NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260507_1200
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 05/07 12Z 0.90 2717  96  26  -2 0.00
Thu 05/07 13Z 0.98 2817  84  27  -2 0.00
Thu 05/07 14Z 2.90 2818  82  36  -3 0.00
Thu 05/07 15Z 9.10 2817  79  38  -3 0.00
Thu 05/07 16Z 11.90 2716  74  36  -2 0.00
Thu 05/07 17Z 12.27 2817  67  39  -1 0.00
Thu 05/07 18Z 12.55 2817  60  40  -1 0.00
Thu 05/07 19Z 12.74 2818  55  38   0 0.00
Thu 05/07 20Z 12.99 2818  54  39   0 0.00
Thu 05/07 21Z 12.41 2818  54  39   0 0.00
Thu 05/07 22Z 11.09 2817  55  34   0 0.00
Thu 05/07 23Z 6.75 2916  55  30   0 0.00
Fri 05/08 00Z 1.91 2916  56  30   0 0.00
Fri 05/08 01Z 1.90 2917  58  35   0 0.00
Fri 05/08 02Z 1.35 2918  61  33  -1 0.00
Fri 05/08 03Z 1.19 2917  66  40  -1 0.00
Fri 05/08 04Z 1.35 2917  69  66  -2 0.00
Fri 05/08 05Z 1.16 2817  71  69  -2 0.00
Fri 05/08 06Z 1.01 2917  73  55  -2 0.00
Fri 05/08 07Z 0.99 2818  75  44  -2 0.00
Fri 05/08 08Z 0.98 2919  72  49  -2 0.00
Fri 05/08 09Z 0.87 2918  69  58  -2 0.00
Fri 05/08 10Z 0.78 2918  69  63  -3 0.00
Fri 05/08 11Z 0.75 2916  72  67  -3 0.00
Fri 05/08 12Z 1.38 2913  73  65  -3 0.00
Fri 05/08 13Z 4.80 2811  71  63  -3 0.00
Fri 05/08 14Z 6.85 2710  67  62  -3 0.00
Fri 05/08 15Z 6.94 2710  66  61  -2 0.00
Fri 05/08 16Z 7.06 2710  66  62  -2 0.00
Fri 05/08 17Z 7.14 2710  65  69  -1 0.00
Fri 05/08 18Z 7.36 2710  64  78  -1 0.00
Fri 05/08 19Z 7.41 2711  64  83  -1 0.00
Fri 05/08 20Z 6.41 2812  67  83  -1 0.00
Fri 05/08 21Z 6.29 2812  69  77  -1 0.00
Fri 05/08 22Z 3.35 2812  70  59  -1 0.00
Fri 05/08 23Z 1.31 2812  66  35  -1 0.00
Sat 05/09 00Z 1.16 2813  65  33  -1 0.00
Sat 05/09 01Z 1.08 2813  62  36  -1 0.00
Sat 05/09 02Z 1.41 2813  62  41  -1 0.00
Sat 05/09 03Z 1.10 2714  62  40  -1 0.00
Sat 05/09 04Z 1.02 2715  61  43  -1 0.00
Sat 05/09 05Z 0.81 2715  54  55  -1 0.00
Sat 05/09 06Z 0.72 2715  55  57  -1 0.00
Sat 05/09 07Z 0.66 2714  65  44  -1 0.00
Sat 05/09 08Z 0.59 2712  63  22  -1 0.00
Sat 05/09 09Z 0.50 2610  61  18  -1 0.00
Sat 05/09 10Z 0.45 2509  60  16  -1 0.00
Sat 05/09 11Z 0.40 2309  60  12  -1 0.00
Sat 05/09 12Z 0.37 2109  57   7  -1 0.00
Sat 05/09 13Z 0.76 1910  55  11  -1 0.00
Sat 05/09 14Z 0.85 1810  52  14   0 0.00
Sat 05/09 15Z 0.74 1811  50  18   0 0.00
Sat 05/09 16Z 0.98 1711  51  31   0 0.00
Sat 05/09 17Z 1.07 1711  56  51   0 0.00
Sat 05/09 18Z 1.28 1613  62  64   1 0.00
Sat 05/09 19Z 1.34 1614  68  71   1 0.00
Sat 05/09 20Z 1.03 1615  76  80   1 0.00
Sat 05/09 21Z 0.37 1717  82  79   1 0.01
Sat 05/09 22Z 0.19 1718  90  77   1 0.01
Sat 05/09 23Z 0.08 1720  94  78   2 0.01

Run total areal avg QPF for the greens is: 0.03 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