Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260817_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)
=== ===== === ==== ==== === === === ====
Mon 08/17 18Z 0.27 2706 97 80 14 0.00
Mon 08/17 19Z 0.16 2804 95 71 15 0.00
Mon 08/17 20Z 0.13 2903 93 76 15 0.01
Mon 08/17 21Z 0.14 3004 93 72 15 0.02
Mon 08/17 22Z 0.15 3105 93 68 15 0.03
Mon 08/17 23Z 0.11 3305 93 65 14 0.06
Tue 08/18 00Z 0.09 0004 93 72 14 0.01
Tue 08/18 01Z 0.07 0003 91 74 14 0.03
Tue 08/18 02Z 0.09 0103 89 84 14 0.13
Tue 08/18 03Z 0.09 0303 91 86 14 0.07
Tue 08/18 04Z 0.09 0502 92 81 14 0.01
Tue 08/18 05Z 0.06 0101 94 73 14 0.01
Tue 08/18 06Z 0.05 0002 94 58 14 0.01
Tue 08/18 07Z 0.05 0004 95 49 14 0.01
Tue 08/18 08Z 0.04 0005 93 47 13 0.01
Tue 08/18 09Z 0.03 0005 92 35 13 0.01
Tue 08/18 10Z 0.03 3506 90 32 13 0.00
Tue 08/18 11Z 0.03 3505 88 27 13 0.00
Tue 08/18 12Z 0.03 3505 88 23 13 0.00
Tue 08/18 13Z 0.02 3505 89 15 13 0.00
Tue 08/18 14Z 0.02 0004 89 8 13 0.00
Tue 08/18 15Z 0.02 3503 90 4 12 0.00
Tue 08/18 16Z 0.04 3402 91 4 12 0.01
Tue 08/18 17Z 0.05 3101 91 3 13 0.01
Tue 08/18 18Z 0.12 2903 91 3 13 0.01
Tue 08/18 19Z 0.13 2904 87 3 13 0.00
Tue 08/18 20Z 0.16 2905 86 3 13 0.00
Tue 08/18 21Z 0.23 2907 83 3 13 0.00
Tue 08/18 22Z 0.11 2903 78 3 13 0.00
Tue 08/18 23Z 0.11 2803 78 3 13 0.00
Wed 08/19 00Z 0.16 2704 79 7 12 0.00
Wed 08/19 01Z 0.13 2804 79 6 12 0.00
Wed 08/19 02Z 0.17 2705 81 11 12 0.00
Wed 08/19 03Z 0.15 2804 83 8 12 0.00
Wed 08/19 04Z 0.16 2704 87 6 11 0.00
Wed 08/19 05Z 0.21 2806 90 7 11 0.00
Wed 08/19 06Z 0.17 2805 91 12 11 0.00
Wed 08/19 07Z 0.18 2805 91 16 11 0.00
Wed 08/19 08Z 0.20 2806 91 14 11 0.00
Wed 08/19 09Z 0.18 2705 89 13 11 0.00
Wed 08/19 10Z 0.20 2707 87 11 11 0.00
Wed 08/19 11Z 0.20 2607 83 10 11 0.01
Wed 08/19 12Z 0.19 2507 75 23 12 0.00
Wed 08/19 13Z 0.22 2509 72 42 12 0.00
Wed 08/19 14Z 0.22 2509 70 58 12 0.00
Wed 08/19 15Z 0.23 2409 69 69 12 0.00
Wed 08/19 16Z 0.23 2409 66 64 13 0.00
Wed 08/19 17Z 0.31 2311 62 66 13 0.00
Wed 08/19 18Z 0.40 2314 57 55 14 0.00
Wed 08/19 19Z 0.49 2315 57 51 14 0.00
Wed 08/19 20Z 0.61 2415 70 50 14 0.00
Wed 08/19 21Z 1.85 2418 84 45 13 0.00
Wed 08/19 22Z 0.79 2519 92 34 13 0.01
Wed 08/19 23Z 0.68 2517 93 34 13 0.01
Thu 08/20 00Z 0.68 2518 93 50 13 0.01
Thu 08/20 01Z 0.63 2617 91 39 13 0.03
Thu 08/20 02Z 0.54 2616 87 33 13 0.01
Thu 08/20 03Z 0.48 2615 84 40 13 0.01
Thu 08/20 04Z 0.44 2715 80 32 13 0.01
Thu 08/20 05Z 0.39 2714 78 32 13 0.01
Run total areal avg QPF for the greens is: 0.56 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