NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260409_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)
=== ===== === ==== ==== === === === ====
Thu 04/09 12Z 0.22 2021  31  56   2 0.00
Thu 04/09 13Z 0.23 2025  26  57   2 0.00
Thu 04/09 14Z 0.24 2026  24  55   2 0.00
Thu 04/09 15Z 0.23 2026  24  53   3 0.00
Thu 04/09 16Z 0.22 2026  25  48   3 0.00
Thu 04/09 17Z 0.24 2027  24  27   4 0.00
Thu 04/09 18Z 0.29 2028  24   8   4 0.00
Thu 04/09 19Z 0.34 2030  23  20   4 0.00
Thu 04/09 20Z 0.34 2029  23  32   5 0.00
Thu 04/09 21Z 0.33 2028  25  43   6 0.00
Thu 04/09 22Z 0.32 2028  28  53   7 0.00
Thu 04/09 23Z 0.33 2129  35  56   7 0.00
Fri 04/10 00Z 0.39 2130  39  56   7 0.00
Fri 04/10 01Z 0.45 2230  45  51   7 0.00
Fri 04/10 02Z 0.49 2330  52  56   6 0.00
Fri 04/10 03Z 0.53 2330  59  68   6 0.00
Fri 04/10 04Z 0.60 2431  63  64   6 0.00
Fri 04/10 05Z 0.65 2431  68  60   6 0.00
Fri 04/10 06Z 0.66 2530  73  37   6 0.00
Fri 04/10 08Z 0.65 2527  75  56   6 0.00
Fri 04/10 09Z 0.58 2525  83  52   5 0.00
Fri 04/10 10Z 0.56 2524  84  84   6 0.00
Fri 04/10 11Z 0.54 2523  91  77   5 0.00
Fri 04/10 12Z 0.50 2422  91  48   5 0.01
Fri 04/10 13Z 0.48 2422  87  51   6 0.00
Fri 04/10 14Z 0.48 2423  88  85   6 0.00
Fri 04/10 15Z 0.50 2423  89  97   6 0.00
Fri 04/10 16Z 0.53 2325  88  78   7 0.00
Fri 04/10 17Z 0.66 2326  86  64   7 0.00
Fri 04/10 18Z 0.83 2426  82  43   7 0.00
Fri 04/10 19Z 0.85 2327  71  44   8 0.00
Fri 04/10 20Z 0.89 2227  69  48   7 0.00
Fri 04/10 21Z 0.93 2128  70  48   7 0.00
Fri 04/10 22Z 0.93 2131  70  63   7 0.00
Fri 04/10 23Z 1.02 2234  74  93   7 0.00
Sat 04/11 00Z 0.96 2335  87  95   6 0.01
Sat 04/11 01Z 0.76 2332  93  96   6 0.04
Sat 04/11 02Z 0.62 2526  95  96   5 0.09
Sat 04/11 03Z 0.59 2725  98  96   3 0.14
Sat 04/11 04Z 0.53 2925  98  94   1 0.12
Sat 04/11 05Z 0.55 3026  97  65   0 0.09
Sat 04/11 06Z 0.67 3127  97  31  -2 0.04
Sat 04/11 07Z 0.74 3027  94  10  -4 0.03
Sat 04/11 08Z 0.78 3028  92   7  -5 0.02
Sat 04/11 09Z 0.74 3028  91   5  -5 0.01
Sat 04/11 10Z 0.67 3128  89   5  -6 0.01
Sat 04/11 11Z 0.63 3128  88   7  -6 0.01
Sat 04/11 12Z 0.61 3128  87  10  -7 0.01
Sat 04/11 13Z 0.65 3128  86  14  -7 0.01
Sat 04/11 14Z 0.75 3128  87  14  -8 0.01
Sat 04/11 15Z 0.75 3127  87  16  -8 0.01
Sat 04/11 16Z 0.81 3126  85  15  -8 0.00
Sat 04/11 17Z 1.07 3125  84  16  -8 0.00
Sat 04/11 18Z 1.66 3124  81  14  -7 0.00
Sat 04/11 19Z 3.59 3123  79  11  -7 0.00
Sat 04/11 20Z 5.66 3121  80  11  -7 0.00
Sat 04/11 21Z 5.18 3121  80  11  -7 0.00
Sat 04/11 22Z 2.91 3121  78  10  -7 0.00
Sat 04/11 23Z 1.31 3121  78   9  -6 0.00

Run total areal avg QPF for the greens is: 0.66 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