Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260311_1800
Using the MB925 to MB850 layer.
Assumes Central_Greens are orthogonal to 260 degree winds.

925-850 mb Layer Average Wind/RH. Avg QPF over the Central_Greens.
Date   Time        Wind RH% 700 850T QPF
Day mm/dd hhZ Frd# DDFF (%) RH% (C) (in)
=== ===== === ==== ==== === === === ====
Wed 03/11 18Z 0.15 1832  75  84  11 0.00
Wed 03/11 19Z 0.19 1834  75  75  11 0.00
Wed 03/11 20Z 0.22 1835  77  90  12 0.00
Wed 03/11 21Z 0.26 1936  80  88  11 0.00
Wed 03/11 22Z 0.24 1838  93  90  11 0.03
Wed 03/11 23Z 0.36 1940  97  86  11 0.04
Thu 03/12 00Z 0.58 2142  99  95  11 0.09
Thu 03/12 01Z 0.84 2239  98  94  11 0.08
Thu 03/12 02Z 0.91 2239  98  93  11 0.09
Thu 03/12 03Z 0.94 2237  96  91  11 0.07
Thu 03/12 04Z 0.83 2233  97  82  11 0.10
Thu 03/12 05Z 1.05 2333  97  79  10 0.10
Thu 03/12 06Z 0.83 2327  96  79  10 0.06
Thu 03/12 07Z 0.92 2330  95  95   9 0.05
Thu 03/12 08Z 0.90 2330  96  85   9 0.08
Thu 03/12 09Z 1.37 2632  97  79   5 0.08
Thu 03/12 10Z 0.99 2632  93  55   1 0.04
Thu 03/12 11Z 0.94 2732  90  58  -2 0.01
Thu 03/12 12Z 0.85 2730  91  47  -4 0.00
Thu 03/12 13Z 1.00 2730  92  28  -7 0.00
Thu 03/12 14Z 1.07 2828  91  20  -8 0.00
Thu 03/12 15Z 1.15 2827  91  32  -9 0.00
Thu 03/12 16Z 1.65 2826  90  19 -10 0.00
Thu 03/12 17Z 2.51 2824  90  11 -10 0.00
Thu 03/12 18Z 1.94 2822  88  12 -10 0.00
Thu 03/12 19Z 2.17 2821  88  11 -11 0.00
Thu 03/12 20Z 2.32 2920  88  23 -11 0.00
Thu 03/12 21Z 1.84 2919  87  25 -11 0.00
Thu 03/12 22Z 1.91 2921  88  36 -12 0.00
Thu 03/12 23Z 1.22 2922  88  43 -12 0.00
Fri 03/13 00Z 1.16 2923  90  51 -12 0.00
Fri 03/13 01Z 1.30 2925  91  55 -12 0.00
Fri 03/13 02Z 1.17 2925  92  63 -12 0.00
Fri 03/13 03Z 1.04 2924  94  64 -12 0.00
Fri 03/13 04Z 1.05 2923  96  65 -12 0.00
Fri 03/13 05Z 1.05 2921  97  60 -13 0.00
Fri 03/13 06Z 1.18 2922  96  61 -13 0.00
Fri 03/13 07Z 1.06 2921  95  57 -13 0.00
Fri 03/13 08Z 1.00 2922  93  51 -12 0.00
Fri 03/13 09Z 0.75 2818  92  36 -12 0.00
Fri 03/13 10Z 0.49 2814  90  10 -12 0.00
Fri 03/13 11Z 0.53 2715  89   5 -12 0.00
Fri 03/13 12Z 0.27 2609  80   4 -10 0.00
Fri 03/13 13Z 0.26 2409  73   4 -10 0.00
Fri 03/13 14Z 0.13 2008  57  12  -9 0.00
Fri 03/13 15Z 0.11 1812  49  37  -8 0.00
Fri 03/13 16Z 0.26 1916  49  86  -8 0.00
Fri 03/13 17Z 0.41 1919  49  86  -8 0.00
Fri 03/13 18Z 2.82 1822  70  90 -10 0.00
Fri 03/13 19Z 0.14 1730  93  88  -9 0.04
Fri 03/13 20Z 0.24 1833  92  92  -9 0.05
Fri 03/13 21Z 0.43 1833  94  88  -9 0.02
Fri 03/13 22Z 0.50 1933  93  90  -8 0.01
Fri 03/13 23Z 0.53 1933  93  86  -7 0.00
Sat 03/14 00Z 0.52 1933  92  80  -6 0.00
Sat 03/14 01Z 0.43 1933  90  88  -5 0.00
Sat 03/14 02Z 0.61 2034  93  77  -5 0.01
Sat 03/14 03Z 0.91 2136  95  33  -5 0.02
Sat 03/14 04Z 1.18 2335  97  12  -6 0.01
Sat 03/14 05Z 1.79 2433  97   7  -8 0.02

Run total areal avg QPF for the Central_Greens is: 1.10 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