NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20250501_1800
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 05/01 18Z 0.96 2103  34   4   5 0.00
Thu 05/01 21Z 0.43 1907  31  13   7 0.00
Fri 05/02 00Z 0.08 1815  40  71   8 0.00
Fri 05/02 03Z 0.16 1925  54  97  10 0.00
Fri 05/02 06Z 0.39 2038  63  91  11 0.02
Fri 05/02 09Z 0.38 2135  76  79  13 0.00
Fri 05/02 12Z 0.35 2129  98  48  11 0.02
Fri 05/02 15Z 0.53 2327  98  87  10 0.06
Fri 05/02 18Z 0.78 2523  90  69  10 0.03
Fri 05/02 21Z 7.04 2422  85  80  10 0.02
Sat 05/03 00Z 2.89 2426  79  68  10 0.02
Sat 05/03 03Z 1.43 2631  79  14   8 0.00
Sat 05/03 06Z 0.83 2727  85  15   6 0.00
Sat 05/03 09Z 0.50 2722  80   9   6 0.00
Sat 05/03 12Z 0.34 2817  76  24   5 0.00
Sat 05/03 15Z 0.28 2712  69  71   6 0.00
Sat 05/03 18Z 0.19 2407  65  91   6 0.00
Sat 05/03 21Z 0.18 2405  78  96   6 0.00
Sun 05/04 00Z 0.12 1508  95  97   8 0.08
Sun 05/04 03Z 0.12 2007  97  97   8 0.31
Sun 05/04 06Z 0.20 2508  98  93   7 0.02
Sun 05/04 09Z 0.22 2910  95  92   6 0.02
Sun 05/04 12Z 0.17 3109  83  64   6 0.01
Sun 05/04 15Z 0.20 2908  92  83   6 0.01
Sun 05/04 18Z 0.23 3010  61  50   7 0.02
Sun 05/04 21Z 0.47 3211  54  44   6 0.01
Mon 05/05 00Z 0.17 3514  55  27   5 0.00

Run total areal avg QPF for the greens is: 0.65 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