NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260116_0000
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)
=== ===== === ==== ==== === === === ====
Fri 01/16 00Z 0.60 2916  89  24 -13 0.00
Fri 01/16 03Z 5.06 2818  86  56 -16 0.00
Fri 01/16 06Z 7.25 2717  84  53 -18 0.00
Fri 01/16 09Z 7.27 2922  87  35 -20 0.00
Fri 01/16 12Z 0.80 2824  85  51 -20 0.00
Fri 01/16 15Z 0.57 2926  84  54 -16 0.00
Fri 01/16 18Z 0.58 2826  62  11 -13 0.00
Fri 01/16 21Z 0.46 2920  44  16 -12 0.00
Sat 01/17 00Z 0.34 2714  38  56 -11 0.00
Sat 01/17 03Z 0.24 2214  41  83 -10 0.00
Sat 01/17 06Z 0.28 2122  90  87 -10 0.01
Sat 01/17 09Z 0.42 2223  90  67  -9 0.04
Sat 01/17 12Z 0.28 2119  87  60  -8 0.01
Sat 01/17 15Z 0.24 2021  88  86  -6 0.00
Sat 01/17 18Z 0.34 2124  93  87  -6 0.05
Sat 01/17 21Z 0.51 2223  93  87  -7 0.08
Sun 01/18 00Z 0.65 2623  88  71  -7 0.03
Sun 01/18 03Z 0.71 2624  80  59  -7 0.00
Sun 01/18 06Z 0.91 2527  88  60  -8 0.00
Sun 01/18 09Z 1.18 2523  77  57 -10 0.00
Sun 01/18 12Z 1.18 2423  82  31 -12 0.00
Sun 01/18 15Z 1.82 2521  82  10 -14 0.00
Sun 01/18 18Z 7.31 2416  85  10 -14 0.00
Sun 01/18 21Z 1.45 2420  83  17 -13 0.00
Mon 01/19 00Z 1.38 2613  77  25 -14 0.00
Mon 01/19 03Z 0.60 2711  73  31 -13 0.00
Mon 01/19 06Z 0.41 2509  71  36 -13 0.00
Mon 01/19 09Z 0.25 2112  78  32 -12 0.00
Mon 01/19 12Z 0.22 1922  90  19 -11 0.00

Run total areal avg QPF for the greens is: 0.22 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