NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260417_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)
=== ===== === ==== ==== === === === ====
Fri 04/17 06Z 0.31 3012  84  87   9 0.00
Fri 04/17 09Z 0.10 3514  85  83   9 0.00
Fri 04/17 12Z 0.11 0117  78  75   8 0.00
Fri 04/17 15Z 0.15 0213  70  57   8 0.00
Fri 04/17 18Z 0.11 0211  60  60   8 0.00
Fri 04/17 21Z 0.10 0211  45  15   9 0.00
Sat 04/18 00Z 0.16 0410  37   4  10 0.00
Sat 04/18 03Z 0.22 1010  45   6   9 0.00
Sat 04/18 06Z 0.31 1316  70   4   8 0.00
Sat 04/18 09Z 0.20 1517  68   3   7 0.00
Sat 04/18 12Z 0.13 1619  68   3   7 0.00
Sat 04/18 15Z 0.16 1622  72   8   7 0.00
Sat 04/18 18Z 0.27 1622  81  18   5 0.00
Sat 04/18 21Z 0.08 1725  71   6  10 0.00
Sun 04/19 00Z 0.01 1827  71  30  11 0.00
Sun 04/19 03Z 0.18 2031  76  38  12 0.00
Sun 04/19 06Z 0.35 2131  98  97   9 0.12
Sun 04/19 09Z 0.48 2324  98  93   6 0.09
Sun 04/19 12Z 0.58 2717  98  96   1 0.09
Sun 04/19 15Z 0.42 3022  97  97  -2 0.23
Sun 04/19 18Z 0.55 3022  94  93  -5 0.29
Sun 04/19 21Z 0.61 3021  93  78  -7 0.11
Mon 04/20 00Z 2.72 3025  87  28  -8 0.00
Mon 04/20 03Z 0.73 3023  91  66  -7 0.00
Mon 04/20 06Z 0.85 3220  70  71 -10 0.04
Mon 04/20 09Z 0.36 3313  66  79 -11 0.00
Mon 04/20 12Z 0.56 3211  75  82 -11 0.00
Mon 04/20 15Z 4.96 2912  83  66 -12 0.00
Mon 04/20 18Z 8.87 2913  68  59 -10 0.00

Run total areal avg QPF for the greens is: 0.97 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