NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260218_1200
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 02/18 12Z 0.40 3017  83  36  -5 0.00
Wed 02/18 13Z 0.38 3119  72  40  -5 0.00
Wed 02/18 14Z 0.31 3118  64  42  -5 0.00
Wed 02/18 15Z 0.26 3116  55  39  -5 0.00
Wed 02/18 16Z 0.23 3215  47  34  -4 0.00
Wed 02/18 17Z 0.19 3213  41  33  -5 0.00
Wed 02/18 18Z 0.11 3311  37  31  -5 0.00
Wed 02/18 19Z 0.09 3309  37  30  -5 0.00
Wed 02/18 20Z 0.06 3408  37  26  -5 0.00
Wed 02/18 21Z 0.04 3508  39  24  -5 0.00
Wed 02/18 22Z 0.04 0009  39  15  -5 0.00
Wed 02/18 23Z 0.08 0110  35   9  -5 0.00
Thu 02/19 00Z 0.12 0211  32   9  -5 0.00
Thu 02/19 01Z 0.14 0312  32  11  -5 0.00
Thu 02/19 02Z 0.15 0312  32  13  -5 0.00
Thu 02/19 03Z 0.14 0312  33  14  -6 0.00
Thu 02/19 04Z 0.12 0211  32  18  -6 0.00
Thu 02/19 05Z 0.10 0211  30  22  -5 0.00
Thu 02/19 06Z 0.12 0211  28  23  -5 0.00
Thu 02/19 07Z 0.14 0311  27  27  -5 0.00
Thu 02/19 08Z 0.10 0308  25  35  -6 0.00
Thu 02/19 09Z 0.09 0306  23  39  -6 0.00
Thu 02/19 10Z 0.09 0405  22  39  -6 0.00
Thu 02/19 11Z 0.09 0404  20  40  -5 0.00
Thu 02/19 12Z 0.08 0404  20  36  -5 0.00
Thu 02/19 13Z 0.05 0502  19  29  -5 0.00
Thu 02/19 14Z 0.04 0701  19  28  -5 0.00
Thu 02/19 15Z 0.02 1200  21  26  -4 0.00
Thu 02/19 16Z 0.02 1800  24  31  -4 0.00
Thu 02/19 17Z 0.03 2101  28  34  -4 0.00
Thu 02/19 18Z 0.04 2202  33  38  -4 0.00
Thu 02/19 19Z 0.05 2103  38  39  -4 0.00
Thu 02/19 20Z 0.07 2204  41  35  -3 0.00
Thu 02/19 21Z 0.06 2203  44  34  -3 0.00
Thu 02/19 22Z 0.04 2003  47  36  -3 0.00
Thu 02/19 23Z 0.03 2003  51  34  -3 0.00
Fri 02/20 00Z 0.04 2004  56  30  -3 0.00
Fri 02/20 01Z 0.03 2003  61  30  -3 0.00
Fri 02/20 02Z 0.02 1804  65  35  -3 0.00
Fri 02/20 03Z 0.05 1907  69  35  -4 0.00
Fri 02/20 04Z 0.04 1907  75  30  -4 0.00
Fri 02/20 05Z 0.07 1908  80  31  -4 0.00
Fri 02/20 06Z 0.09 2009  84  31  -4 0.00
Fri 02/20 07Z 0.12 2010  87  32  -3 0.00
Fri 02/20 08Z 0.11 2010  87  31  -3 0.00
Fri 02/20 09Z 0.08 2009  85  27  -2 0.00
Fri 02/20 10Z 0.07 2008  84  21  -2 0.00
Fri 02/20 11Z 0.06 2006  85  22  -2 0.00
Fri 02/20 12Z 0.04 1806  84  26  -1 0.00
Fri 02/20 13Z 0.03 1707  84  29  -1 0.00
Fri 02/20 14Z 0.04 1608  82  29   0 0.00
Fri 02/20 15Z 0.05 1610  81  37   0 0.00
Fri 02/20 16Z 0.08 1612  81  57   0 0.00
Fri 02/20 17Z 0.12 1514  85  73   0 0.00
Fri 02/20 18Z 0.18 1417  90  82   0 0.01
Fri 02/20 19Z 0.26 1321  93  91   0 0.02
Fri 02/20 20Z 0.31 1323  95  97   0 0.04
Fri 02/20 21Z 0.35 1324  96  97  -1 0.12
Fri 02/20 22Z 0.40 1224  95  98  -2 0.15
Fri 02/20 23Z 0.42 1224  96  98  -3 0.12

Run total areal avg QPF for the greens is: 0.46 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