Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260311_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)
=== ===== === ==== ==== === === === ====
Wed 03/11 18Z 0.09 1834  76  83  10 0.00
Wed 03/11 19Z 0.11 1936  78  80  11 0.00
Wed 03/11 20Z 0.14 1937  78  90  12 0.00
Wed 03/11 21Z 0.16 1938  82  87  11 0.00
Wed 03/11 22Z 0.15 1940  92  91  11 0.02
Wed 03/11 23Z 0.24 1942  96  89  11 0.04
Thu 03/12 00Z 0.41 2043  98  96  11 0.08
Thu 03/12 01Z 0.65 2242  98  95  11 0.10
Thu 03/12 02Z 0.80 2240  97  92  11 0.08
Thu 03/12 03Z 0.81 2237  96  91  11 0.07
Thu 03/12 04Z 0.70 2232  97  83  11 0.08
Thu 03/12 05Z 0.92 2332  97  79  10 0.07
Thu 03/12 06Z 0.77 2327  96  72  10 0.04
Thu 03/12 07Z 0.83 2329  95  92   9 0.04
Thu 03/12 08Z 0.80 2329  97  88   9 0.08
Thu 03/12 09Z 1.25 2634  96  84   5 0.09
Thu 03/12 10Z 1.05 2633  93  60   0 0.05
Thu 03/12 11Z 1.10 2633  91  53  -3 0.01
Thu 03/12 12Z 0.98 2730  92  38  -5 0.00
Thu 03/12 13Z 1.20 2730  94  23  -7 0.00
Thu 03/12 14Z 1.33 2728  93  15  -9 0.00
Thu 03/12 15Z 1.57 2828  92  22 -10 0.00
Thu 03/12 16Z 2.20 2827  92  13 -11 0.00
Thu 03/12 17Z 2.81 2825  92   8 -11 0.00
Thu 03/12 18Z 2.52 2823  90  12 -12 0.00
Thu 03/12 19Z 2.26 2823  90  15 -12 0.00
Thu 03/12 20Z 2.34 2921  90  25 -12 0.00
Thu 03/12 21Z 2.04 2921  89  26 -12 0.00
Thu 03/12 22Z 2.00 2922  90  36 -13 0.00
Thu 03/12 23Z 1.48 2923  90  41 -13 0.00
Fri 03/13 00Z 1.47 2925  91  48 -13 0.00
Fri 03/13 01Z 1.66 2927  92  52 -13 0.00
Fri 03/13 02Z 1.51 2927  93  60 -13 0.00
Fri 03/13 03Z 1.36 2926  94  62 -13 0.00
Fri 03/13 04Z 1.46 2924  95  64 -13 0.00
Fri 03/13 05Z 1.36 3022  95  60 -13 0.00
Fri 03/13 06Z 1.26 2922  95  60 -13 0.00
Fri 03/13 07Z 1.09 2922  94  56 -13 0.00
Fri 03/13 08Z 1.02 2922  93  52 -13 0.00
Fri 03/13 09Z 0.74 2819  90  40 -12 0.00
Fri 03/13 10Z 0.52 2815  89  19 -12 0.00
Fri 03/13 11Z 0.57 2717  89  13 -12 0.00
Fri 03/13 12Z 0.31 2710  82   5 -11 0.00
Fri 03/13 13Z 0.25 2409  76   4 -10 0.00
Fri 03/13 14Z 0.09 2008  63  10  -9 0.00
Fri 03/13 15Z 0.04 1813  56  30  -8 0.00
Fri 03/13 16Z 0.11 1817  55  81  -8 0.00
Fri 03/13 17Z 0.22 1920  53  86  -8 0.00
Fri 03/13 18Z 0.59 1823  68  89 -10 0.00
Fri 03/13 19Z 0.13 1729  91  88  -9 0.03
Fri 03/13 20Z 0.11 1733  92  92  -9 0.05
Fri 03/13 21Z 0.15 1833  93  88  -9 0.03
Fri 03/13 22Z 0.25 1933  94  90  -8 0.01
Fri 03/13 23Z 0.32 1934  93  88  -7 0.01
Sat 03/14 00Z 0.33 1934  92  81  -6 0.00
Sat 03/14 01Z 0.27 1934  90  89  -5 0.00
Sat 03/14 02Z 0.38 2034  93  82  -5 0.01
Sat 03/14 03Z 0.61 2134  95  54  -5 0.02
Sat 03/14 04Z 0.88 2233  97  34  -6 0.02
Sat 03/14 05Z 1.44 2332  97  28  -8 0.02

Run total areal avg QPF for the greens is: 1.05 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