NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260327_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)
=== ===== === ==== ==== === === === ====
Fri 03/27 06Z 0.40 3117  98  71  -1 0.00
Fri 03/27 07Z 0.35 3220  97  63  -3 0.01
Fri 03/27 08Z 0.29 3323  95  57  -5 0.01
Fri 03/27 09Z 0.29 3325  93  53  -6 0.01
Fri 03/27 10Z 0.31 3326  91  44  -8 0.00
Fri 03/27 11Z 0.32 3327  90  23 -10 0.00
Fri 03/27 12Z 0.32 3327  88  13 -11 0.00
Fri 03/27 13Z 0.31 3326  79   8 -12 0.00
Fri 03/27 14Z 0.27 3326  67   6 -12 0.00
Fri 03/27 15Z 0.24 3324  56   4 -12 0.00
Fri 03/27 16Z 0.28 3321  52   2 -13 0.00
Fri 03/27 17Z 0.55 3318  52   2 -13 0.00
Fri 03/27 18Z 0.80 3415  52   3 -13 0.00
Fri 03/27 19Z 0.85 3313  50   3 -13 0.00
Fri 03/27 20Z 0.94 3311  48   4 -13 0.00
Fri 03/27 21Z 0.58 3212  44   6 -13 0.00
Fri 03/27 22Z 0.28 3212  41   7 -13 0.00
Fri 03/27 23Z 0.27 3212  44   7 -12 0.00
Sat 03/28 00Z 0.24 3212  47   8 -12 0.00
Sat 03/28 01Z 0.25 3112  52  10 -12 0.00
Sat 03/28 02Z 0.29 3113  58  13 -13 0.00
Sat 03/28 03Z 0.28 3214  60  12 -13 0.00
Sat 03/28 04Z 0.26 3214  59  12 -14 0.00
Sat 03/28 05Z 0.27 3215  58  18 -14 0.00
Sat 03/28 06Z 0.29 3215  61  20 -15 0.00
Sat 03/28 07Z 0.31 3215  65  32 -15 0.00
Sat 03/28 08Z 0.33 3216  69  52 -16 0.00
Sat 03/28 09Z 0.35 3216  72  68 -17 0.00
Sat 03/28 10Z 0.34 3315  76  80 -18 0.00
Sat 03/28 11Z 0.33 3314  78  79 -18 0.00
Sat 03/28 12Z 0.38 3214  80  75 -19 0.00
Sat 03/28 13Z 0.59 3213  80  68 -19 0.00
Sat 03/28 14Z 0.84 3212  76  65 -19 0.00
Sat 03/28 15Z 0.86 3111  66  67 -18 0.00
Sat 03/28 16Z 0.69 3210  56  74 -16 0.00
Sat 03/28 17Z 1.52 3110  52  74 -16 0.00
Sat 03/28 18Z 3.77 3010  53  69 -16 0.00
Sat 03/28 19Z 4.97 2910  54  62 -15 0.00
Sat 03/28 20Z 5.45 2710  57  61 -15 0.00
Sat 03/28 21Z 4.98 2611  67  48 -15 0.00
Sat 03/28 22Z 2.99 2713  73  41 -14 0.00
Sat 03/28 23Z 4.62 2817  71  40 -14 0.00
Sun 03/29 00Z 3.94 2820  72  36 -15 0.00
Sun 03/29 01Z 3.58 2821  77  28 -15 0.00
Sun 03/29 02Z 1.91 2820  79  23 -15 0.00
Sun 03/29 03Z 2.45 2722  80  21 -15 0.00
Sun 03/29 04Z 1.39 2721  68  30 -14 0.00
Sun 03/29 05Z 1.21 2822  64  60 -14 0.00
Sun 03/29 06Z 1.14 2824  63  70 -14 0.00
Sun 03/29 07Z 1.05 2724  69  68 -13 0.00
Sun 03/29 08Z 0.94 2723  79  81 -13 0.00
Sun 03/29 09Z 0.81 2621  87  86 -12 0.00
Sun 03/29 10Z 0.73 2621  91  89 -12 0.00
Sun 03/29 11Z 0.60 2620  82  85 -10 0.01
Sun 03/29 12Z 0.59 2522  60  78 -10 0.00
Sun 03/29 13Z 0.59 2423  68  67  -9 0.00
Sun 03/29 14Z 0.58 2422  67  43  -8 0.00
Sun 03/29 15Z 0.60 2323  52  16  -8 0.00
Sun 03/29 16Z 0.87 2224  49  35  -8 0.00
Sun 03/29 17Z 3.46 2222  50  68  -7 0.00

Run total areal avg QPF for the greens is: 0.04 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