NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260912_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)
=== ===== === ==== ==== === === === ====
Sat 09/12 18Z 0.21 1807  69  20  10 0.00
Sat 09/12 19Z 0.31 1909  68  18  11 0.00
Sat 09/12 20Z 0.34 1911  67  17  11 0.00
Sat 09/12 21Z 0.25 1912  64  15  11 0.00
Sat 09/12 22Z 0.19 1914  60  12  12 0.00
Sat 09/12 23Z 0.20 1916  56   9  12 0.00
Sun 09/13 00Z 0.25 2019  57   5  12 0.00
Sun 09/13 01Z 0.31 2021  61   3  12 0.00
Sun 09/13 02Z 0.40 2024  69  10  12 0.00
Sun 09/13 03Z 0.42 2125  78  29  13 0.00
Sun 09/13 04Z 0.45 2125  86  58  13 0.00
Sun 09/13 05Z 0.42 2126  91  86  13 0.01
Sun 09/13 06Z 0.43 2128  93  91  13 0.07
Sun 09/13 07Z 0.42 2226  95  90  14 0.06
Sun 09/13 08Z 0.43 2227  97  92  14 0.17
Sun 09/13 09Z 0.41 2226  96  92  14 0.10
Sun 09/13 10Z 0.36 2126  97  92  14 0.15
Sun 09/13 11Z 0.45 2131  99  92  14 0.26
Sun 09/13 12Z 0.51 2228  98  89  14 0.17
Sun 09/13 13Z 0.55 2425  98  90  14 0.06
Sun 09/13 14Z 0.54 2522  98  69  14 0.05
Sun 09/13 15Z 0.51 2519  96  50  14 0.03
Sun 09/13 16Z 0.48 2518  93  35  14 0.01
Sun 09/13 17Z 0.48 2516  92  48  14 0.00
Sun 09/13 18Z 0.48 2514  92  66  14 0.00
Sun 09/13 19Z 0.51 2513  93  46  14 0.00
Sun 09/13 20Z 0.56 2414  95  29  14 0.00
Sun 09/13 21Z 0.57 2415  96  41  14 0.01
Sun 09/13 22Z 0.66 2418  95  59  14 0.02
Sun 09/13 23Z 0.78 2521  93  69  14 0.02
Mon 09/14 00Z 0.66 2620  88  68  15 0.03
Mon 09/14 01Z 0.67 2620  87  72  14 0.03
Mon 09/14 02Z 0.70 2720  81  71  14 0.02
Mon 09/14 03Z 0.74 2721  79  74  12 0.02
Mon 09/14 04Z 0.83 2823  75  76  11 0.01
Mon 09/14 05Z 0.71 2923  79  79  11 0.01
Mon 09/14 06Z 0.57 2920  77  83  10 0.01
Mon 09/14 07Z 0.56 2920  80  72   9 0.00
Mon 09/14 08Z 0.66 2922  87  66   8 0.01
Mon 09/14 09Z 0.74 3025  90  56   7 0.01
Mon 09/14 10Z 0.65 3025  92  30   7 0.01
Mon 09/14 11Z 0.60 3124  91  25   6 0.02
Mon 09/14 12Z 0.55 3124  89  21   5 0.02
Mon 09/14 13Z 0.47 3223  89  22   5 0.01
Mon 09/14 14Z 0.39 3222  87  21   5 0.01
Mon 09/14 15Z 0.38 3220  86  18   5 0.01
Mon 09/14 16Z 0.41 3219  86  13   4 0.01
Mon 09/14 17Z 0.44 3219  85  11   4 0.01
Mon 09/14 18Z 0.44 3218  84  12   4 0.01
Mon 09/14 19Z 0.48 3218  85  18   4 0.01
Mon 09/14 20Z 0.49 3218  84  20   4 0.01
Mon 09/14 21Z 0.44 3217  82  20   4 0.00
Mon 09/14 22Z 0.41 3217  79  18   4 0.00
Mon 09/14 23Z 0.30 3317  77  15   5 0.00
Tue 09/15 00Z 0.21 3418  76  16   5 0.00
Tue 09/15 01Z 0.16 3419  77  16   6 0.00
Tue 09/15 02Z 0.06 3517  72  14   7 0.00
Tue 09/15 03Z 0.04 3516  69  13   7 0.00
Tue 09/15 04Z 0.05 0014  62  11   8 0.00
Tue 09/15 05Z 0.06 0013  60  10   8 0.00

Run total areal avg QPF for the greens is: 1.47 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