NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260403_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)
=== ===== === ==== ==== === === === ====
Fri 04/03 12Z 0.39 2237  88  93  10 0.00
Fri 04/03 13Z 0.45 2238  88  97  10 0.02
Fri 04/03 14Z 0.47 2336  88  95  11 0.02
Fri 04/03 15Z 0.51 2336  90  82  11 0.00
Fri 04/03 16Z 0.55 2435  92  76  11 0.00
Fri 04/03 17Z 0.63 2436  92  84  11 0.00
Fri 04/03 18Z 0.70 2537  92  66  11 0.00
Fri 04/03 19Z 0.76 2536  91  56  11 0.00
Fri 04/03 20Z 0.85 2536  89  56  11 0.01
Fri 04/03 21Z 0.93 2536  88  45  11 0.00
Fri 04/03 22Z 1.08 2537  89  29  10 0.00
Fri 04/03 23Z 1.14 2539  92  26  10 0.00
Sat 04/04 00Z 1.18 2541  95  24   9 0.01
Sat 04/04 01Z 1.32 2542  97  23   9 0.02
Sat 04/04 02Z 1.37 2641  98  17   8 0.03
Sat 04/04 03Z 1.51 2740  98  13   7 0.03
Sat 04/04 04Z 1.47 2738  97  17   5 0.03
Sat 04/04 05Z 1.24 2836  96  18   3 0.03
Sat 04/04 07Z 0.62 3029  94  18   0 0.02
Sat 04/04 09Z 0.26 3325  89  16   0 0.01
Sat 04/04 10Z 0.13 3422  83  13   1 0.01
Sat 04/04 14Z 0.20 0514  56  12   4 0.01
Sat 04/04 15Z 0.21 0712  54  12   3 0.00
Sat 04/04 16Z 0.22 1012  55  14   3 0.00
Sat 04/04 17Z 0.21 1213  58  17   3 0.00
Sat 04/04 18Z 0.22 1314  61  26   2 0.00
Sat 04/04 19Z 0.25 1416  64  47   2 0.00
Sat 04/04 20Z 0.42 1418  66  59   1 0.00
Sat 04/04 21Z 0.71 1420  69  61   1 0.00
Sat 04/04 22Z 0.43 1524  72  46   2 0.00
Sat 04/04 23Z 0.27 1528  72  80   4 0.00
Sun 04/05 00Z 0.17 1630  65  91   5 0.00
Sun 04/05 01Z 0.12 1633  63  85   5 0.00
Sun 04/05 02Z 0.11 1634  70  80   6 0.00
Sun 04/05 03Z 0.10 1635  77  89   7 0.00
Sun 04/05 04Z 0.07 1737  85  93   7 0.00
Sun 04/05 05Z 0.06 1740  92  95   8 0.00
Sun 04/05 06Z 0.05 1743  96  90   7 0.00
Sun 04/05 07Z 0.05 1844  95  68   8 0.00
Sun 04/05 08Z 0.06 1845  95  76   8 0.00
Sun 04/05 09Z 0.08 1844  95  80   9 0.00
Sun 04/05 10Z 0.12 1943  96  84   9 0.02
Sun 04/05 11Z 0.18 1941  97  70   8 0.07
Sun 04/05 12Z 0.28 2038  98  92   7 0.02
Sun 04/05 13Z 0.38 2135  98  97   6 0.01
Sun 04/05 14Z 0.50 2332  97  95   5 0.01
Sun 04/05 15Z 0.54 2429  92  87   4 0.03
Sun 04/05 17Z 0.89 2526  96  20   0 0.05
Sun 04/05 18Z 1.40 2527  95  22  -1 0.07
Sun 04/05 19Z 3.08 2526  94  21  -2 0.09
Sun 04/05 21Z 8.69 2627  87  13  -3 0.08
Sun 04/05 22Z 6.76 2628  87  16  -4 0.03
Sun 04/05 23Z 3.49 2629  87  17  -4 0.01

Run total areal avg QPF for the greens is: 0.74 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