NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260218_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)
=== ===== === ==== ==== === === === ====
Wed 02/18 06Z 0.79 2923  90  18  -4 0.00
Wed 02/18 09Z 0.67 3023  85  20  -5 0.00
Wed 02/18 12Z 0.41 3123  68  23  -5 0.00
Wed 02/18 15Z 0.21 3216  49  24  -5 0.00
Wed 02/18 18Z 0.10 3312  37  20  -5 0.00
Wed 02/18 21Z 0.04 0011  40  11  -6 0.00
Thu 02/19 00Z 0.15 0213  34   9  -6 0.00
Thu 02/19 03Z 0.21 0315  33  12  -6 0.00
Thu 02/19 06Z 0.19 0411  31  14  -6 0.00
Thu 02/19 09Z 0.14 0506  27   5  -6 0.00
Thu 02/19 12Z 0.04 0402  23   5  -5 0.00
Thu 02/19 15Z 0.03 1701  21   7  -5 0.00
Thu 02/19 18Z 0.07 2005  25  27  -4 0.00
Thu 02/19 21Z 0.17 2308  35  18  -4 0.00
Fri 02/20 00Z 0.23 2409  63  15  -3 0.00
Fri 02/20 03Z 0.17 2606  72  11  -3 0.00
Fri 02/20 06Z 0.17 2308  79  24  -3 0.00
Fri 02/20 09Z 0.11 2207  81  43  -1 0.00
Fri 02/20 12Z 0.02 1806  73  30   1 0.00
Fri 02/20 15Z 0.02 1712  78  20   0 0.00
Fri 02/20 18Z 0.11 1616  85  68   0 0.00
Fri 02/20 21Z 0.19 1417  96  93   0 0.29
Sat 02/21 00Z 0.26 1424  96  96  -1 0.28
Sat 02/21 03Z 0.18 1519  92  86   0 0.09
Sat 02/21 06Z 0.04 1718  95  96  -5 0.22
Sat 02/21 09Z 0.16 2014  96  79  -5 0.05
Sat 02/21 12Z 0.17 2112  97  61  -5 0.02
Sat 02/21 15Z 0.05 1910  97  29  -6 0.02
Sat 02/21 18Z 0.18 2112  97  48  -6 0.02

Run total areal avg QPF for the greens is: 0.99 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