NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260309_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)
=== ===== === ==== ==== === === === ====
Mon 03/09 18Z 0.75 2228  38  40   6 0.00
Mon 03/09 19Z 0.80 2231  35  36   7 0.00
Mon 03/09 20Z 0.87 2232  32  37   7 0.00
Mon 03/09 21Z 0.94 2232  33  41   7 0.00
Mon 03/09 22Z 0.97 2334  36  46   7 0.00
Mon 03/09 23Z 0.95 2335  37  48   7 0.00
Tue 03/10 00Z 1.03 2337  38  50   7 0.00
Tue 03/10 01Z 1.10 2438  40  46   7 0.00
Tue 03/10 02Z 1.17 2438  41  46   7 0.00
Tue 03/10 03Z 1.21 2537  41  44   7 0.00
Tue 03/10 04Z 1.30 2637  42  40   7 0.00
Tue 03/10 05Z 1.41 2635  46  39   6 0.00
Tue 03/10 06Z 1.42 2732  53  40   6 0.00
Tue 03/10 07Z 1.22 2829  59  44   5 0.00
Tue 03/10 08Z 0.97 2825  63  46   5 0.00
Tue 03/10 09Z 0.72 2821  66  50   6 0.00
Tue 03/10 10Z 0.55 2718  67  54   7 0.00
Tue 03/10 11Z 0.44 2615  66  59   7 0.00
Tue 03/10 12Z 0.37 2514  65  62   7 0.00
Tue 03/10 13Z 0.35 2414  64  60   8 0.00
Tue 03/10 14Z 0.39 2317  64  50   7 0.00
Tue 03/10 15Z 0.43 2219  64  45   7 0.00
Tue 03/10 16Z 0.51 2222  65  42   7 0.00
Tue 03/10 17Z 0.63 2223  66  45   7 0.00
Tue 03/10 18Z 0.72 2223  68  45   7 0.00
Tue 03/10 19Z 0.75 2222  72  41   7 0.00
Tue 03/10 20Z 0.86 2321  78  39   7 0.00
Tue 03/10 21Z 0.74 2421  82  46   7 0.00
Tue 03/10 22Z 0.70 2423  87  42   8 0.00
Wed 03/11 00Z 0.62 2523  96  51   8 0.01
Wed 03/11 01Z 0.54 2521  98  63   8 0.02
Wed 03/11 02Z 0.45 2619  98  69   8 0.02
Wed 03/11 03Z 0.35 2716  98  73   7 0.03
Wed 03/11 04Z 0.25 2913  98  67   7 0.03
Wed 03/11 05Z 0.17 3010  98  74   7 0.02
Wed 03/11 06Z 0.11 3006  97  76   7 0.01
Wed 03/11 07Z 0.07 2903  97  74   7 0.01
Wed 03/11 08Z 0.06 2101  97  74   7 0.00
Wed 03/11 09Z 0.07 1705  98  75   7 0.01
Wed 03/11 10Z 0.06 1709  98  75   7 0.02
Wed 03/11 11Z 0.05 1714  98  87   8 0.01
Wed 03/11 12Z 0.03 1720  98  82   8 0.01
Wed 03/11 13Z 0.05 1726  97  75   9 0.02
Wed 03/11 14Z 0.02 1730  97  70   9 0.03
Wed 03/11 15Z 0.04 1833  97  69   9 0.03
Wed 03/11 16Z 0.10 1837  97  81   9 0.03
Wed 03/11 17Z 0.18 1939  97  59   9 0.03
Wed 03/11 18Z 0.21 1938  96  65  10 0.03
Wed 03/11 19Z 0.20 1940  96  61  10 0.02
Wed 03/11 20Z 0.24 2042  96  79  10 0.01
Wed 03/11 21Z 0.26 2043  97  88  10 0.02
Wed 03/11 22Z 0.24 2040  93  89  11 0.04
Wed 03/11 23Z 0.25 2041  95  91  11 0.03
Thu 03/12 00Z 0.27 2043  94  96  11 0.01
Thu 03/12 01Z 0.27 2045  95  97  11 0.03
Thu 03/12 02Z 0.40 2047  97  92  11 0.08
Thu 03/12 03Z 0.58 2148  98  86  11 0.12
Thu 03/12 04Z 0.73 2243  98  84  10 0.06
Thu 03/12 05Z 0.68 2239  96  91  11 0.05

Run total areal avg QPF for the greens is: 0.84 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