Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260311_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 03/11 12Z 0.11 1625  84  84   8 0.00
Wed 03/11 13Z 0.11 1630  86  83   9 0.00
Wed 03/11 14Z 0.06 1733  85  72  10 0.00
Wed 03/11 15Z 0.04 1735  83  85  10 0.00
Wed 03/11 16Z 0.04 1836  85  92  10 0.02
Wed 03/11 17Z 0.10 1836  84  90  10 0.02
Wed 03/11 18Z 0.15 1937  82  69  11 0.00
Wed 03/11 19Z 0.19 1938  80  58  12 0.00
Wed 03/11 20Z 0.20 1936  80  75  12 0.00
Wed 03/11 21Z 0.22 1939  83  92  12 0.00
Wed 03/11 22Z 0.27 2041  90  88  11 0.01
Wed 03/11 23Z 0.35 2042  93  79  11 0.00
Thu 03/12 00Z 0.42 2043  94  85  11 0.01
Thu 03/12 02Z 0.53 2141  94  91  12 0.02
Thu 03/12 03Z 0.64 2139  97  83  10 0.03
Thu 03/12 04Z 0.84 2334  95  91  10 0.10
Thu 03/12 05Z 0.65 2327  88  78  10 0.11
Thu 03/12 06Z 0.70 2327  94  75   8 0.11
Thu 03/12 07Z 0.82 2328  97  67   8 0.00
Thu 03/12 08Z 0.97 2329  98  87   7 0.02
Thu 03/12 09Z 1.58 2633  98  91   3 0.04
Thu 03/12 10Z 1.03 2634  94  63   0 0.12
Thu 03/12 11Z 0.94 2732  90  54  -3 0.06
Thu 03/12 12Z 1.03 2732  91  36  -6 0.01
Thu 03/12 14Z 1.23 2730  91   4  -9 0.00
Thu 03/12 15Z 1.42 2728  91   9 -10 0.00
Thu 03/12 16Z 2.03 2728  93   3 -11 0.00
Thu 03/12 17Z 3.59 2827  93   2 -11 0.00
Thu 03/12 18Z 4.40 2827  91  11 -12 0.00
Thu 03/12 19Z 3.48 2924  89  24 -12 0.00
Thu 03/12 20Z 3.17 2923  88  20 -12 0.00
Thu 03/12 21Z 2.69 2922  88  28 -13 0.00
Thu 03/12 22Z 1.48 2921  88  36 -13 0.00
Thu 03/12 23Z 1.23 2922  89  41 -13 0.00
Fri 03/13 00Z 1.36 2924  90  50 -13 0.00
Fri 03/13 01Z 1.51 2925  92  55 -13 0.00
Fri 03/13 02Z 1.45 2925  92  55 -13 0.00
Fri 03/13 03Z 1.29 2924  93  55 -13 0.00
Fri 03/13 04Z 1.18 2924  94  53 -13 0.00
Fri 03/13 05Z 1.17 2924  94  54 -13 0.00
Fri 03/13 06Z 1.07 2922  93  50 -13 0.00
Fri 03/13 07Z 0.95 2921  94  40 -13 0.00
Fri 03/13 08Z 0.83 2918  93  30 -13 0.00
Fri 03/13 09Z 0.63 2815  92  16 -12 0.00
Fri 03/13 10Z 0.62 2716  91   8 -12 0.00
Fri 03/13 11Z 0.43 2613  86   4 -11 0.00
Fri 03/13 12Z 0.28 2510  79   3 -11 0.00
Fri 03/13 13Z 0.13 2108  71   7 -10 0.00
Fri 03/13 14Z 0.11 1911  64  11 -10 0.00
Fri 03/13 15Z 0.06 1814  54  21  -9 0.00
Fri 03/13 16Z 0.11 1916  50  54  -8 0.00
Fri 03/13 17Z 0.23 1919  52  86  -8 0.00
Fri 03/13 18Z 0.32 1822  60  89  -9 0.00
Fri 03/13 19Z 0.15 1725  92  88  -9 0.01
Fri 03/13 21Z 0.20 1832  94  88  -8 0.01
Fri 03/13 22Z 0.28 1931  93  81  -8 0.02
Fri 03/13 23Z 0.35 1931  92  91  -7 0.06

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