NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260410_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 04/10 06Z 0.58 2429  53  44   6 0.00
Fri 04/10 09Z 0.73 2529  77  52   5 0.00
Fri 04/10 12Z 0.52 2520  77  47   6 0.00
Fri 04/10 15Z 0.43 2225  71  89   7 0.00
Fri 04/10 18Z 0.58 2225  62  88   9 0.00
Fri 04/10 21Z 0.63 2221  64  51   8 0.00
Sat 04/11 00Z 0.57 2225  79  89   7 0.01
Sat 04/11 03Z 0.45 2616  96  97   3 0.16
Sat 04/11 06Z 0.37 3223  98  90   0 0.18
Sat 04/11 09Z 0.87 3129  95  11  -6 0.02
Sat 04/11 12Z 0.91 3129  91   6  -8 0.00
Sat 04/11 15Z 2.00 3124  91   9  -8 0.00
Sat 04/11 18Z 7.00 3021  88  13  -7 0.00
Sat 04/11 21Z 8.61 3121  81  10  -7 0.00
Sun 04/12 00Z 2.60 3124  78  11  -7 0.00
Sun 04/12 03Z 4.58 3225  75  14  -7 0.00
Sun 04/12 06Z 0.69 3322  70  14  -7 0.00
Sun 04/12 09Z 0.29 3314  66  21  -6 0.00
Sun 04/12 12Z 0.20 2907  59  26  -4 0.00
Sun 04/12 15Z 0.21 2208  52  29  -3 0.00
Sun 04/12 18Z 0.27 2014  46  26  -1 0.00
Sun 04/12 21Z 0.53 1920  90  97  -2 0.01
Mon 04/13 00Z 0.41 2136  98  98   1 0.36
Mon 04/13 03Z 0.29 2023  78  73   0 0.26
Mon 04/13 06Z 0.76 2546  95  94   9 0.21
Mon 04/13 09Z 0.93 2647  96  95  10 0.07
Mon 04/13 12Z 0.94 2645  97  94  10 0.16
Mon 04/13 15Z 1.00 2643  96  93  10 0.08
Mon 04/13 18Z 1.07 2638  97  69   9 0.03

Run total areal avg QPF for the greens is: 1.55 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