38 3120 82 59 -3 0.00
Fri 05/01 10Z 0.38 3120 82 47 -3 0.00
Fri 05/01 11Z 0.38 3119 82 41 -3 0.00
Fri 05/01 12Z 0.42 3118 83 35 -3 0.00
Fri 05/01 13Z 0.47 3116 85 43 -4 0.00
Fri 05/01 14Z 0.61 3114 84 49 -4 0.00
Fri 05/01 15Z 2.33 3112 81 47 -4 0.00
Fri 05/01 16Z 3.88 3010 78 46 -3 0.00
Fri 05/01 17Z 4.63 2909 75 39 -2 0.00
Fri 05/01 18Z 5.14 2809 73 37 -2 0.00
Fri 05/01 19Z 5.35 2808 73 36 -1 0.00
Fri 05/01 20Z 4.54 2708 75 35 -1 0.00
Fri 05/01 21Z 3.89 2707 74 52 -1 0.00
Fri 05/01 22Z 2.48 2706 76 52 -1 0.00
Fri 05/01 23Z 0.80 2706 78 53 -1 0.00
Sat 05/02 00Z 0.44 2706 77 54 -1 0.00
Sat 05/02 01Z 0.40 2707 76 58 -1 0.00
Sat 05/02 02Z 0.37 2706 76 68 -1 0.00
Sat 05/02 03Z 0.29 2705 77 74 -1 0.00
Sat 05/02 04Z 0.15 2702 74 81 -1 0.00
Sat 05/02 05Z 0.10 3000 75 88 -1 0.00
Sat 05/02 06Z 0.07 1700 84 93 -2 0.01
Sat 05/02 07Z 0.07 1701 91 94 -2 0.02
Sat 05/02 08Z 0.06 1801 94 95 -2 0.03
Sat 05/02 09Z 0.04 1701 97 93 -2 0.03
Sat 05/02 10Z 0.04 1601 97 90 -2 0.02
Sat 05/02 11Z 0.03 1501 98 89 -2 0.01
Sat 05/02 12Z 0.03 1601 99 90 -2 0.01
Sat 05/02 13Z 0.03 1901 100 89 -2 0.01
Sat 05/02 14Z 0.05 2301 100 85 -2 0.01
Sat 05/02 15Z 0.11 2602 100 80 -3 0.01
Sat 05/02 16Z 0.18 2804 99 68 -3 0.01
Sat 05/02 17Z 0.26 2805 96 53 -3 0.01
Run total areal avg QPF for the greens is: 0.76 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