NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260508_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)
=== ===== === ==== ==== === === === ====
Fri 05/08 12Z 0.90 2910  81  76  -3 0.00
Fri 05/08 13Z 4.45 2811  82  75  -3 0.00
Fri 05/08 14Z 7.28 2811  79  77  -3 0.00
Fri 05/08 15Z 6.52 2710  74  69  -2 0.00
Fri 05/08 16Z 7.13 2612  75  71  -2 0.00
Fri 05/08 17Z 8.09 2612  72  85  -2 0.00
Fri 05/08 18Z 5.61 2713  76  88  -2 0.01
Fri 05/08 19Z 5.86 2713  76  86  -1 0.00
Fri 05/08 20Z 6.54 2712  73  80  -1 0.00
Fri 05/08 21Z 4.26 2713  72  73  -1 0.01
Fri 05/08 22Z 3.83 2713  68  56  -1 0.00
Fri 05/08 23Z 2.50 2713  71  41  -1 0.00
Sat 05/09 00Z 1.13 2713  65  30  -1 0.00
Sat 05/09 01Z 1.10 2712  63  29  -1 0.00
Sat 05/09 02Z 1.23 2713  62  31  -1 0.00
Sat 05/09 03Z 1.27 2613  62  37  -1 0.00
Sat 05/09 04Z 1.05 2513  61  48  -1 0.00
Sat 05/09 05Z 0.88 2514  56  46  -1 0.00
Sat 05/09 06Z 0.78 2514  49  33  -1 0.00
Sat 05/09 07Z 0.82 2514  52  16  -1 0.00
Sat 05/09 08Z 0.85 2514  60  16  -1 0.00
Sat 05/09 09Z 0.68 2512  60  26  -1 0.00
Sat 05/09 10Z 0.52 2409  60  18  -1 0.00
Sat 05/09 11Z 0.30 2109  60  13   0 0.00
Sat 05/09 12Z 0.14 1810  63  31   0 0.00
Sat 05/09 13Z 0.27 1712  68  42   0 0.00
Sat 05/09 14Z 0.81 1613  74  56   0 0.01
Sat 05/09 15Z 0.88 1615  81  72   0 0.01
Sat 05/09 16Z 0.74 1618  87  79   1 0.01
Sat 05/09 17Z 0.72 1619  90  83   1 0.02
Sat 05/09 18Z 0.44 1619  93  91   1 0.03
Sat 05/09 19Z 0.14 1721  96  88   2 0.03
Sat 05/09 20Z 0.09 1721  97  80   2 0.02
Sat 05/09 21Z 0.07 1820  97  74   3 0.01
Sat 05/09 22Z 0.08 1820  97  55   3 0.01
Sat 05/09 23Z 0.12 1920  97  46   3 0.01
Sun 05/10 00Z 0.19 1921  97  36   4 0.01
Sun 05/10 01Z 0.21 1921  96  39   4 0.01
Sun 05/10 02Z 0.26 2023  96  50   4 0.01
Sun 05/10 03Z 0.31 2024  96  49   4 0.01
Sun 05/10 04Z 0.38 2125  95  56   4 0.01
Sun 05/10 05Z 0.39 2126  94  57   5 0.01
Sun 05/10 06Z 0.38 2127  93  67   6 0.01
Sun 05/10 07Z 0.41 2129  94  85   6 0.01
Sun 05/10 08Z 0.51 2131  95  94   6 0.02
Sun 05/10 09Z 0.78 2233  96  91   5 0.04
Sun 05/10 10Z 0.90 2329  97  72   5 0.04
Sun 05/10 11Z 1.04 2429  94  49   4 0.02
Sun 05/10 12Z 1.17 2526  93  37   3 0.01
Sun 05/10 13Z 0.98 2623  80  39   3 0.01
Sun 05/10 14Z 1.03 2620  76  35   2 0.01
Sun 05/10 15Z 5.19 2617  76  32   2 0.00
Sun 05/10 16Z 8.16 2615  74  29   2 0.00
Sun 05/10 17Z 8.02 2514  71  26   2 0.00
Sun 05/10 18Z 7.21 2513  69  45   2 0.00
Sun 05/10 19Z 6.58 2512  68  54   2 0.00
Sun 05/10 20Z 5.63 2511  69  52   2 0.00
Sun 05/10 21Z 4.05 2512  71  71   2 0.00
Sun 05/10 22Z 1.71 2611  72  64   2 0.00
Sun 05/10 23Z 0.87 2709  72  41   2 0.00

Run total areal avg QPF for the greens is: 0.41 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