Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260218_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)
=== ===== === ==== ==== === === === ====
Wed 02/18 18Z 0.17 3211 55 25 -6 0.00
Wed 02/18 19Z 0.11 3411 48 17 -6 0.00
Wed 02/18 20Z 0.07 3410 46 16 -6 0.00
Wed 02/18 21Z 0.06 3509 46 23 -6 0.00
Wed 02/18 22Z 0.05 0010 47 25 -5 0.00
Thu 02/19 00Z 0.10 0113 41 32 -5 0.00
Thu 02/19 01Z 0.14 0214 40 33 -6 0.00
Thu 02/19 02Z 0.18 0215 41 34 -6 0.00
Thu 02/19 03Z 0.17 0215 41 33 -6 0.00
Thu 02/19 04Z 0.14 0214 40 33 -7 0.00
Thu 02/19 05Z 0.13 0214 37 32 -7 0.00
Thu 02/19 06Z 0.10 0212 33 32 -6 0.00
Thu 02/19 07Z 0.11 0211 30 31 -6 0.00
Thu 02/19 08Z 0.10 0210 27 26 -6 0.00
Thu 02/19 09Z 0.09 0209 26 19 -6 0.00
Thu 02/19 10Z 0.07 0207 24 12 -6 0.00
Thu 02/19 11Z 0.08 0305 22 9 -6 0.00
Thu 02/19 12Z 0.07 0404 20 15 -5 0.00
Thu 02/19 13Z 0.06 0404 19 14 -5 0.00
Thu 02/19 14Z 0.05 0403 19 14 -5 0.00
Thu 02/19 15Z 0.04 0601 19 16 -4 0.00
Thu 02/19 16Z 0.02 1400 22 20 -4 0.00
Thu 02/19 17Z 0.02 1902 25 29 -4 0.00
Thu 02/19 18Z 0.04 2103 29 38 -4 0.00
Thu 02/19 19Z 0.07 2203 34 45 -3 0.00
Thu 02/19 20Z 0.11 2305 37 49 -4 0.00
Thu 02/19 21Z 0.12 2305 35 42 -3 0.00
Thu 02/19 22Z 0.12 2305 32 46 -3 0.00
Thu 02/19 23Z 0.12 2306 31 48 -3 0.00
Fri 02/20 00Z 0.10 2205 34 32 -2 0.00
Fri 02/20 01Z 0.08 2204 39 19 -2 0.00
Fri 02/20 02Z 0.09 2106 44 9 -3 0.00
Fri 02/20 03Z 0.09 2106 50 12 -2 0.00
Fri 02/20 04Z 0.13 2108 58 12 -3 0.00
Fri 02/20 05Z 0.14 2109 67 12 -3 0.00
Fri 02/20 06Z 0.13 2110 73 18 -2 0.00
Fri 02/20 07Z 0.10 2010 77 20 -2 0.00
Fri 02/20 08Z 0.08 2009 77 18 -1 0.00
Fri 02/20 09Z 0.07 2009 75 19 -1 0.00
Fri 02/20 10Z 0.04 1908 72 21 -1 0.00
Fri 02/20 11Z 0.02 1808 72 17 -1 0.00
Fri 02/20 12Z 0.02 1709 73 17 -1 0.00
Fri 02/20 13Z 0.04 1711 73 17 -1 0.00
Fri 02/20 14Z 0.05 1613 69 19 -1 0.00
Fri 02/20 15Z 0.08 1616 66 33 -1 0.00
Fri 02/20 16Z 0.08 1618 64 51 -1 0.00
Fri 02/20 17Z 0.08 1618 77 66 -1 0.00
Fri 02/20 18Z 0.08 1618 84 74 -1 0.01
Fri 02/20 19Z 0.13 1518 88 87 0 0.04
Fri 02/20 20Z 0.20 1419 92 96 0 0.08
Fri 02/20 21Z 0.27 1322 94 98 0 0.13
Fri 02/20 22Z 0.31 1324 96 98 -1 0.12
Fri 02/20 23Z 0.35 1326 95 98 -1 0.13
Sat 02/21 00Z 0.39 1327 95 96 -1 0.12
Sat 02/21 01Z 0.43 1327 96 93 -2 0.10
Sat 02/21 02Z 0.41 1325 96 90 -3 0.08
Sat 02/21 03Z 0.36 1419 97 92 -5 0.07
Sat 02/21 04Z 0.31 1417 95 94 -6 0.06
Sat 02/21 05Z 0.23 1515 95 94 -6 0.04
Run total areal avg QPF for the greens is: 0.98 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