Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260419_1200
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 04/19 12Z 0.66 2918 98 99 1 0.00
Sun 04/19 13Z 0.56 3020 98 99 0 0.06
Sun 04/19 14Z 0.60 3120 98 99 -1 0.11
Sun 04/19 15Z 0.51 3120 97 99 -2 0.07
Sun 04/19 16Z 0.53 3021 97 99 -3 0.08
Sun 04/19 17Z 0.54 3021 96 96 -4 0.07
Sun 04/19 18Z 0.59 3122 96 94 -6 0.05
Sun 04/19 19Z 0.65 3022 96 91 -6 0.04
Sun 04/19 20Z 0.64 3122 83 88 -7 0.04
Sun 04/19 21Z 0.61 3121 89 77 -7 0.01
Sun 04/19 22Z 0.79 3020 90 18 -6 0.00
Sun 04/19 23Z 0.80 2920 91 20 -6 0.00
Mon 04/20 00Z 0.98 2922 94 27 -6 0.00
Mon 04/20 01Z 1.24 2825 94 39 -6 0.01
Mon 04/20 02Z 1.14 2924 86 31 -6 0.01
Mon 04/20 03Z 1.24 2924 78 45 -6 0.00
Mon 04/20 04Z 1.28 2822 75 49 -6 0.00
Mon 04/20 05Z 1.34 2721 76 56 -6 0.00
Mon 04/20 06Z 1.02 2720 83 39 -7 0.00
Mon 04/20 07Z 0.85 2720 85 53 -7 0.00
Mon 04/20 08Z 0.70 2819 80 44 -7 0.01
Mon 04/20 09Z 0.60 2918 83 56 -8 0.02
Mon 04/20 10Z 0.59 3018 86 62 -9 0.01
Mon 04/20 11Z 0.53 3117 86 54 -9 0.01
Mon 04/20 12Z 0.50 3115 87 51 -10 0.01
Mon 04/20 13Z 0.85 3213 91 42 -11 0.00
Mon 04/20 14Z 2.21 3212 89 40 -11 0.00
Mon 04/20 15Z 2.97 3111 83 38 -11 0.00
Mon 04/20 16Z 4.45 3110 77 39 -11 0.00
Mon 04/20 17Z 5.31 3010 71 39 -10 0.00
Mon 04/20 18Z 5.81 3011 67 36 -10 0.00
Mon 04/20 19Z 5.84 3010 64 31 -10 0.00
Mon 04/20 20Z 5.31 3011 64 28 -10 0.00
Mon 04/20 21Z 4.97 3011 62 24 -9 0.00
Mon 04/20 22Z 4.21 3111 60 18 -9 0.00
Mon 04/20 23Z 2.18 3111 59 13 -10 0.00
Tue 04/21 00Z 1.08 3211 56 13 -10 0.00
Tue 04/21 01Z 1.11 3212 54 12 -10 0.00
Tue 04/21 02Z 1.43 3213 53 11 -10 0.00
Tue 04/21 03Z 1.48 3213 52 10 -10 0.00
Tue 04/21 04Z 1.25 3212 52 11 -11 0.00
Tue 04/21 05Z 1.01 3212 52 12 -11 0.00
Tue 04/21 06Z 0.74 3111 52 11 -11 0.00
Tue 04/21 07Z 0.56 3109 50 10 -11 0.00
Tue 04/21 08Z 0.56 3109 50 11 -11 0.00
Tue 04/21 09Z 0.49 3009 49 11 -11 0.00
Tue 04/21 10Z 0.42 2907 48 11 -11 0.00
Tue 04/21 11Z 0.38 2807 47 11 -11 0.00
Tue 04/21 12Z 0.44 2706 47 11 -10 0.00
Tue 04/21 13Z 1.43 2506 48 10 -10 0.00
Tue 04/21 14Z 2.75 2405 47 8 -10 0.00
Tue 04/21 15Z 3.33 2306 45 8 -9 0.00
Tue 04/21 16Z 3.24 2206 42 9 -8 0.00
Tue 04/21 17Z 3.52 2207 40 17 -7 0.00
Tue 04/21 18Z 3.78 2307 39 40 -6 0.00
Tue 04/21 19Z 3.49 2307 38 80 -6 0.00
Tue 04/21 20Z 3.40 2307 39 85 -5 0.00
Tue 04/21 21Z 2.29 2307 39 81 -5 0.00
Tue 04/21 22Z 0.99 2207 38 75 -5 0.00
Tue 04/21 23Z 0.39 2208 39 72 -4 0.00
Run total areal avg QPF for the greens is: 0.61 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