Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260403_1200
Using the MB925 to MB850 layer.
Assumes Central_Greens are orthogonal to 260 degree winds.

925-850 mb Layer Average Wind/RH. Avg QPF over the Central_Greens.
Date   Time        Wind RH% 700 850T QPF
Day mm/dd hhZ Frd# DDFF (%) RH% (C) (in)
=== ===== === ==== ==== === === === ====
Fri 04/03 12Z 0.42 2234  85  90  10 0.00
Fri 04/03 13Z 0.49 2236  91  97  11 0.01
Fri 04/03 14Z 0.49 2335  91  95  11 0.01
Fri 04/03 15Z 0.52 2434  90  81  11 0.00
Fri 04/03 16Z 0.55 2433  92  74  11 0.00
Fri 04/03 17Z 0.62 2434  92  85  11 0.00
Fri 04/03 18Z 0.69 2535  93  75  11 0.00
Fri 04/03 19Z 0.72 2534  91  44  11 0.00
Fri 04/03 20Z 0.79 2534  89  49  11 0.00
Fri 04/03 21Z 0.84 2535  87  48  11 0.00
Fri 04/03 22Z 0.98 2537  88  24  11 0.00
Fri 04/03 23Z 1.12 2539  91  21  10 0.00
Sat 04/04 00Z 1.09 2540  95  21  10 0.01
Sat 04/04 01Z 1.26 2542  97  26   9 0.02
Sat 04/04 02Z 1.35 2642  97  19   8 0.03
Sat 04/04 03Z 1.49 2641  97  12   8 0.03
Sat 04/04 04Z 1.52 2739  96  18   6 0.02
Sat 04/04 05Z 1.26 2835  96  17   5 0.02
Sat 04/04 07Z 0.60 3030  92  12   2 0.01
Sat 04/04 09Z 0.25 3227  89  15   1 0.01
Sat 04/04 10Z 0.09 3323  83  16   2 0.01
Sat 04/04 14Z 0.18 0512  59  19   4 0.01
Sat 04/04 15Z 0.19 0711  58  19   4 0.00
Sat 04/04 16Z 0.18 1111  61  21   4 0.00
Sat 04/04 17Z 0.16 1313  64  26   3 0.00
Sat 04/04 18Z 0.18 1314  63  36   3 0.00
Sat 04/04 19Z 0.20 1416  65  60   2 0.00
Sat 04/04 20Z 0.34 1417  69  66   2 0.00
Sat 04/04 21Z 0.57 1420  72  52   2 0.00
Sat 04/04 22Z 0.27 1423  77  35   2 0.00
Sat 04/04 23Z 0.16 1526  75  88   4 0.00
Sun 04/05 00Z 0.10 1528  64  91   6 0.00
Sun 04/05 01Z 0.08 1629  65  81   5 0.00
Sun 04/05 02Z 0.07 1630  72  81   5 0.00
Sun 04/05 03Z 0.05 1631  81  89   6 0.00
Sun 04/05 04Z 0.04 1733  91  94   7 0.00
Sun 04/05 05Z 0.04 1736  96  95   7 0.00
Sun 04/05 06Z 0.07 1739  97  91   7 0.00
Sun 04/05 07Z 0.09 1740  96  63   7 0.00
Sun 04/05 08Z 0.12 1840  97  74   8 0.00
Sun 04/05 09Z 0.15 1841  97  76   8 0.00
Sun 04/05 10Z 0.19 1840  97  88   9 0.03
Sun 04/05 11Z 0.25 1938  98  61   8 0.08
Sun 04/05 12Z 0.34 2036  98  90   7 0.02
Sun 04/05 13Z 0.47 2234  98  97   6 0.02
Sun 04/05 14Z 0.56 2332  96  94   5 0.02
Sun 04/05 15Z 0.58 2430  90  92   5 0.03
Sun 04/05 17Z 0.81 2526  95  17   1 0.05
Sun 04/05 18Z 1.20 2626  97  20   0 0.07
Sun 04/05 19Z 1.71 2626  94  19  -2 0.08
Sun 04/05 21Z 6.94 2627  87  10  -3 0.06
Sun 04/05 22Z 6.86 2627  88   9  -4 0.02
Sun 04/05 23Z 3.57 2629  89   9  -4 0.01

Run total areal avg QPF for the Central_Greens is: 0.68 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