Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20261015_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)
=== ===== === ==== ==== === === === ====
Thu 10/15 18Z 0.10 3404  76   6   1 0.00
Thu 10/15 19Z 0.12 3303  75   5   1 0.00
Thu 10/15 20Z 0.12 3104  73   3   2 0.00
Thu 10/15 21Z 0.09 3003  67   2   2 0.00
Thu 10/15 22Z 0.05 3002  58   3   3 0.00
Thu 10/15 23Z 0.04 2801  48   3   4 0.00
Fri 10/16 00Z 0.02 1901  43   3   5 0.00
Fri 10/16 01Z 0.02 1805  43   4   5 0.00
Fri 10/16 02Z 0.03 1808  45   4   6 0.00
Fri 10/16 03Z 0.03 1811  47   4   6 0.00
Fri 10/16 04Z 0.04 1815  52  10   6 0.00
Fri 10/16 05Z 0.05 1818  57  28   6 0.00
Fri 10/16 06Z 0.07 1822  63  48   5 0.00
Fri 10/16 07Z 0.05 1826  65  82   5 0.00
Fri 10/16 08Z 0.08 1829  70  96   4 0.00
Fri 10/16 09Z 0.09 1830  79  98   4 0.01
Fri 10/16 10Z 0.07 1831  89  98   5 0.03
Fri 10/16 11Z 0.08 1834  96  98   5 0.11
Fri 10/16 12Z 0.07 1839  98  99   5 0.17
Fri 10/16 13Z 0.14 1842  98  97   6 0.20
Fri 10/16 14Z 0.27 1941  98  96   7 0.15
Fri 10/16 15Z 0.37 2041  98  86   9 0.07
Fri 10/16 16Z 0.41 2139  98  90  10 0.06
Fri 10/16 17Z 0.44 2136  98  93  12 0.07
Fri 10/16 18Z 0.49 2232  98  95  13 0.07
Fri 10/16 19Z 0.58 2331  97  94  13 0.07
Fri 10/16 20Z 0.58 2330  94  85  13 0.05
Fri 10/16 21Z 0.68 2332  95  80  12 0.02
Fri 10/16 22Z 0.83 2331  93  75  11 0.03
Fri 10/16 23Z 1.04 2334  97  68  10 0.05
Sat 10/17 00Z 1.18 2431  98  66   9 0.06
Sat 10/17 01Z 1.06 2631  96  66   8 0.08
Sat 10/17 02Z 0.98 2730  96  44   6 0.05
Sat 10/17 03Z 0.89 2828  97  55   3 0.03
Sat 10/17 04Z 0.84 2928  96  61   0 0.02
Sat 10/17 05Z 0.89 3028  97  55  -1 0.02
Sat 10/17 06Z 0.68 3227  96  54  -2 0.02
Sat 10/17 07Z 0.58 3228  97  54  -3 0.02
Sat 10/17 08Z 0.56 3328  97  53  -3 0.02
Sat 10/17 09Z 0.52 3329  97  55  -4 0.01
Sat 10/17 10Z 0.54 3329  97  59  -5 0.01
Sat 10/17 11Z 0.66 3329  98  65  -5 0.02
Sat 10/17 12Z 0.69 3330  98  69  -6 0.02
Sat 10/17 13Z 0.73 3329  97  70  -7 0.01
Sat 10/17 14Z 0.77 3227  97  72  -7 0.02
Sat 10/17 15Z 0.63 3226  97  58  -8 0.01
Sat 10/17 16Z 0.60 3225  95  32  -8 0.01
Sat 10/17 17Z 1.17 3225  89  10  -9 0.00
Sat 10/17 18Z 5.84 3226  79   2  -9 0.00
Sat 10/17 19Z 10.93 3227  72   4  -9 0.00
Sat 10/17 20Z 10.59 3227  74   5  -9 0.00
Sat 10/17 21Z 7.80 3127  78   5  -9 0.00
Sat 10/17 22Z 5.30 3128  83   4  -9 0.00
Sat 10/17 23Z 2.16 3129  82   6 -10 0.00
Sun 10/18 00Z 2.27 3129  80   5 -10 0.00
Sun 10/18 01Z 1.94 3129  74   5 -10 0.00
Sun 10/18 02Z 1.06 3128  62   6 -10 0.00
Sun 10/18 03Z 0.86 3227  54   9  -9 0.00
Sun 10/18 04Z 0.76 3126  48  14  -9 0.00
Sun 10/18 05Z 0.75 3126  52  13  -8 0.00

Run total areal avg QPF for the greens is: 1.59 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