NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260423_0000
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)
=== ===== === ==== ==== === === === ====
Thu 04/23 00Z 0.17 2306  87  76   1 0.00
Thu 04/23 01Z 0.23 2508  87  62   2 0.00
Thu 04/23 02Z 0.24 2609  84  57   2 0.00
Thu 04/23 03Z 0.28 2810  82  52   2 0.00
Thu 04/23 04Z 0.31 2911  80  64   2 0.00
Thu 04/23 05Z 0.36 2912  76  69   2 0.00
Thu 04/23 06Z 0.41 2914  71  83   2 0.00
Thu 04/23 07Z 0.48 2916  72  85   2 0.00
Thu 04/23 08Z 0.53 2917  72  84   2 0.00
Thu 04/23 09Z 0.52 3019  76  83   1 0.00
Thu 04/23 10Z 0.47 3121  80  84   0 0.00
Thu 04/23 11Z 0.37 3221  78  64   0 0.00
Thu 04/23 12Z 0.34 3321  77  42  -1 0.00
Thu 04/23 13Z 0.46 3222  82  24  -2 0.00
Thu 04/23 14Z 1.00 3223  87  17  -3 0.00
Thu 04/23 15Z 3.68 3223  84  14  -3 0.00
Thu 04/23 16Z 5.82 3222  81  11  -3 0.00
Thu 04/23 17Z 6.49 3220  72   7  -3 0.00
Thu 04/23 18Z 7.41 3220  64   5  -2 0.00
Thu 04/23 19Z 7.60 3220  61   5  -2 0.00
Thu 04/23 20Z 7.46 3220  61   7  -2 0.00
Thu 04/23 21Z 7.25 3221  57   6  -1 0.00
Thu 04/23 22Z 5.79 3221  54   5  -2 0.00
Thu 04/23 23Z 2.78 3322  54   5  -2 0.00
Fri 04/24 00Z 1.08 3324  53   7  -2 0.00
Fri 04/24 02Z 0.53 3423  57   7  -3 0.00
Fri 04/24 03Z 0.38 3421  61   8  -4 0.00
Fri 04/24 05Z 0.13 3516  68   8  -5 0.00
Fri 04/24 06Z 0.12 3514  72   8  -5 0.00
Fri 04/24 07Z 0.14 3413  76   9  -6 0.00
Fri 04/24 08Z 0.16 3414  80   8  -6 0.00
Fri 04/24 09Z 0.19 3416  83   8  -7 0.00
Fri 04/24 13Z 0.15 3417  81  55  -6 0.00
Fri 04/24 16Z 1.16 3314  75  46  -5 0.00
Fri 04/24 19Z 2.04 3413  59  39  -4 0.00
Fri 04/24 20Z 1.38 3412  59  36  -3 0.00
Fri 04/24 21Z 0.80 3512  63  25  -3 0.00
Fri 04/24 22Z 0.34 3511  67  14  -3 0.00
Fri 04/24 23Z 0.30 0011  70   9  -3 0.00
Sat 04/25 00Z 0.31 0211  73   9  -4 0.00
Sat 04/25 01Z 0.54 0313  74   9  -4 0.00
Sat 04/25 02Z 0.74 0416  74  11  -4 0.00
Sat 04/25 03Z 0.69 0517  73  14  -4 0.00
Sat 04/25 04Z 0.53 0516  73  16  -4 0.00
Sat 04/25 05Z 0.37 0513  72  16  -4 0.00
Sat 04/25 06Z 0.26 0610  69  17  -3 0.00
Sat 04/25 07Z 0.19 0708  66  18  -3 0.00
Sat 04/25 08Z 0.16 0607  62  19  -2 0.00
Sat 04/25 09Z 0.16 0707  60  21  -2 0.00
Sat 04/25 10Z 0.15 0707  58  25  -2 0.00
Sat 04/25 11Z 0.17 0807  58  28  -2 0.00

Run total areal avg QPF for the greens is: 0.00 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