Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260720_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)
=== ===== === ==== ==== === === === ====
Mon 07/20 12Z 0.40 3016 78 72 10 0.00
Mon 07/20 13Z 0.41 3015 75 70 10 0.00
Mon 07/20 14Z 0.47 2914 72 69 10 0.00
Mon 07/20 15Z 0.56 2913 69 71 10 0.00
Mon 07/20 16Z 0.97 2812 67 65 11 0.00
Mon 07/20 17Z 2.12 2711 69 57 11 0.00
Mon 07/20 18Z 4.32 2510 71 55 11 0.00
Mon 07/20 19Z 5.37 2409 70 55 11 0.00
Mon 07/20 20Z 4.96 2410 70 54 11 0.00
Mon 07/20 21Z 3.76 2311 72 49 11 0.00
Mon 07/20 22Z 2.09 2312 74 45 12 0.00
Mon 07/20 23Z 0.73 2214 74 38 12 0.00
Tue 07/21 00Z 0.60 2215 72 29 12 0.00
Tue 07/21 01Z 0.63 2217 72 24 12 0.00
Tue 07/21 02Z 0.63 2218 71 19 12 0.00
Tue 07/21 03Z 0.52 2117 69 18 12 0.00
Tue 07/21 04Z 0.58 2220 71 21 12 0.00
Tue 07/21 05Z 0.58 2120 73 21 12 0.00
Tue 07/21 06Z 0.58 2122 73 21 12 0.00
Tue 07/21 07Z 0.63 2124 73 19 12 0.00
Tue 07/21 08Z 0.64 2126 75 15 12 0.00
Tue 07/21 09Z 0.56 2125 79 14 12 0.00
Tue 07/21 10Z 0.50 2125 79 13 12 0.00
Tue 07/21 11Z 0.46 2125 80 11 12 0.00
Tue 07/21 12Z 0.35 2026 83 9 12 0.01
Tue 07/21 13Z 0.37 2026 86 8 12 0.01
Tue 07/21 14Z 0.38 2027 84 11 12 0.00
Tue 07/21 15Z 0.45 1927 88 33 11 0.00
Tue 07/21 16Z 0.26 1928 89 59 12 0.00
Tue 07/21 17Z 0.12 1829 91 76 12 0.01
Tue 07/21 18Z 0.05 1831 95 63 13 0.01
Tue 07/21 19Z 0.06 1734 94 51 14 0.01
Tue 07/21 20Z 0.10 1735 96 66 14 0.02
Tue 07/21 21Z 0.07 1734 96 55 14 0.02
Tue 07/21 22Z 0.08 1733 97 73 15 0.04
Tue 07/21 23Z 0.09 1733 95 85 15 0.21
Wed 07/22 00Z 0.10 1831 95 82 15 0.20
Wed 07/22 01Z 0.11 1830 94 72 16 0.11
Wed 07/22 02Z 0.13 1929 95 69 16 0.02
Wed 07/22 03Z 0.30 2024 82 62 15 0.32
Wed 07/22 04Z 0.45 2126 85 59 15 0.04
Wed 07/22 05Z 0.55 2227 90 62 15 0.00
Wed 07/22 06Z 0.46 2224 92 58 15 0.00
Wed 07/22 07Z 0.46 2224 91 58 14 0.00
Wed 07/22 08Z 0.47 2223 91 62 14 0.00
Wed 07/22 09Z 0.48 2323 87 63 15 0.00
Wed 07/22 10Z 0.47 2321 87 60 15 0.00
Wed 07/22 11Z 0.47 2419 91 59 15 0.00
Wed 07/22 12Z 0.42 2317 95 59 14 0.00
Wed 07/22 13Z 0.40 2316 97 65 14 0.01
Wed 07/22 14Z 0.40 2215 97 68 13 0.01
Wed 07/22 15Z 0.44 2215 97 68 13 0.01
Wed 07/22 16Z 0.48 2116 95 75 14 0.00
Wed 07/22 17Z 0.61 2116 94 83 14 0.00
Wed 07/22 18Z 0.97 2117 92 72 14 0.01
Wed 07/22 19Z 1.25 2017 89 61 15 0.00
Wed 07/22 20Z 1.15 2115 90 66 15 0.09
Wed 07/22 21Z 0.54 2316 89 79 14 0.24
Wed 07/22 22Z 0.55 2517 84 56 14 0.20
Wed 07/22 23Z 0.59 2519 86 36 13 0.01
Run total areal avg QPF for the greens is: 1.61 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