Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260114_0600
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 01/14 06Z 0.46 2129  79  66  -2 0.00
Wed 01/14 07Z 0.59 2232  77  77  -1 0.00
Wed 01/14 08Z 0.65 2232  76  74  -1 0.00
Wed 01/14 09Z 0.66 2330  78  80  -1 0.00
Wed 01/14 10Z 0.66 2330  78  83  -1 0.00
Wed 01/14 11Z 0.67 2329  78  72  -1 0.00
Wed 01/14 13Z 0.63 2327  80  79   0 0.00
Wed 01/14 14Z 0.62 2327  79  68   0 0.00
Wed 01/14 15Z 0.57 2325  81  61   0 0.00
Wed 01/14 16Z 0.55 2224  81  62   0 0.00
Wed 01/14 17Z 0.56 2223  81  62   0 0.00
Wed 01/14 18Z 0.61 2222  81  65   0 0.00
Wed 01/14 19Z 0.59 2222  82  64   0 0.00
Wed 01/14 20Z 0.62 2221  83  75  -1 0.00
Wed 01/14 22Z 0.45 2118  87  91  -1 0.00
Wed 01/14 23Z 0.37 2117  93  93  -1 0.00
Thu 01/15 00Z 0.32 2117  95  95  -1 0.00
Thu 01/15 01Z 0.26 2116  97  95  -1 0.01
Thu 01/15 02Z 0.16 2014  98  94  -1 0.01
Thu 01/15 03Z 0.08 1813  98  93  -1 0.01
Thu 01/15 04Z 0.07 1712  97  94  -1 0.02
Thu 01/15 05Z 0.07 1713  96  94  -1 0.03
Thu 01/15 07Z 0.06 1715  98  95  -1 0.03
Thu 01/15 08Z 0.05 1715  98  95  -1 0.03
Thu 01/15 09Z 0.05 1715  98  95  -1 0.03
Thu 01/15 10Z 0.04 1816  98  94  -1 0.03
Thu 01/15 11Z 0.07 1815  98  95  -1 0.04
Thu 01/15 12Z 0.11 1915  99  93  -2 0.04
Thu 01/15 13Z 0.23 2016  99  90  -2 0.04
Thu 01/15 14Z 0.35 2116  98  79  -3 0.03
Thu 01/15 15Z 0.44 2216  98  73  -3 0.03
Thu 01/15 16Z 0.40 2316  97  74  -4 0.03
Thu 01/15 17Z 0.37 2317  96  57  -5 0.02
Thu 01/15 18Z 0.42 2418  95  40  -7 0.02
Thu 01/15 19Z 0.79 2519  95  22 -10 0.02
Thu 01/15 20Z 1.61 2519  93  14 -12 0.02
Thu 01/15 21Z 2.65 2521  91  20 -14 0.02
Thu 01/15 22Z 2.88 2521  91  35 -15 0.01
Thu 01/15 23Z 2.94 2522  91  48 -16 0.01
Fri 01/16 00Z 3.97 2521  89  64 -17 0.01
Fri 01/16 01Z 3.13 2522  86  67 -17 0.01
Fri 01/16 02Z 3.75 2522  86  68 -18 0.01
Fri 01/16 03Z 4.43 2522  85  72 -19 0.01
Fri 01/16 04Z 4.84 2622  85  75 -20 0.01
Fri 01/16 05Z 4.21 2621  87  77 -20 0.01
Fri 01/16 06Z 5.33 2722  88  77 -21 0.01
Fri 01/16 08Z 1.53 2824  88  76 -21 0.01
Fri 01/16 09Z 1.01 2825  88  68 -20 0.01
Fri 01/16 10Z 0.91 2826  88  38 -20 0.01
Fri 01/16 11Z 0.71 2825  88  21 -20 0.01
Fri 01/16 12Z 0.53 2924  85  14 -18 0.01
Fri 01/16 13Z 0.42 2923  72  11 -17 0.01
Fri 01/16 14Z 0.34 3023  55  13 -14 0.00
Fri 01/16 15Z 0.30 3021  44   9 -13 0.00
Fri 01/16 16Z 0.27 3018  41   7 -13 0.00
Fri 01/16 17Z 0.27 2916  38   9 -12 0.00

Run total areal avg QPF for the greens is: 0.66 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