NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260204_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 02/04 06Z 0.51 2612  88  76 -13 0.00
Wed 02/04 09Z 0.43 2912  87  74 -14 0.00
Wed 02/04 12Z 0.41 2911  85  77 -15 0.01
Wed 02/04 15Z 0.31 2911  84  70 -15 0.00
Wed 02/04 18Z 0.37 3013  83  44 -14 0.00
Wed 02/04 21Z 0.43 3016  89  30 -13 0.00
Thu 02/05 00Z 0.47 3018  91  32 -13 0.00
Thu 02/05 03Z 0.37 3017  87  31 -13 0.00
Thu 02/05 06Z 0.41 3018  83  37 -13 0.00
Thu 02/05 09Z 0.47 2919  84  63 -14 0.00
Thu 02/05 12Z 0.48 3020  90  32 -15 0.00
Thu 02/05 15Z 0.32 3116  87   8 -15 0.00
Thu 02/05 18Z 0.26 3112  76   6 -14 0.00
Thu 02/05 21Z 0.19 3211  65   7 -13 0.00
Fri 02/06 00Z 0.11 3210  48   7 -11 0.00
Fri 02/06 03Z 0.09 3005  47  11 -10 0.00
Fri 02/06 06Z 0.11 2506  52  17 -10 0.00
Fri 02/06 09Z 0.16 2309  52  10 -10 0.00
Fri 02/06 12Z 0.15 2209  61  13 -11 0.00
Fri 02/06 15Z 0.13 2013  66  89 -10 0.00
Fri 02/06 18Z 0.16 2015  58  88 -10 0.00
Fri 02/06 21Z 0.15 1917  65  92 -10 0.00
Sat 02/07 00Z 0.21 1919  82  93 -11 0.00
Sat 02/07 03Z 0.17 1917  68  91 -10 0.04
Sat 02/07 06Z 0.23 2113  90  85 -11 0.01
Sat 02/07 09Z 0.29 2608  91  83 -12 0.05
Sat 02/07 12Z 0.23 3321  86  82 -14 0.05
Sat 02/07 15Z 0.17 3323  81  80 -17 0.02
Sat 02/07 18Z 0.24 3225  82  77 -20 0.02

Run total areal avg QPF for the greens is: 0.20 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