NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260118_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)
=== ===== === ==== ==== === === === ====
Sun 01/18 06Z 0.75 2423  90  67  -7 0.00
Sun 01/18 09Z 0.72 2419  88  53  -8 0.00
Sun 01/18 12Z 0.71 2421  79  78  -9 0.00
Sun 01/18 15Z 1.08 2420  74  84 -11 0.00
Sun 01/18 18Z 1.87 2514  78  87 -12 0.00
Sun 01/18 21Z 0.44 2805  75  45 -12 0.00
Mon 01/19 00Z 0.09 0103  69  22 -11 0.00
Mon 01/19 03Z 0.04 3504  74  34 -11 0.00
Mon 01/19 06Z 0.12 3411  82  45 -12 0.00
Mon 01/19 09Z 0.18 3207  85  58 -13 0.00
Mon 01/19 12Z 0.36 2710  82  25 -14 0.00
Mon 01/19 15Z 0.45 2312  86  58 -14 0.00
Mon 01/19 18Z 2.15 2116  88  82 -14 0.00
Mon 01/19 21Z 1.43 2223  90  88 -13 0.01
Tue 01/20 00Z 2.15 2328  84  77 -14 0.01
Tue 01/20 03Z 2.38 2432  86  67 -15 0.00
Tue 01/20 06Z 1.42 2432  81  61 -16 0.00
Tue 01/20 09Z 2.19 2629  78  37 -18 0.00
Tue 01/20 12Z 1.34 2624  80  24 -19 0.00
Tue 01/20 15Z 1.15 2618  82  46 -19 0.00
Tue 01/20 18Z 2.46 2517  84  75 -20 0.00
Tue 01/20 21Z 3.42 2621  82  72 -20 0.01
Wed 01/21 00Z 1.68 2725  84  64 -21 0.01
Wed 01/21 03Z 1.79 2621  83  70 -20 0.01
Wed 01/21 06Z 1.16 2726  77  48 -21 0.01
Wed 01/21 09Z 0.84 2724  75  21 -20 0.00
Wed 01/21 12Z 0.71 2623  78  48 -19 0.00
Wed 01/21 15Z 0.38 2319  68  42 -16 0.00
Wed 01/21 18Z 0.30 2027  45  85 -14 0.00

Run total areal avg QPF for the greens is: 0.06 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