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Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260418_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)
=== ===== === ==== ==== === === === ====
Sat 04/18 06Z 0.14 1514 67 8 9 0.00
Sat 04/18 07Z 0.17 1518 66 7 9 0.00
Sat 04/18 08Z 0.14 1619 67 6 9 0.00
Sat 04/18 09Z 0.10 1719 69 6 8 0.00
Sat 04/18 10Z 0.10 1721 72 6 8 0.00
Sat 04/18 11Z 0.12 1723 70 6 8 0.00
Sat 04/18 12Z 0.11 1723 65 6 8 0.00
Sat 04/18 13Z 0.08 1722 58 6 9 0.00
Sat 04/18 14Z 0.07 1720 53 6 9 0.00
Sat 04/18 15Z 0.10 1621 49 6 9 0.00
Sat 04/18 16Z 0.14 1622 49 7 9 0.00
Sat 04/18 17Z 0.16 1623 51 8 9 0.00
Sat 04/18 18Z 0.16 1624 55 10 9 0.00
Sat 04/18 19Z 0.17 1626 60 10 9 0.00
Sat 04/18 20Z 0.16 1627 60 6 10 0.00
Sat 04/18 21Z 0.13 1629 59 3 10 0.00
Sat 04/18 22Z 0.09 1730 59 4 10 0.00
Sat 04/18 23Z 0.04 1730 61 5 10 0.01
Sun 04/19 00Z 0.03 1731 64 6 10 0.01
Sun 04/19 01Z 0.05 1832 68 7 10 0.01
Sun 04/19 02Z 0.06 1832 70 10 11 0.01
Sun 04/19 03Z 0.11 1933 72 27 11 0.00
Sun 04/19 04Z 0.15 1934 75 35 11 0.00
Sun 04/19 05Z 0.21 1936 79 42 11 0.00
Sun 04/19 06Z 0.24 2036 86 84 10 0.01
Sun 04/19 07Z 0.34 2038 97 96 8 0.10
Sun 04/19 08Z 0.47 2233 97 89 7 0.05
Sun 04/19 09Z 0.52 2327 97 93 6 0.02
Sun 04/19 10Z 0.48 2421 98 95 5 0.02
Sun 04/19 11Z 0.53 2417 99 95 3 0.03
Sun 04/19 12Z 0.55 2713 99 97 1 0.05
Sun 04/19 13Z 0.82 2919 98 98 -1 0.10
Sun 04/19 14Z 0.50 3022 97 99 -1 0.13
Sun 04/19 15Z 0.57 2925 97 99 -2 0.12
Sun 04/19 16Z 0.63 3025 97 99 -3 0.09
Sun 04/19 17Z 0.68 3025 96 96 -5 0.07
Sun 04/19 18Z 0.63 3024 89 92 -6 0.08
Sun 04/19 19Z 0.53 3122 69 82 -6 0.01
Sun 04/19 20Z 0.55 3122 54 34 -5 0.00
Sun 04/19 21Z 0.71 3021 63 12 -6 0.00
Sun 04/19 22Z 1.28 2920 80 15 -6 0.00
Sun 04/19 23Z 1.13 2920 80 14 -6 0.00
Mon 04/20 00Z 1.41 2820 77 21 -6 0.00
Mon 04/20 01Z 2.47 2822 75 29 -6 0.00
Mon 04/20 02Z 2.61 2823 76 39 -6 0.00
Mon 04/20 03Z 2.36 2823 77 48 -7 0.00
Mon 04/20 04Z 1.49 2823 80 48 -7 0.00
Mon 04/20 05Z 1.28 2921 72 42 -8 0.00
Mon 04/20 06Z 0.96 2919 71 44 -8 0.00
Mon 04/20 07Z 0.81 2918 71 53 -8 0.00
Mon 04/20 08Z 0.72 2917 75 65 -9 0.00
Mon 04/20 09Z 0.64 2915 78 60 -9 0.00
Mon 04/20 10Z 0.61 3014 82 39 -10 0.00
Mon 04/20 11Z 0.59 3113 85 38 -11 0.00
Mon 04/20 12Z 0.71 3112 85 41 -11 0.00
Mon 04/20 13Z 0.83 3111 83 46 -11 0.00
Mon 04/20 14Z 1.49 3111 83 50 -11 0.00
Mon 04/20 15Z 2.67 3111 81 53 -11 0.00
Mon 04/20 16Z 3.62 3010 75 54 -11 0.00
Mon 04/20 17Z 5.52 3010 69 55 -11 0.00
Run total areal avg QPF for the greens is: 0.92 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