Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260518_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)
=== ===== === ==== ==== === === === ====
Mon 05/18 06Z 0.13 2505 61 42 12 0.00
Mon 05/18 07Z 0.14 2306 61 47 12 0.00
Mon 05/18 08Z 0.11 2107 62 52 13 0.00
Mon 05/18 09Z 0.12 2009 63 64 13 0.00
Mon 05/18 10Z 0.13 2011 64 84 13 0.00
Mon 05/18 11Z 0.15 2014 68 92 12 0.00
Mon 05/18 12Z 0.05 1818 72 87 13 0.01
Mon 05/18 13Z 0.23 2022 88 88 11 0.11
Mon 05/18 14Z 0.27 2025 89 78 12 0.10
Mon 05/18 15Z 0.34 2125 92 57 12 0.00
Mon 05/18 16Z 0.36 2222 92 36 13 0.00
Mon 05/18 17Z 0.37 2219 88 26 13 0.00
Mon 05/18 18Z 0.46 2318 83 19 14 0.00
Mon 05/18 19Z 0.72 2320 85 22 14 0.00
Mon 05/18 20Z 1.27 2321 85 21 14 0.00
Mon 05/18 21Z 1.00 2320 82 21 14 0.00
Mon 05/18 22Z 0.95 2321 85 21 15 0.00
Mon 05/18 23Z 1.05 2323 86 18 15 0.00
Tue 05/19 00Z 1.04 2323 87 18 15 0.00
Tue 05/19 01Z 1.09 2324 88 22 15 0.00
Tue 05/19 02Z 1.14 2327 89 19 15 0.00
Tue 05/19 03Z 1.23 2429 87 17 15 0.00
Tue 05/19 04Z 1.29 2432 82 15 16 0.00
Tue 05/19 05Z 1.39 2433 80 18 16 0.01
Tue 05/19 06Z 1.44 2536 79 37 15 0.00
Tue 05/19 07Z 1.60 2636 81 37 15 0.00
Tue 05/19 08Z 1.35 2634 82 29 15 0.01
Tue 05/19 09Z 1.23 2633 79 43 15 0.01
Tue 05/19 10Z 1.27 2633 78 39 15 0.00
Tue 05/19 11Z 1.24 2632 80 44 15 0.00
Tue 05/19 12Z 1.08 2630 82 48 15 0.00
Tue 05/19 13Z 1.07 2529 83 63 15 0.00
Tue 05/19 14Z 1.09 2529 82 68 15 0.00
Tue 05/19 15Z 1.18 2526 87 77 15 0.00
Tue 05/19 16Z 1.19 2523 91 76 14 0.00
Tue 05/19 17Z 2.11 2523 91 78 14 0.01
Tue 05/19 18Z 1.62 2524 87 76 14 0.01
Tue 05/19 19Z 3.12 2622 84 64 14 0.00
Tue 05/19 20Z 6.33 2622 85 72 15 0.00
Tue 05/19 21Z 6.57 2522 83 65 15 0.00
Tue 05/19 22Z 4.82 2523 82 66 15 0.00
Tue 05/19 23Z 2.27 2426 84 72 15 0.00
Wed 05/20 00Z 1.61 2430 80 63 16 0.00
Wed 05/20 01Z 1.57 2532 81 65 15 0.00
Wed 05/20 02Z 1.39 2433 84 65 16 0.00
Wed 05/20 03Z 1.47 2435 88 60 16 0.00
Wed 05/20 04Z 1.46 2436 91 57 16 0.00
Wed 05/20 05Z 1.31 2536 90 57 16 0.00
Wed 05/20 06Z 1.41 2539 92 63 15 0.00
Wed 05/20 07Z 1.20 2638 87 64 16 0.02
Wed 05/20 08Z 1.09 2536 86 61 16 0.02
Wed 05/20 09Z 1.36 2537 92 58 14 0.00
Wed 05/20 10Z 1.11 2635 88 65 14 0.01
Wed 05/20 11Z 1.09 2633 94 67 14 0.00
Wed 05/20 12Z 1.01 2631 94 59 14 0.01
Wed 05/20 13Z 1.09 2629 96 70 12 0.02
Wed 05/20 14Z 1.20 2628 97 76 12 0.05
Wed 05/20 15Z 1.27 2626 94 75 11 0.04
Wed 05/20 16Z 1.50 2625 92 79 10 0.02
Wed 05/20 17Z 4.84 2723 89 65 9 0.02
Run total areal avg QPF for the greens is: 0.48 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