NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260422_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)
=== ===== === ==== ==== === === === ====
Wed 04/22 18Z 0.33 1805  87  77   0 0.00
Wed 04/22 19Z 0.21 1807  88  76   0 0.00
Wed 04/22 20Z 0.23 1910  91  64   0 0.00
Wed 04/22 21Z 0.17 2011  94  56   0 0.00
Wed 04/22 22Z 0.23 2111  97  77   0 0.00
Wed 04/22 23Z 0.22 2211  97  49   1 0.01
Thu 04/23 00Z 0.24 2310  95  44   1 0.00
Thu 04/23 01Z 0.26 2510  94  24   2 0.00
Thu 04/23 02Z 0.30 2611  93  23   2 0.00
Thu 04/23 03Z 0.34 2813  93  30   1 0.00
Thu 04/23 04Z 0.34 2913  90  52   1 0.00
Thu 04/23 05Z 0.37 2914  88  65   1 0.00
Thu 04/23 06Z 0.44 2915  88  76   1 0.00
Thu 04/23 07Z 0.51 3016  86  84   1 0.00
Thu 04/23 08Z 0.57 3019  88  85   0 0.00
Thu 04/23 09Z 0.52 3120  88  84   0 0.00
Thu 04/23 10Z 0.47 3222  87  80   0 0.01
Thu 04/23 11Z 0.42 3223  86  63  -1 0.00
Thu 04/23 12Z 0.41 3323  87  30  -2 0.00
Thu 04/23 13Z 0.51 3323  88  17  -3 0.00
Thu 04/23 14Z 1.50 3322  88  19  -3 0.00
Thu 04/23 15Z 4.56 3222  84  16  -3 0.00
Thu 04/23 16Z 6.53 3222  85  14  -3 0.00
Thu 04/23 17Z 7.73 3222  80  14  -3 0.00
Thu 04/23 18Z 7.93 3222  73   9  -2 0.00
Thu 04/23 19Z 7.87 3221  68   7  -2 0.00
Thu 04/23 20Z 7.12 3220  64   5  -1 0.00
Thu 04/23 21Z 6.38 3220  59   5  -1 0.00
Thu 04/23 22Z 4.98 3321  57   4  -1 0.00
Thu 04/23 23Z 2.20 3321  57   3  -2 0.00
Fri 04/24 00Z 0.83 3421  58   2  -2 0.00
Fri 04/24 01Z 0.50 3421  59   2  -3 0.00
Fri 04/24 02Z 0.35 3519  61   2  -3 0.00
Fri 04/24 03Z 0.24 3517  65   3  -4 0.00
Fri 04/24 04Z 0.13 3514  69   5  -4 0.00
Fri 04/24 05Z 0.08 3512  73   7  -4 0.00
Fri 04/24 06Z 0.11 3411  76  10  -5 0.00
Fri 04/24 07Z 0.17 3411  78  12  -5 0.00
Fri 04/24 08Z 0.16 3412  79  14  -5 0.00
Fri 04/24 09Z 0.13 3413  80  14  -6 0.00
Fri 04/24 10Z 0.13 3415  80  11  -6 0.00
Fri 04/24 11Z 0.12 3416  78  12  -6 0.00
Fri 04/24 12Z 0.13 3415  80  15  -6 0.00
Fri 04/24 13Z 0.12 3413  81  24  -6 0.00
Fri 04/24 14Z 0.20 3412  79  38  -5 0.00
Fri 04/24 15Z 0.64 3310  77  47  -5 0.00
Fri 04/24 16Z 0.94 3310  74  49  -5 0.00
Fri 04/24 17Z 1.63 3310  71  42  -4 0.00
Fri 04/24 18Z 2.22 3310  67  34  -4 0.00
Fri 04/24 19Z 2.43 3310  63  27  -3 0.00
Fri 04/24 20Z 2.43 3311  57  24  -3 0.00
Fri 04/24 21Z 1.69 3312  51  22  -2 0.00
Fri 04/24 22Z 0.76 3413  50  19  -2 0.00
Fri 04/24 23Z 0.13 0012  54  12  -2 0.00
Sat 04/25 00Z 0.20 0112  60  10  -3 0.00
Sat 04/25 01Z 0.34 0213  66  13  -3 0.00
Sat 04/25 02Z 0.44 0414  69  17  -3 0.00
Sat 04/25 03Z 0.47 0513  71  20  -4 0.00
Sat 04/25 04Z 0.39 0611  73  27  -4 0.00
Sat 04/25 05Z 0.38 0611  71  32  -4 0.00

Run total areal avg QPF for the greens is: 0.02 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