NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20250306_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)
=== ===== === ==== ==== === === === ====
Thu 03/06 12Z 0.30 2116  98  98   4 0.00
Thu 03/06 13Z 0.41 2513  98  93   2 0.04
Thu 03/06 14Z 0.27 2710  97  84   2 0.01
Thu 03/06 15Z 0.38 2714  97  94   1 0.01
Thu 03/06 16Z 0.48 2716  96  89   0 0.02
Thu 03/06 17Z 0.61 2616  96  89   0 0.01
Thu 03/06 18Z 0.61 2515  96  85  -1 0.01
Thu 03/06 19Z 0.68 2515  96  81  -1 0.01
Thu 03/06 20Z 0.78 2617  95  80  -2 0.01
Thu 03/06 21Z 0.72 2617  93  77  -3 0.01
Thu 03/06 22Z 0.76 2618  93  71  -4 0.00
Thu 03/06 23Z 0.83 2620  93  67  -5 0.00
Fri 03/07 00Z 1.05 2622  94  64  -7 0.00
Fri 03/07 01Z 1.06 2724  93  69  -9 0.01
Fri 03/07 02Z 1.04 2723  95  78 -10 0.00
Fri 03/07 03Z 1.02 2822  96  88 -11 0.01
Fri 03/07 04Z 1.39 2823  96  91 -12 0.01
Fri 03/07 05Z 1.54 2926  94  94 -13 0.01
Fri 03/07 06Z 1.50 2928  91  92 -14 0.02
Fri 03/07 07Z 2.02 2931  90  87 -15 0.03
Fri 03/07 08Z 2.14 2932  90  82 -15 0.02
Fri 03/07 09Z 1.75 2933  89  81 -16 0.02
Fri 03/07 10Z 1.73 2934  90  78 -15 0.03
Fri 03/07 11Z 2.00 2935  91  74 -15 0.02
Fri 03/07 12Z 2.07 2935  94  67 -15 0.01
Fri 03/07 13Z 2.38 2935  94  60 -15 0.01
Fri 03/07 14Z 3.99 2834  91  52 -15 0.00
Fri 03/07 15Z 10.38 2833  84  47 -15 0.00
Fri 03/07 16Z 13.90 2832  76  46 -14 0.00
Fri 03/07 17Z 17.21 2831  70  35 -13 0.00
Fri 03/07 18Z 19.46 2832  66  27 -12 0.00
Fri 03/07 19Z 20.65 2833  68  23 -11 0.00
Fri 03/07 20Z 18.91 2833  75  20 -11 0.00
Fri 03/07 21Z 12.26 2833  82  15 -11 0.00
Fri 03/07 22Z 5.15 2734  88  14 -11 0.00
Fri 03/07 23Z 2.89 2734  89   8 -11 0.00
Sat 03/08 00Z 2.24 2733  91   6 -12 0.01
Sat 03/08 01Z 2.71 2733  91   7 -12 0.01
Sat 03/08 02Z 3.59 2833  89  12 -13 0.01
Sat 03/08 03Z 3.29 2832  90  13 -14 0.01
Sat 03/08 04Z 2.49 2731  92  24 -14 0.01
Sat 03/08 05Z 2.33 2731  94  37 -15 0.01
Sat 03/08 06Z 2.30 2730  93  49 -15 0.00
Sat 03/08 07Z 2.35 2730  91  54 -15 0.00
Sat 03/08 08Z 2.15 2730  91  59 -16 0.00
Sat 03/08 09Z 1.91 2730  92  62 -16 0.00
Sat 03/08 10Z 1.96 2731  92  67 -16 0.00
Sat 03/08 11Z 2.03 2731  93  68 -17 0.00
Sat 03/08 12Z 2.06 2832  92  69 -17 0.01
Sat 03/08 13Z 2.80 2833  92  69 -17 0.01
Sat 03/08 14Z 8.82 2833  91  65 -17 0.01
Sat 03/08 15Z 12.72 2833  90  62 -17 0.00
Sat 03/08 16Z 13.50 2832  89  64 -16 0.00
Sat 03/08 17Z 13.09 2832  89  67 -16 0.00
Sat 03/08 18Z 13.54 2831  88  70 -15 0.00
Sat 03/08 19Z 13.62 2831  86  69 -15 0.00
Sat 03/08 20Z 13.34 2832  84  68 -14 0.00
Sat 03/08 21Z 11.76 2932  83  68 -13 0.00
Sat 03/08 22Z 5.14 2933  80  68 -13 0.00
Sat 03/08 23Z 2.32 2934  76  67 -13 0.00

Run total areal avg QPF for the greens is: 0.42 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