Subject: NAM3 EXPERIMENTAL Froude Number for the greens
Model run: BTV_GRID_D2D_NAM3_20260430_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)
=== ===== === ==== ==== === === === ====
Thu 04/30 12Z 0.15 1916 98 97 3 0.00
Thu 04/30 13Z 0.20 2015 97 95 3 0.01
Thu 04/30 14Z 0.25 2113 98 92 2 0.01
Thu 04/30 15Z 0.28 2211 99 92 2 0.01
Thu 04/30 16Z 0.33 2310 99 91 2 0.01
Thu 04/30 17Z 0.36 2509 99 92 1 0.02
Thu 04/30 18Z 0.38 2708 99 91 1 0.02
Thu 04/30 19Z 0.35 2908 97 90 1 0.01
Thu 04/30 20Z 0.27 2907 95 87 1 0.01
Thu 04/30 21Z 0.26 3007 94 84 1 0.01
Thu 04/30 22Z 0.39 3010 94 76 0 0.01
Thu 04/30 23Z 0.39 3112 95 61 0 0.01
Fri 05/01 00Z 0.42 3115 95 54 0 0.01
Fri 05/01 01Z 0.46 3117 95 55 0 0.01
Fri 05/01 02Z 0.45 3117 95 60 -1 0.01
Fri 05/01 03Z 0.42 3117 94 67 -1 0.01
Fri 05/01 04Z 0.38 3117 93 69 -1 0.01
Fri 05/01 05Z 0.37 3218 93 68 -1 0.01
Fri 05/01 06Z 0.36 3218 91 71 -2 0.01
Fri 05/01 07Z 0.34 3218 87 78 -2 0.00
Fri 05/01 08Z 0.36 3219 83 79 -2 0.00
Fri 05/01 09Z 0.36 3220 81 76 -2 0.00
Fri 05/01 10Z 0.36 3220 80 65 -2 0.00
Fri 05/01 11Z 0.38 3221 79 62 -3 0.00
Fri 05/01 12Z 0.40 3220 79 53 -3 0.00
Fri 05/01 13Z 0.48 3118 80 44 -4 0.00
Fri 05/01 14Z 1.43 3117 79 41 -4 0.00
Fri 05/01 15Z 3.28 3115 76 47 -4 0.00
Fri 05/01 16Z 4.21 3112 72 47 -3 0.00
Fri 05/01 17Z 4.88 3011 69 47 -2 0.00
Fri 05/01 18Z 5.37 3010 67 45 -1 0.00
Fri 05/01 19Z 4.90 3008 64 35 -1 0.00
Fri 05/01 20Z 4.65 2908 63 34 0 0.00
Fri 05/01 21Z 4.24 2907 65 38 0 0.00
Fri 05/01 22Z 3.06 2807 67 41 0 0.00
Fri 05/01 23Z 1.15 2706 68 54 0 0.00
Sat 05/02 00Z 0.60 2706 69 63 0 0.00
Sat 05/02 01Z 0.52 2706 70 71 0 0.00
Sat 05/02 02Z 0.41 2604 71 77 0 0.00
Sat 05/02 03Z 0.20 2702 72 80 0 0.00
Sat 05/02 04Z 0.15 0700 72 83 0 0.00
Sat 05/02 05Z 0.28 0904 75 90 -1 0.00
Sat 05/02 06Z 0.21 0904 83 92 -1 0.03
Sat 05/02 07Z 0.17 1005 91 92 -1 0.07
Sat 05/02 08Z 0.16 1205 93 91 -2 0.06
Sat 05/02 09Z 0.13 1305 95 90 -2 0.03
Sat 05/02 10Z 0.11 1505 97 88 -2 0.02
Sat 05/02 11Z 0.12 1705 99 83 -2 0.02
Sat 05/02 12Z 0.13 1904 99 82 -2 0.01
Sat 05/02 13Z 0.13 2104 99 81 -2 0.01
Sat 05/02 14Z 0.16 2304 99 84 -3 0.01
Sat 05/02 15Z 0.21 2505 99 77 -3 0.00
Sat 05/02 16Z 0.26 2706 98 71 -3 0.00
Sat 05/02 17Z 0.27 2906 97 61 -3 0.00
Sat 05/02 18Z 0.32 3105 94 48 -2 0.00
Sat 05/02 19Z 0.47 3104 90 35 -2 0.00
Sat 05/02 20Z 0.58 3103 86 30 -1 0.00
Sat 05/02 21Z 0.82 3004 84 40 -1 0.00
Sat 05/02 22Z 0.60 3005 86 63 -1 0.00
Sat 05/02 23Z 0.25 3206 87 69 -1 0.01
Run total areal avg QPF for the greens is: 0.47 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