Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260315_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)
=== ===== === ==== ==== === === === ====
Sun 03/15 18Z 0.11 1613  68  95  -6 0.00
Sun 03/15 19Z 0.19 1619  65  97  -6 0.00
Sun 03/15 20Z 0.23 1624  62  97  -5 0.00
Sun 03/15 21Z 0.29 1628  59  94  -5 0.00
Sun 03/15 22Z 0.28 1632  59  87  -5 0.00
Sun 03/15 23Z 0.16 1737  64  81  -4 0.00
Mon 03/16 02Z 0.13 1845  55  68   0 0.00
Mon 03/16 06Z 0.07 1853  92  79   4 0.00
Mon 03/16 07Z 0.07 1854  92  64   4 0.00
Mon 03/16 09Z 0.08 1752  93  70   6 0.00
Mon 03/16 10Z 0.07 1752  94  85   7 0.00
Mon 03/16 11Z 0.09 1753  94  88   8 0.00
Mon 03/16 12Z 0.08 1752  95  93   9 0.01
Mon 03/16 13Z 0.06 1751  95  93   9 0.01
Mon 03/16 14Z 0.03 1849  97  53   9 0.01
Mon 03/16 15Z 0.07 1849  96  64  10 0.01
Mon 03/16 16Z 0.11 1848  94  62  10 0.02
Mon 03/16 17Z 0.16 1846  95  30  10 0.05
Mon 03/16 18Z 0.23 1944  95  29  10 0.05
Mon 03/16 19Z 0.24 1943  92  53  10 0.15
Mon 03/16 20Z 0.29 1942  91  49  10 0.05
Mon 03/16 21Z 0.23 1943  93  72  10 0.02
Mon 03/16 22Z 0.11 1842  94  84  10 0.01
Mon 03/16 23Z 0.11 1843  96  76   9 0.01
Tue 03/17 00Z 0.11 1846  93  55   9 0.01
Tue 03/17 01Z 0.08 1846  93  52   9 0.00
Tue 03/17 02Z 0.05 1746  95  63   9 0.00
Tue 03/17 03Z 0.12 1746  96  87   9 0.01
Tue 03/17 04Z 0.19 1745  96  95   9 0.02
Tue 03/17 05Z 0.17 1746  96  95   9 0.01
Tue 03/17 06Z 0.21 1743  96  93   9 0.01
Tue 03/17 07Z 0.70 2325  98  95   3 0.01
Tue 03/17 08Z 1.04 2527  96  96  -3 0.01
Tue 03/17 09Z 1.54 2531  96  61  -7 0.02
Tue 03/17 10Z 1.98 2431  96  16  -9 0.03
Tue 03/17 11Z 9.75 2532  86   8 -11 0.04
Tue 03/17 12Z 7.16 2433  76  17 -14 0.04
Tue 03/17 13Z 14.94 2432  74  20 -16 0.17
Tue 03/17 14Z 17.60 2432  75  17 -16 0.09
Tue 03/17 15Z 19.10 2431  74  18 -17 0.03
Tue 03/17 16Z 18.81 2429  73  24 -16 0.03
Tue 03/17 17Z 18.38 2429  73  25 -16 0.01
Tue 03/17 18Z 18.39 2429  73  28 -16 0.00
Tue 03/17 19Z 19.28 2529  72  35 -15 0.00
Tue 03/17 20Z 19.31 2530  76  46 -16 0.00
Tue 03/17 21Z 18.51 2530  78  52 -16 0.00
Tue 03/17 22Z 17.04 2630  81  49 -17 0.00
Tue 03/17 23Z 12.49 2630  84  50 -18 0.00
Wed 03/18 00Z 10.22 2728  86  54 -18 0.00
Wed 03/18 01Z 5.60 2726  86  62 -18 0.00
Wed 03/18 02Z 2.61 2724  86  64 -18 0.00
Wed 03/18 03Z 3.22 2723  87  62 -18 0.00
Wed 03/18 04Z 3.64 2722  86  63 -18 0.00
Wed 03/18 05Z 1.99 2723  83  46 -18 0.00

Run total areal avg QPF for the greens is: 0.94 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