NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20251230_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)
=== ===== === ==== ==== === === === ====
Tue 12/30 12Z 1.69 2826  83  64 -18 0.00
Tue 12/30 13Z 2.11 2830  85  62 -19 0.00
Tue 12/30 14Z 13.52 2731  85  71 -20 0.00
Tue 12/30 15Z 16.04 2729  84  70 -21 0.00
Tue 12/30 16Z 15.64 2727  84  69 -21 0.00
Tue 12/30 17Z 16.08 2727  82  69 -21 0.00
Tue 12/30 18Z 15.45 2726  80  70 -20 0.00
Tue 12/30 19Z 14.58 2726  80  72 -20 0.00
Tue 12/30 20Z 12.06 2726  81  76 -20 0.00
Tue 12/30 21Z 7.37 2727  82  78 -20 0.00
Tue 12/30 22Z 3.99 2730  83  79 -20 0.00
Tue 12/30 23Z 3.57 2730  82  78 -20 0.00
Wed 12/31 00Z 2.72 2729  84  77 -20 0.00
Wed 12/31 01Z 2.07 2729  85  76 -20 0.00
Wed 12/31 02Z 2.33 2730  85  74 -20 0.00
Wed 12/31 03Z 2.72 2731  85  73 -19 0.00
Wed 12/31 04Z 2.94 2732  85  72 -20 0.00
Wed 12/31 05Z 2.34 2732  85  65 -19 0.00
Wed 12/31 06Z 1.88 2731  83  55 -19 0.00
Wed 12/31 07Z 1.64 2731  79  49 -19 0.00
Wed 12/31 08Z 1.31 2732  75  38 -18 0.00
Wed 12/31 09Z 1.18 2731  72  28 -18 0.00
Wed 12/31 10Z 1.14 2630  70  22 -17 0.00
Wed 12/31 11Z 1.15 2630  69  28 -17 0.00
Wed 12/31 12Z 1.12 2629  74  38 -17 0.00
Wed 12/31 13Z 1.09 2527  79  47 -17 0.00
Wed 12/31 14Z 1.20 2526  84  54 -16 0.00
Wed 12/31 15Z 1.75 2425  86  57 -16 0.00
Wed 12/31 16Z 3.07 2422  86  59 -16 0.00
Wed 12/31 17Z 4.48 2421  87  60 -15 0.00
Wed 12/31 18Z 5.15 2321  86  65 -15 0.00
Wed 12/31 19Z 4.25 2321  85  74 -14 0.00
Wed 12/31 20Z 3.26 2321  85  76 -14 0.00
Wed 12/31 21Z 1.75 2321  86  75 -14 0.00
Wed 12/31 22Z 1.31 2322  85  77 -14 0.00
Wed 12/31 23Z 1.28 2422  85  77 -14 0.01
Thu 01/01 00Z 0.99 2420  85  78 -14 0.00
Thu 01/01 01Z 0.77 2319  87  79 -14 0.00
Thu 01/01 02Z 0.66 2219  88  79 -13 0.00
Thu 01/01 03Z 0.57 2219  90  80 -13 0.01
Thu 01/01 04Z 0.50 2218  92  82 -13 0.01
Thu 01/01 05Z 0.47 2216  92  81 -13 0.02
Thu 01/01 06Z 0.50 2313  93  81 -13 0.02
Thu 01/01 07Z 0.60 2512  93  81 -13 0.02
Thu 01/01 08Z 0.69 2812  91  78 -14 0.02
Thu 01/01 09Z 0.72 2914  90  80 -15 0.02
Thu 01/01 10Z 0.62 2914  84  77 -16 0.01
Thu 01/01 11Z 0.59 2914  82  65 -17 0.00
Thu 01/01 12Z 0.93 2815  88  49 -18 0.00
Thu 01/01 13Z 0.93 2817  83  47 -19 0.00
Thu 01/01 14Z 0.76 2817  78  57 -19 0.00
Thu 01/01 15Z 0.68 2816  77  64 -19 0.00
Thu 01/01 16Z 0.74 2716  74  70 -19 0.00
Thu 01/01 17Z 1.46 2716  70  71 -19 0.00
Thu 01/01 18Z 3.41 2617  68  52 -19 0.00
Thu 01/01 19Z 7.27 2619  66  18 -19 0.00
Thu 01/01 20Z 7.01 2621  61  11 -19 0.00
Thu 01/01 21Z 2.80 2723  54  10 -19 0.00
Thu 01/01 22Z 1.38 2825  42  11 -18 0.00
Thu 01/01 23Z 1.02 2925  44  31 -18 0.00

Run total areal avg QPF for the greens is: 0.14 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