NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260908_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)
=== ===== === ==== ==== === === === ====
Tue 09/08 18Z 0.35 3211  90  40   9 0.00
Tue 09/08 19Z 0.31 3311  88  21   9 0.00
Tue 09/08 20Z 0.16 3410  82   7   9 0.00
Tue 09/08 21Z 0.09 3408  76   2  10 0.00
Tue 09/08 22Z 0.07 3406  70   1  10 0.00
Tue 09/08 23Z 0.04 3404  62   1  11 0.00
Wed 09/09 00Z 0.04 3302  58   1  11 0.00
Wed 09/09 01Z 0.03 3100  57   1  11 0.00
Wed 09/09 02Z 0.04 2001  58   2  11 0.00
Wed 09/09 03Z 0.04 2003  61   4  11 0.00
Wed 09/09 04Z 0.05 2004  64   7  11 0.00
Wed 09/09 05Z 0.09 2006  70   9  10 0.00
Wed 09/09 06Z 0.13 2108  78  11  10 0.00
Wed 09/09 07Z 0.14 2009  85  24  10 0.00
Wed 09/09 08Z 0.18 2111  92  59  10 0.00
Wed 09/09 09Z 0.22 2114  96  74  11 0.01
Wed 09/09 10Z 0.30 2116  97  80  10 0.01
Wed 09/09 11Z 0.29 2117  96  75  11 0.00
Wed 09/09 12Z 0.29 2117  95  72  11 0.00
Wed 09/09 13Z 0.28 2018  93  65  11 0.01
Wed 09/09 14Z 0.30 2022  92  61  11 0.00
Wed 09/09 15Z 0.29 2023  89  53  12 0.00
Wed 09/09 16Z 0.32 2024  92  45  12 0.00
Wed 09/09 17Z 0.39 2026  96  34  12 0.01
Wed 09/09 18Z 0.43 2126  97  42  12 0.02
Wed 09/09 19Z 0.35 2027  94  89  12 0.05
Wed 09/09 20Z 0.30 1929  96  98  12 0.12
Wed 09/09 21Z 0.28 1933  97  99  13 0.17
Wed 09/09 22Z 0.42 2037  99  96  14 0.27
Wed 09/09 23Z 0.64 2137  99  97  14 0.18
Thu 09/10 00Z 0.79 2236  98  97  15 0.11
Thu 09/10 01Z 0.85 2338  98  93  15 0.15
Thu 09/10 02Z 0.93 2341  98  90  15 0.16
Thu 09/10 03Z 1.00 2341  97  88  16 0.10
Thu 09/10 04Z 0.98 2342  97  83  16 0.08
Thu 09/10 05Z 0.97 2340  97  84  16 0.14
Thu 09/10 06Z 1.05 2440  98  89  16 0.18
Thu 09/10 07Z 1.07 2536  97  94  15 0.16
Thu 09/10 08Z 1.08 2532  97  79  14 0.11
Thu 09/10 09Z 1.10 2630  96  62  13 0.07
Thu 09/10 10Z 1.01 2830  81  33  12 0.03
Thu 09/10 11Z 0.97 2930  73  15  11 0.01
Thu 09/10 12Z 1.11 2830  77  17  11 0.00
Thu 09/10 13Z 1.45 2730  82  11  10 0.00
Thu 09/10 14Z 1.41 2728  84  12   9 0.00
Thu 09/10 15Z 1.46 2727  88   9   9 0.00
Thu 09/10 16Z 3.66 2725  86  10   9 0.00
Thu 09/10 17Z 7.84 2725  81   8   9 0.00
Thu 09/10 18Z 11.01 2726  78   5   9 0.01
Thu 09/10 19Z 11.60 2726  80   6   8 0.01
Thu 09/10 20Z 9.98 2827  82   6   8 0.01
Thu 09/10 21Z 7.06 2827  85   7   8 0.01
Thu 09/10 22Z 3.16 2828  91  11   7 0.01
Thu 09/10 23Z 1.44 2928  93  14   6 0.01
Fri 09/11 00Z 1.24 2930  95  18   6 0.01
Fri 09/11 01Z 1.18 3030  95  19   5 0.01
Fri 09/11 02Z 1.25 3029  95  20   4 0.01
Fri 09/11 03Z 1.10 3028  94  21   3 0.00
Fri 09/11 04Z 0.93 3128  94  19   2 0.00
Fri 09/11 05Z 0.74 3127  92   7   2 0.00

Run total areal avg QPF for the greens is: 2.24 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