NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260411_0600
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 06Z 0.43 3119  96  70   0 0.00
Sat 04/11 07Z 0.50 3122  91  60  -2 0.01
Sat 04/11 08Z 0.59 3124  91  50  -3 0.01
Sat 04/11 09Z 0.87 3126  90  23  -4 0.01
Sat 04/11 10Z 0.96 3128  92   7  -5 0.00
Sat 04/11 11Z 0.99 3129  94   3  -6 0.00
Sat 04/11 12Z 1.21 3129  94   6  -7 0.01
Sat 04/11 13Z 1.19 3128  92   9  -7 0.00
Sat 04/11 14Z 1.21 3127  92  10  -8 0.00
Sat 04/11 15Z 1.24 3125  91  11  -8 0.00
Sat 04/11 16Z 1.43 3123  89  12  -8 0.00
Sat 04/11 17Z 2.61 3122  87  13  -7 0.00
Sat 04/11 18Z 4.87 3121  85  14  -7 0.00
Sat 04/11 19Z 6.08 3120  83  13  -7 0.00
Sat 04/11 20Z 7.06 3120  81  13  -6 0.00
Sat 04/11 21Z 7.41 3120  79  12  -6 0.00
Sat 04/11 22Z 6.91 3121  78  11  -6 0.00
Sat 04/11 23Z 4.91 3123  78  11  -6 0.00
Sun 04/12 00Z 2.51 3125  79  11  -6 0.00
Sun 04/12 01Z 2.17 3126  80  11  -6 0.00
Sun 04/12 02Z 1.93 3227  80  14  -7 0.00
Sun 04/12 03Z 1.39 3228  77  14  -7 0.00
Sun 04/12 04Z 0.83 3228  73  13  -7 0.00
Sun 04/12 05Z 0.62 3327  71  14  -7 0.00
Sun 04/12 06Z 0.48 3326  68  14  -6 0.00
Sun 04/12 07Z 0.34 3324  67  13  -6 0.00
Sun 04/12 08Z 0.23 3420  67  13  -6 0.00
Sun 04/12 09Z 0.18 3317  66  15  -5 0.00
Sun 04/12 10Z 0.14 3314  61  16  -4 0.00
Sun 04/12 11Z 0.11 3311  57  16  -3 0.00
Sun 04/12 12Z 0.10 3208  53  17  -3 0.00
Sun 04/12 13Z 0.11 3006  50  19  -2 0.00
Sun 04/12 14Z 0.08 2703  45  19  -1 0.00
Sun 04/12 15Z 0.09 2304  42  22   0 0.00
Sun 04/12 16Z 0.13 2206  43  29  -1 0.00
Sun 04/12 17Z 0.18 2108  42  36  -1 0.00
Sun 04/12 18Z 0.42 2109  43  51  -1 0.00
Sun 04/12 19Z 0.20 2011  38  79   0 0.00
Sun 04/12 20Z 0.13 1914  35  96   0 0.00
Sun 04/12 21Z 0.18 1918  37  94   0 0.00
Sun 04/12 22Z 0.20 1921  36  95   1 0.00
Sun 04/12 23Z 0.37 2025  43  98   0 0.00
Mon 04/13 00Z 1.08 2026  65  97   0 0.00
Mon 04/13 01Z 0.36 2029  83  98   0 0.02
Mon 04/13 02Z 0.35 2031  91  99   2 0.04
Mon 04/13 03Z 0.36 2134  96  98   4 0.10
Mon 04/13 04Z 0.36 2136  96  98   6 0.13
Mon 04/13 05Z 0.45 2141  97  97   6 0.20
Mon 04/13 06Z 0.57 2245  97  96   7 0.16
Mon 04/13 07Z 0.68 2447  97  94   9 0.11
Mon 04/13 08Z 0.74 2447  96  93   9 0.08
Mon 04/13 09Z 0.80 2548  97  92  10 0.07
Mon 04/13 10Z 0.83 2548  96  90  10 0.06
Mon 04/13 11Z 0.86 2647  95  92  10 0.06
Mon 04/13 12Z 0.89 2647  95  91  10 0.06
Mon 04/13 13Z 0.95 2648  96  93  10 0.07
Mon 04/13 14Z 1.02 2647  94  90   9 0.09
Mon 04/13 15Z 1.02 2643  86  85  10 0.05
Mon 04/13 16Z 0.97 2539  88  89   9 0.01
Mon 04/13 17Z 1.02 2538  89  92   8 0.01

Run total areal avg QPF for the greens is: 1.36 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