NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20251007_0000
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)
=== ===== === ==== ==== === === === ====
Tue 10/07 00Z 0.71 2317  52   6  14 0.00
Tue 10/07 03Z 1.21 2519  54   5  13 0.00
Tue 10/07 06Z 1.13 2619  62  10  13 0.00
Tue 10/07 09Z 0.82 2517  66  19  13 0.00
Tue 10/07 12Z 0.79 2418  65  22  13 0.00
Tue 10/07 15Z 0.60 2220  67  55  13 0.00
Tue 10/07 18Z 0.99 2123  78  73  13 0.00
Tue 10/07 21Z 0.71 2228  95  93  12 0.05
Wed 10/08 00Z 0.78 2230  95  80  12 0.13
Wed 10/08 03Z 0.67 2135  95  74  12 0.11
Wed 10/08 06Z 0.70 2139  97  92  12 0.26
Wed 10/08 09Z 0.43 3018  98  96   9 0.55
Wed 10/08 12Z 0.30 3321  85  77   5 0.05
Wed 10/08 15Z 0.36 3319  86  28   2 0.01
Wed 10/08 18Z 5.23 3220  81   4   1 0.00
Wed 10/08 21Z 2.78 3224  88   8   0 0.00
Thu 10/09 00Z 0.60 3326  93   5  -2 0.00
Thu 10/09 03Z 0.47 3325  94   7  -4 0.00
Thu 10/09 06Z 0.30 3424  94   9  -5 0.00
Thu 10/09 09Z 0.20 3423  89  10  -6 0.00
Thu 10/09 12Z 0.21 3426  83  13  -7 0.00
Thu 10/09 15Z 0.35 3423  77  13  -7 0.00
Thu 10/09 18Z 3.81 3320  70   6  -6 0.00
Thu 10/09 21Z 0.34 3423  77  11  -7 0.00
Fri 10/10 00Z 0.73 3323  55   4  -4 0.00
Fri 10/10 03Z 0.25 3419  48   7  -2 0.00
Fri 10/10 06Z 0.19 3314  39  10  -1 0.00
Fri 10/10 09Z 0.27 3115  30  13   0 0.00
Fri 10/10 12Z 0.29 3016  25  12   1 0.00

Run total areal avg QPF for the greens is: 1.16 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