Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260115_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)
=== ===== === ==== ==== === === === ====
Thu 01/15 06Z 0.14 1410  97  92  -1 0.00
Thu 01/15 07Z 0.15 1511  97  95  -1 0.03
Thu 01/15 08Z 0.14 1512  96  93  -1 0.03
Thu 01/15 09Z 0.11 1613  95  90  -1 0.02
Thu 01/15 10Z 0.08 1612  94  84  -1 0.01
Thu 01/15 11Z 0.07 1712  95  81  -1 0.01
Thu 01/15 13Z 0.06 1811  95  92  -1 0.01
Thu 01/15 14Z 0.09 1912  97  92  -2 0.01
Thu 01/15 15Z 0.18 2012  99  92  -2 0.02
Thu 01/15 16Z 0.25 2112  98  80  -2 0.02
Thu 01/15 17Z 0.32 2214  98  71  -3 0.01
Thu 01/15 18Z 0.34 2216  97  56  -3 0.01
Thu 01/15 19Z 0.48 2216  97  40  -4 0.01
Thu 01/15 20Z 0.51 2318  97  19  -6 0.01
Thu 01/15 21Z 0.50 2419  95  13  -8 0.01
Thu 01/15 22Z 0.56 2519  91   9 -10 0.01
Thu 01/15 23Z 0.79 2619  87  18 -12 0.01
Fri 01/16 00Z 1.37 2619  88  26 -14 0.01
Fri 01/16 01Z 1.67 2618  87  41 -15 0.01
Fri 01/16 02Z 1.65 2618  85  55 -16 0.01
Fri 01/16 03Z 2.50 2518  86  61 -17 0.00
Fri 01/16 04Z 3.70 2619  88  50 -18 0.00
Fri 01/16 05Z 4.75 2721  88  47 -18 0.00
Fri 01/16 06Z 6.10 2720  85  51 -20 0.00
Fri 01/16 07Z 4.16 2819  85  74 -20 0.00
Fri 01/16 08Z 2.66 2820  86  74 -20 0.00
Fri 01/16 09Z 1.29 2821  87  75 -20 0.00
Fri 01/16 10Z 0.93 2823  87  74 -20 0.00
Fri 01/16 11Z 0.72 2824  89  66 -19 0.01
Fri 01/16 12Z 0.63 2924  90  59 -18 0.01
Fri 01/16 13Z 0.57 2825  88  50 -17 0.00
Fri 01/16 14Z 0.52 2825  84  35 -17 0.00
Fri 01/16 15Z 0.48 2823  79  17 -16 0.00
Fri 01/16 16Z 0.43 2821  71   7 -15 0.00
Fri 01/16 17Z 0.41 2820  58   7 -14 0.00
Fri 01/16 18Z 0.40 2820  45   5 -12 0.00
Fri 01/16 19Z 0.40 2820  40   5 -12 0.00
Fri 01/16 20Z 0.39 2818  39   7 -12 0.00
Fri 01/16 21Z 0.33 2715  37  11 -11 0.00
Fri 01/16 22Z 0.31 2713  34  25 -11 0.00
Fri 01/16 23Z 0.25 2611  32  56 -10 0.00
Sat 01/17 00Z 0.21 2410  31  76 -10 0.00
Sat 01/17 01Z 0.19 2211  32  92 -10 0.00
Sat 01/17 02Z 0.28 2215  46  91 -11 0.00
Sat 01/17 03Z 0.32 2117  77  87 -12 0.00
Sat 01/17 04Z 0.36 2119  88  86 -12 0.00
Sat 01/17 05Z 0.41 2219  91  86 -12 0.01
Sat 01/17 06Z 0.49 2222  90  78 -12 0.01
Sat 01/17 07Z 0.47 2221  88  36 -11 0.00
Sat 01/17 08Z 0.46 2220  85  38 -11 0.00
Sat 01/17 09Z 0.38 2218  83  46 -10 0.00
Sat 01/17 10Z 0.37 2218  82  42 -10 0.00
Sat 01/17 11Z 0.35 2218  81  50  -9 0.00
Sat 01/17 12Z 0.37 2218  80  61  -9 0.00
Sat 01/17 13Z 0.36 2218  80  67  -9 0.00
Sat 01/17 14Z 0.39 2219  81  71  -9 0.00
Sat 01/17 15Z 0.39 2219  81  72  -8 0.00
Sat 01/17 16Z 0.41 2218  81  77  -8 0.00
Sat 01/17 17Z 0.39 2217  80  69  -8 0.00

Run total areal avg QPF for the greens is: 0.30 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