NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20250523_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)
=== ===== === ==== ==== === === === ====
Fri 05/23 18Z 0.32 2406  98  86   0 0.00
Fri 05/23 19Z 0.42 2507  98  86   0 0.01
Fri 05/23 20Z 0.40 2607  98  89   0 0.01
Fri 05/23 21Z 0.35 2607  98  91   1 0.02
Fri 05/23 22Z 0.32 2607  98  93   1 0.02
Fri 05/23 23Z 0.29 2607  97  95   1 0.01
Sat 05/24 00Z 0.28 2607  97  96   1 0.01
Sat 05/24 01Z 0.28 2607  97  97   1 0.01
Sat 05/24 02Z 0.28 2507  97  96   1 0.01
Sat 05/24 03Z 0.26 2507  98  94   1 0.02
Sat 05/24 04Z 0.26 2607  98  95   1 0.02
Sat 05/24 05Z 0.26 2707  98  95   1 0.03
Sat 05/24 06Z 0.27 2808  98  95   1 0.03
Sat 05/24 07Z 0.27 2909  98  96   0 0.02
Sat 05/24 08Z 0.26 3009  98  96   0 0.02
Sat 05/24 09Z 0.24 3110  98  95   0 0.02
Sat 05/24 10Z 0.24 3212  97  94   0 0.01
Sat 05/24 11Z 0.23 3213  97  93   0 0.01
Sat 05/24 12Z 0.25 3213  97  93   0 0.01
Sat 05/24 13Z 0.31 3214  97  94  -1 0.01
Sat 05/24 14Z 0.35 3214  97  94  -1 0.01
Sat 05/24 15Z 0.42 3214  96  94  -1 0.01
Sat 05/24 16Z 0.88 3215  95  97  -1 0.01
Sat 05/24 17Z 1.09 3215  95  97  -1 0.01
Sat 05/24 18Z 0.63 3215  95  95  -1 0.01
Sat 05/24 19Z 0.52 3215  93  94  -1 0.00
Sat 05/24 20Z 0.66 3214  93  95   0 0.00
Sat 05/24 21Z 0.73 3114  94  95   0 0.01
Sat 05/24 22Z 0.51 3115  94  94   0 0.01
Sat 05/24 23Z 0.47 3115  93  93   0 0.01
Sun 05/25 00Z 0.45 3116  95  95   0 0.01
Sun 05/25 01Z 0.41 3217  95  94   0 0.01
Sun 05/25 02Z 0.38 3216  95  91   0 0.01
Sun 05/25 03Z 0.37 3216  95  89   0 0.01
Sun 05/25 04Z 0.37 3217  97  90   0 0.01
Sun 05/25 05Z 0.37 3217  97  92  -1 0.02
Sun 05/25 06Z 0.39 3218  98  92  -1 0.02
Sun 05/25 07Z 0.33 3319  96  89  -1 0.02
Sun 05/25 08Z 0.34 3319  96  86  -1 0.01
Sun 05/25 09Z 0.37 3219  96  86  -1 0.01
Sun 05/25 10Z 0.41 3220  97  83  -1 0.02
Sun 05/25 11Z 0.39 3220  97  79  -1 0.01
Sun 05/25 12Z 0.37 3219  96  76  -1 0.01
Sun 05/25 13Z 0.38 3219  96  74   0 0.01
Sun 05/25 14Z 0.37 3317  97  71  -1 0.01
Sun 05/25 15Z 0.48 3316  97  71  -1 0.01
Sun 05/25 16Z 0.75 3215  97  70  -1 0.01
Sun 05/25 17Z 0.93 3315  96  67   0 0.01
Sun 05/25 18Z 0.81 3315  95  66   0 0.01
Sun 05/25 19Z 0.78 3314  93  68   0 0.00
Sun 05/25 20Z 0.77 3414  90  69   0 0.00
Sun 05/25 21Z 0.51 3414  86  71   1 0.00
Sun 05/25 22Z 0.31 3513  85  75   1 0.00
Sun 05/25 23Z 0.12 3513  86  80   1 0.00
Mon 05/26 00Z 0.07 3514  86  79   1 0.00
Mon 05/26 01Z 0.04 3513  87  77   2 0.00
Mon 05/26 02Z 0.06 3512  89  80   1 0.00
Mon 05/26 03Z 0.09 3511  91  79   1 0.00
Mon 05/26 04Z 0.10 3511  92  77   1 0.00
Mon 05/26 05Z 0.08 3511  92  75   1 0.00

Run total areal avg QPF for the greens is: 0.61 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