NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260411_0000
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 04/11 00Z 0.64 2620  78  97   6 0.00
Sat 04/11 01Z 0.56 2819  87  96   5 0.03
Sat 04/11 02Z 0.48 2818  95  96   4 0.04
Sat 04/11 03Z 0.42 2816  97  92   2 0.06
Sat 04/11 04Z 0.41 2916  98  86   1 0.05
Sat 04/11 05Z 0.39 3018  98  85   0 0.03
Sat 04/11 06Z 0.43 3020  97  92  -1 0.03
Sat 04/11 07Z 0.51 3123  97  93  -2 0.03
Sat 04/11 08Z 0.52 3124  96  60  -3 0.02
Sat 04/11 09Z 0.64 3126  96  38  -4 0.01
Sat 04/11 10Z 0.72 3127  96  10  -5 0.01
Sat 04/11 11Z 0.82 3128  95   7  -6 0.01
Sat 04/11 12Z 0.86 3128  93   8  -6 0.01
Sat 04/11 13Z 0.86 3128  93   9  -7 0.00
Sat 04/11 14Z 1.01 3126  94   9  -7 0.00
Sat 04/11 15Z 1.15 3124  94  11  -7 0.00
Sat 04/11 16Z 1.46 3124  93  12  -7 0.00
Sat 04/11 17Z 1.82 3122  92  13  -7 0.00
Sat 04/11 18Z 3.46 3020  90  13  -7 0.00
Sat 04/11 19Z 5.08 3019  88  13  -6 0.00
Sat 04/11 20Z 5.21 3019  86  13  -6 0.00
Sat 04/11 21Z 4.89 3019  85  13  -6 0.00
Sat 04/11 22Z 4.50 3020  82  13  -6 0.00
Sat 04/11 23Z 2.14 3121  80  12  -5 0.00
Sun 04/12 00Z 1.69 3124  80  12  -6 0.00
Sun 04/12 01Z 1.71 3125  79  12  -6 0.00
Sun 04/12 02Z 1.73 3127  78  12  -6 0.00
Sun 04/12 03Z 1.44 3227  77  11  -6 0.00
Sun 04/12 04Z 1.03 3227  73  10  -6 0.00
Sun 04/12 05Z 0.67 3326  71   9  -6 0.00
Sun 04/12 06Z 0.50 3324  71  10  -6 0.00
Sun 04/12 07Z 0.39 3323  71  12  -6 0.00
Sun 04/12 08Z 0.32 3320  71  11  -6 0.00
Sun 04/12 09Z 0.23 3317  68  12  -5 0.00
Sun 04/12 10Z 0.19 3315  64  12  -5 0.00
Sun 04/12 11Z 0.15 3313  58  14  -4 0.00
Sun 04/12 12Z 0.13 3210  53  15  -3 0.00
Sun 04/12 13Z 0.11 3207  48  16  -2 0.00
Sun 04/12 14Z 0.11 2904  46  17  -2 0.00
Sun 04/12 15Z 0.12 2604  46  19  -2 0.00
Sun 04/12 16Z 0.15 2405  44  23  -1 0.00
Sun 04/12 17Z 0.19 2205  42  28  -1 0.00
Sun 04/12 18Z 0.31 2108  41  38  -1 0.00
Sun 04/12 19Z 0.24 2010  40  83   0 0.00
Sun 04/12 20Z 0.19 2013  39  95   0 0.00
Sun 04/12 21Z 0.36 2016  43  96   0 0.00
Sun 04/12 22Z 2.26 2021  62  98  -2 0.00
Sun 04/12 23Z 0.43 2022  78  98  -1 0.00
Mon 04/13 00Z 0.46 2027  80  98  -1 0.00
Mon 04/13 01Z 0.39 2030  91  99   0 0.02
Mon 04/13 02Z 0.35 2032  88  99   1 0.05
Mon 04/13 03Z 0.36 2133  94  98   3 0.05
Mon 04/13 04Z 0.42 2136  97  98   5 0.10
Mon 04/13 05Z 0.42 2137  97  97   7 0.16
Mon 04/13 06Z 0.56 2245  96  95   7 0.14
Mon 04/13 07Z 0.67 2347  96  95   9 0.07
Mon 04/13 08Z 0.73 2448  96  94   9 0.10
Mon 04/13 09Z 0.79 2548  94  91  10 0.06
Mon 04/13 10Z 0.84 2548  94  93  10 0.05
Mon 04/13 11Z 0.86 2547  92  92  10 0.04

Run total areal avg QPF for the greens is: 1.17 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