Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260403_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 04/03 18Z 0.58 2530  79  92  10 0.00
Fri 04/03 19Z 0.64 2532  83  91  11 0.00
Fri 04/03 20Z 0.74 2433  88  85  11 0.00
Fri 04/03 21Z 0.84 2435  91  68  10 0.00
Fri 04/03 22Z 0.89 2435  92  39  10 0.00
Fri 04/03 23Z 1.06 2441  96  38  10 0.01
Sat 04/04 00Z 1.16 2543  96  39  10 0.01
Sat 04/04 01Z 1.18 2541  97  41   9 0.04
Sat 04/04 02Z 1.35 2640  98  22   8 0.05
Sat 04/04 03Z 1.57 2738  96  23   6 0.04
Sat 04/04 04Z 1.40 2738  95  27   5 0.02
Sat 04/04 05Z 1.23 2837  94  25   3 0.02
Sat 04/04 06Z 0.95 2933  93  20   2 0.01
Sat 04/04 07Z 0.66 3029  93  16   1 0.01
Sat 04/04 08Z 0.43 3126  92  15   0 0.01
Sat 04/04 09Z 0.27 3224  88  15   1 0.01
Sat 04/04 10Z 0.15 3420  83  14   1 0.01
Sat 04/04 11Z 0.10 3517  80  14   2 0.00
Sat 04/04 12Z 0.11 0215  73  13   2 0.00
Sat 04/04 13Z 0.14 0314  66  16   3 0.00
Sat 04/04 14Z 0.20 0515  58  19   4 0.00
Sat 04/04 15Z 0.23 0713  56  20   3 0.00
Sat 04/04 16Z 0.22 1012  57  26   3 0.00
Sat 04/04 17Z 0.20 1213  58  37   3 0.00
Sat 04/04 18Z 0.22 1314  62  55   3 0.00
Sat 04/04 19Z 0.25 1417  64  69   3 0.00
Sat 04/04 20Z 0.40 1419  67  53   2 0.00
Sat 04/04 21Z 0.61 1422  70  59   2 0.00
Sat 04/04 22Z 0.47 1526  72  89   2 0.00
Sat 04/04 23Z 0.28 1529  71  91   3 0.00
Sun 04/05 00Z 0.20 1532  69  92   5 0.00
Sun 04/05 01Z 0.15 1633  77  89   4 0.01
Sun 04/05 02Z 0.11 1634  82  81   5 0.01
Sun 04/05 03Z 0.08 1735  85  74   6 0.00
Sun 04/05 04Z 0.06 1737  91  72   7 0.01
Sun 04/05 05Z 0.05 1740  95  83   8 0.01
Sun 04/05 06Z 0.04 1743  95  91   8 0.03
Sun 04/05 07Z 0.05 1845  96  75   9 0.04
Sun 04/05 08Z 0.05 1845  96  88   9 0.02
Sun 04/05 09Z 0.09 1844  96  89  10 0.03
Sun 04/05 10Z 0.14 1944  97  76  10 0.03
Sun 04/05 11Z 0.20 1942  98  87   8 0.08
Sun 04/05 12Z 0.31 2038  98  96   6 0.13
Sun 04/05 13Z 0.45 2232  98  98   4 0.13
Sun 04/05 14Z 0.53 2329  98  95   4 0.08
Sun 04/05 15Z 0.60 2428  93  81   3 0.04
Sun 04/05 16Z 0.71 2528  91  52   2 0.01
Sun 04/05 17Z 0.87 2526  93  29   0 0.00
Sun 04/05 18Z 1.08 2526  94  21   0 0.01
Sun 04/05 19Z 1.88 2525  94  26  -1 0.01
Sun 04/05 20Z 7.32 2525  89  23  -2 0.01
Sun 04/05 21Z 8.20 2628  87  23  -3 0.00
Sun 04/05 22Z 7.89 2629  86  27  -3 0.00
Sun 04/05 23Z 4.41 2631  86  28  -4 0.00
Mon 04/06 00Z 2.44 2632  88  28  -5 0.00
Mon 04/06 01Z 1.69 2632  87  33  -5 0.01
Mon 04/06 02Z 2.17 2631  91  33  -6 0.01
Mon 04/06 03Z 1.70 2731  91  38  -7 0.00
Mon 04/06 04Z 1.81 2730  95  42  -8 0.00
Mon 04/06 05Z 1.60 2730  95  47  -8 0.00

Run total areal avg QPF for the greens is: 0.95 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