NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260909_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)
=== ===== === ==== ==== === === === ====
Wed 09/09 12Z 0.27 2016  82  69  11 0.00
Wed 09/09 13Z 0.28 2019  80  66  12 0.00
Wed 09/09 14Z 0.30 2020  79  59  12 0.00
Wed 09/09 15Z 0.34 2120  78  51  12 0.00
Wed 09/09 16Z 0.34 2118  80  37  13 0.00
Wed 09/09 17Z 0.33 2119  87  37  13 0.00
Wed 09/09 18Z 0.30 2021  91  46  13 0.01
Wed 09/09 19Z 0.29 2024  91  42  13 0.01
Wed 09/09 20Z 0.32 2025  90  69  13 0.01
Wed 09/09 21Z 0.45 2027  91  91  13 0.01
Wed 09/09 22Z 0.49 2128  96  97  13 0.05
Wed 09/09 23Z 0.56 2130  97  98  13 0.09
Thu 09/10 00Z 0.68 2133  97  98  14 0.11
Thu 09/10 01Z 0.78 2234  97  94  14 0.15
Thu 09/10 02Z 0.81 2335  97  96  15 0.08
Thu 09/10 03Z 0.87 2241  98  94  16 0.17
Thu 09/10 04Z 0.94 2340  97  92  16 0.14
Thu 09/10 05Z 0.99 2340  97  92  16 0.13
Thu 09/10 06Z 1.06 2342  98  93  16 0.18
Thu 09/10 07Z 1.12 2538  98  89  15 0.23
Thu 09/10 08Z 1.09 2533  95  84  15 0.19
Thu 09/10 09Z 0.95 2728  91  70  14 0.13
Thu 09/10 10Z 0.95 2727  89  51  14 0.06
Thu 09/10 11Z 0.95 2727  85  30  13 0.02
Thu 09/10 12Z 1.07 2729  85  37  13 0.01
Thu 09/10 13Z 1.09 2731  76  22  12 0.01
Thu 09/10 14Z 1.09 2730  75   9  12 0.01
Thu 09/10 15Z 1.29 2728  74   8  11 0.01
Thu 09/10 16Z 2.35 2728  71   8  11 0.01
Thu 09/10 17Z 6.33 2727  70   9  10 0.00
Thu 09/10 18Z 12.25 2726  73  10  10 0.00
Thu 09/10 19Z 13.77 2727  71  10  10 0.01
Thu 09/10 20Z 13.01 2828  72  11  10 0.01
Thu 09/10 21Z 8.02 2827  75  10   9 0.01
Thu 09/10 22Z 2.97 2827  81  11   8 0.01
Thu 09/10 23Z 1.77 2927  88   8   7 0.01
Fri 09/11 00Z 1.36 2928  93   9   6 0.01
Fri 09/11 01Z 1.10 3028  94   8   5 0.01
Fri 09/11 02Z 1.00 3028  94   6   5 0.01
Fri 09/11 03Z 0.84 3028  91   5   4 0.01
Fri 09/11 04Z 0.81 3027  90   4   3 0.00
Fri 09/11 05Z 0.77 3027  88   3   3 0.00
Fri 09/11 06Z 0.68 3026  87   2   3 0.00
Fri 09/11 07Z 0.55 3025  85   2   4 0.00
Fri 09/11 08Z 0.50 3025  83   4   4 0.00
Fri 09/11 09Z 0.46 3025  79   3   5 0.00
Fri 09/11 10Z 0.45 3025  76   2   5 0.00
Fri 09/11 11Z 0.46 3025  76   2   5 0.00
Fri 09/11 12Z 0.48 3025  77   3   4 0.00
Fri 09/11 13Z 0.47 3025  73   3   5 0.00
Fri 09/11 14Z 0.45 3025  66   4   5 0.00
Fri 09/11 15Z 0.46 3024  61   4   6 0.00
Fri 09/11 16Z 0.50 3024  58   4   6 0.00
Fri 09/11 17Z 0.57 3022  56   5   6 0.00
Fri 09/11 18Z 0.71 3021  58   5   5 0.00
Fri 09/11 19Z 1.14 2919  57   5   5 0.00
Fri 09/11 20Z 1.46 2919  54   7   6 0.00
Fri 09/11 21Z 1.10 3018  51  10   6 0.00
Fri 09/11 22Z 0.88 2919  51  13   6 0.00
Fri 09/11 23Z 0.82 3020  51  18   7 0.00

Run total areal avg QPF for the greens is: 1.91 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