NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260406_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)
=== ===== === ==== ==== === === === ====
Mon 04/06 12Z 1.81 2923  94  82  -9 0.00
Mon 04/06 13Z 5.17 2924  94  78  -9 0.00
Mon 04/06 14Z 4.69 2924  94  76  -9 0.01
Mon 04/06 15Z 4.89 2923  92  75  -9 0.01
Mon 04/06 16Z 6.05 2922  89  75  -9 0.00
Mon 04/06 17Z 10.66 2921  86  77  -9 0.00
Mon 04/06 18Z 12.53 2820  80  76  -9 0.00
Mon 04/06 19Z 12.70 2819  76  72  -8 0.00
Mon 04/06 20Z 13.10 2820  74  70  -8 0.00
Mon 04/06 21Z 13.04 2821  73  71  -8 0.00
Mon 04/06 22Z 11.26 2821  73  70  -8 0.00
Mon 04/06 23Z 7.14 2821  74  68  -9 0.00
Tue 04/07 00Z 3.86 2921  77  64  -9 0.00
Tue 04/07 01Z 2.36 2920  78  62  -9 0.00
Tue 04/07 02Z 1.73 2918  78  60  -9 0.00
Tue 04/07 03Z 1.20 2916  78  56  -9 0.00
Tue 04/07 04Z 0.80 2914  79  54  -9 0.00
Tue 04/07 05Z 0.55 2911  78  52  -9 0.00
Tue 04/07 06Z 0.38 2909  78  49  -9 0.00
Tue 04/07 07Z 0.24 2906  78  51  -9 0.00
Tue 04/07 08Z 0.17 2904  78  48  -9 0.00
Tue 04/07 09Z 0.13 2803  79  51  -9 0.00
Tue 04/07 10Z 0.10 2502  81  57  -9 0.00
Tue 04/07 11Z 0.08 2302  82  62 -10 0.00
Tue 04/07 12Z 0.08 2001  83  67  -9 0.00
Tue 04/07 13Z 0.10 1901  85  72 -10 0.00
Tue 04/07 14Z 0.19 1200  86  78  -9 0.00
Tue 04/07 15Z 0.23 0700  86  86  -9 0.00
Tue 04/07 16Z 0.27 3502  87  88  -9 0.01
Tue 04/07 17Z 0.22 3405  89  87  -9 0.02
Tue 04/07 18Z 0.17 3408  90  87  -9 0.02
Tue 04/07 19Z 0.19 3308  88  87  -9 0.01
Tue 04/07 20Z 0.42 3309  86  83  -9 0.00
Tue 04/07 21Z 0.61 3212  85  73  -9 0.00
Tue 04/07 22Z 0.34 3214  83  76 -10 0.00
Tue 04/07 23Z 0.37 3217  78  78 -10 0.00
Wed 04/08 00Z 0.50 3219  79  56 -11 0.00
Wed 04/08 01Z 1.18 3221  81  28 -12 0.00
Wed 04/08 02Z 1.17 3121  76  23 -13 0.00
Wed 04/08 03Z 0.78 3120  62  25 -13 0.00
Wed 04/08 04Z 0.61 3120  46  22 -13 0.00
Wed 04/08 05Z 0.53 3120  39  20 -13 0.00
Wed 04/08 06Z 0.46 3119  38  21 -13 0.00
Wed 04/08 07Z 0.44 3118  38  21 -12 0.00
Wed 04/08 08Z 0.42 3119  40  18 -12 0.00
Wed 04/08 09Z 0.38 3119  41  17 -12 0.00
Wed 04/08 10Z 0.32 3117  40  17 -11 0.00
Wed 04/08 11Z 0.31 3016  40  13 -10 0.00
Wed 04/08 12Z 0.27 3014  38   9 -10 0.00
Wed 04/08 13Z 0.25 2912  37   7  -9 0.00
Wed 04/08 14Z 0.24 2910  35   6  -9 0.00
Wed 04/08 15Z 0.22 2808  32   5  -8 0.00
Wed 04/08 16Z 0.21 2706  29   4  -7 0.00
Wed 04/08 17Z 0.25 2506  26   3  -7 0.00
Wed 04/08 18Z 0.44 2406  24   3  -6 0.00
Wed 04/08 19Z 0.48 2307  23   3  -5 0.00
Wed 04/08 20Z 0.41 2208  22   3  -5 0.00
Wed 04/08 21Z 0.31 2110  24   4  -4 0.00
Wed 04/08 22Z 0.29 2113  24   4  -4 0.00
Wed 04/08 23Z 0.32 2116  22   4  -3 0.00

Run total areal avg QPF for the greens is: 0.08 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