Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260219_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)
=== ===== === ==== ==== === === === ====
Thu 02/19 00Z 0.08 0209  52  28  -6 0.00
Thu 02/19 01Z 0.16 0212  53  40  -6 0.00
Thu 02/19 02Z 0.21 0314  54  47  -7 0.00
Thu 02/19 03Z 0.21 0314  54  48  -7 0.00
Thu 02/19 04Z 0.19 0313  51  43  -7 0.00
Thu 02/19 05Z 0.12 0310  46  38  -7 0.00
Thu 02/19 06Z 0.12 0309  42  36  -6 0.00
Thu 02/19 07Z 0.13 0408  37  32  -6 0.00
Thu 02/19 08Z 0.10 0406  33  32  -6 0.00
Thu 02/19 09Z 0.08 0305  30  25  -6 0.00
Thu 02/19 10Z 0.06 0304  27  17  -6 0.00
Thu 02/19 11Z 0.04 0303  25  11  -5 0.00
Thu 02/19 12Z 0.05 0303  24  11  -5 0.00
Thu 02/19 13Z 0.04 0302  24  13  -5 0.00
Thu 02/19 14Z 0.04 0502  24  14  -5 0.00
Thu 02/19 15Z 0.04 0701  25  13  -4 0.00
Thu 02/19 16Z 0.03 1001  28  23  -4 0.00
Thu 02/19 17Z 0.01 1701  32  29  -4 0.00
Thu 02/19 18Z 0.02 1802  36  33  -4 0.00
Thu 02/19 19Z 0.03 1903  40  40  -4 0.00
Thu 02/19 20Z 0.05 2103  44  36  -4 0.00
Thu 02/19 21Z 0.06 2104  47  34  -4 0.00
Thu 02/19 22Z 0.12 2305  51  38  -4 0.00
Thu 02/19 23Z 0.11 2205  53  45  -4 0.00
Fri 02/20 00Z 0.06 2104  58  46  -3 0.00
Fri 02/20 01Z 0.02 1903  62  38  -3 0.00
Fri 02/20 02Z 0.04 1904  64  25  -4 0.00
Fri 02/20 03Z 0.07 2005  69  21  -4 0.00
Fri 02/20 04Z 0.07 2006  74  14  -4 0.00
Fri 02/20 05Z 0.08 2006  79  12  -5 0.00
Fri 02/20 06Z 0.10 2007  82  13  -5 0.00
Fri 02/20 07Z 0.09 2007  82  13  -4 0.00
Fri 02/20 08Z 0.07 2006  83  14  -4 0.00
Fri 02/20 09Z 0.05 1906  81  19  -3 0.00
Fri 02/20 10Z 0.04 1905  80  28  -3 0.00
Fri 02/20 11Z 0.02 1805  79  26  -3 0.00
Fri 02/20 12Z 0.03 1706  80  28  -3 0.00
Fri 02/20 13Z 0.06 1608  82  29  -2 0.00
Fri 02/20 14Z 0.07 1611  84  30  -2 0.00
Fri 02/20 15Z 0.08 1613  83  33  -2 0.00
Fri 02/20 16Z 0.07 1614  82  38  -3 0.00
Fri 02/20 17Z 0.10 1615  78  40  -2 0.00
Fri 02/20 18Z 0.11 1616  75  56  -2 0.00
Fri 02/20 19Z 0.15 1516  78  77  -1 0.00
Fri 02/20 20Z 0.18 1417  85  90  -1 0.01
Fri 02/20 21Z 0.24 1420  89  96  -1 0.05
Fri 02/20 22Z 0.30 1322  94  97  -1 0.09
Fri 02/20 23Z 0.36 1326  95  99  -1 0.13
Sat 02/21 00Z 0.37 1328  95  99  -2 0.11
Sat 02/21 01Z 0.36 1328  95  97  -2 0.08
Sat 02/21 02Z 0.36 1328  95  85  -2 0.04
Sat 02/21 03Z 0.34 1425  96  96  -3 0.04
Sat 02/21 04Z 0.31 1422  95  94  -4 0.06
Sat 02/21 05Z 0.25 1520  95  94  -6 0.05
Sat 02/21 06Z 0.17 1618  95  93  -6 0.02
Sat 02/21 07Z 0.07 1717  95  93  -6 0.02
Sat 02/21 08Z 0.05 1814  95  93  -7 0.02
Sat 02/21 09Z 0.08 1813  96  95  -7 0.02
Sat 02/21 10Z 0.12 1912  96  96  -7 0.02
Sat 02/21 11Z 0.15 2012  97  97  -7 0.02

Run total areal avg QPF for the greens is: 0.78 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