NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260409_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 04/09 18Z 0.28 1920  32  36   3 0.00
Thu 04/09 19Z 0.34 1924  34  30   3 0.00
Thu 04/09 20Z 0.34 1926  33  33   4 0.00
Thu 04/09 21Z 0.33 1928  31  43   4 0.00
Thu 04/09 22Z 0.32 2029  31  50   5 0.00
Fri 04/10 00Z 0.34 2131  34  54   7 0.00
Fri 04/10 01Z 0.44 2233  39  59   7 0.00
Fri 04/10 02Z 0.52 2334  45  76   7 0.00
Fri 04/10 03Z 0.58 2334  54  78   7 0.00
Fri 04/10 04Z 0.65 2435  64  64   6 0.00
Fri 04/10 05Z 0.69 2433  71  61   6 0.00
Fri 04/10 06Z 0.69 2532  73  43   6 0.00
Fri 04/10 07Z 0.67 2629  77  27   5 0.00
Fri 04/10 08Z 0.63 2626  79   9   5 0.00
Fri 04/10 09Z 0.60 2624  82   5   4 0.00
Fri 04/10 10Z 0.57 2523  87   4   4 0.00
Fri 04/10 11Z 0.56 2522  88   8   4 0.00
Fri 04/10 13Z 0.48 2420  88  15   5 0.00
Fri 04/10 14Z 0.45 2420  89  45   5 0.00
Fri 04/10 15Z 0.43 2321  87  68   6 0.00
Fri 04/10 16Z 0.42 2321  89  95   6 0.00
Fri 04/10 17Z 0.43 2222  88  85   6 0.00
Fri 04/10 18Z 0.46 2223  84  55   7 0.00
Fri 04/10 19Z 0.55 2226  79  41   8 0.01
Fri 04/10 20Z 0.64 2229  75  39   8 0.01
Fri 04/10 22Z 0.77 2231  76  73   8 0.00
Fri 04/10 23Z 0.84 2232  85  96   7 0.00
Sat 04/11 00Z 0.79 2430  96  94   6 0.01
Sat 04/11 01Z 0.71 2527  98  91   5 0.00
Sat 04/11 02Z 0.61 2623  98  90   4 0.03
Sat 04/11 03Z 0.51 2821  98  80   3 0.10
Sat 04/11 04Z 0.43 2920  98  83   1 0.10
Sat 04/11 05Z 0.48 3023  97  80   0 0.05
Sat 04/11 06Z 0.53 3025  96  46  -1 0.04
Sat 04/11 07Z 0.60 3027  92  23  -3 0.03
Sat 04/11 08Z 0.60 3028  86   8  -3 0.03
Sat 04/11 09Z 0.63 3029  86   4  -4 0.02
Sat 04/11 10Z 0.66 3029  86   5  -5 0.01
Sat 04/11 11Z 0.62 3029  81   5  -5 0.01
Sat 04/11 12Z 0.58 3128  81   7  -6 0.01
Sat 04/11 13Z 0.60 3127  81   9  -6 0.01
Sat 04/11 14Z 0.59 3126  80  10  -6 0.01
Sat 04/11 15Z 0.63 3125  82  10  -7 0.01
Sat 04/11 16Z 0.73 3124  84  12  -7 0.01
Sat 04/11 17Z 0.99 3123  84  13  -7 0.01
Sat 04/11 18Z 1.63 3122  84  13  -7 0.00
Sat 04/11 19Z 1.42 3121  83  12  -7 0.00
Sat 04/11 21Z 2.25 3120  80  10  -6 0.00
Sat 04/11 22Z 1.75 3120  78   9  -6 0.00
Sat 04/11 23Z 1.31 3121  76   9  -6 0.00
Sun 04/12 00Z 1.09 3122  75   9  -6 0.00
Sun 04/12 01Z 0.82 3122  74   7  -6 0.00
Sun 04/12 02Z 0.57 3222  69   8  -5 0.00
Sun 04/12 03Z 0.42 3221  63   8  -5 0.00
Sun 04/12 04Z 0.35 3220  59   8  -4 0.00
Sun 04/12 05Z 0.29 3218  55   8  -4 0.00

Run total areal avg QPF for the greens is: 0.51 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