NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260405_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)
=== ===== === ==== ==== === === === ====
Sun 04/05 12Z 0.26 2032  98  98   5 0.00
Sun 04/05 13Z 0.30 2129  98  97   6 0.07
Sun 04/05 14Z 0.35 2227  98  98   5 0.15
Sun 04/05 15Z 0.46 2327  97  92   4 0.12
Sun 04/05 16Z 0.51 2425  95  89   4 0.04
Sun 04/05 17Z 0.65 2525  94  64   2 0.01
Sun 04/05 18Z 0.72 2523  95  42   1 0.01
Sun 04/05 19Z 0.90 2624  96  30   0 0.01
Sun 04/05 20Z 1.19 2625  95  20  -1 0.01
Sun 04/05 21Z 1.45 2624  95  17  -2 0.01
Sun 04/05 22Z 1.43 2624  94  13  -2 0.01
Sun 04/05 23Z 1.66 2626  91  12  -3 0.01
Mon 04/06 00Z 2.04 2728  88  16  -4 0.00
Mon 04/06 01Z 2.03 2728  88  24  -5 0.00
Mon 04/06 02Z 1.58 2729  90  32  -5 0.00
Mon 04/06 03Z 1.69 2730  91  36  -6 0.00
Mon 04/06 04Z 2.03 2731  94  36  -7 0.00
Mon 04/06 05Z 2.25 2731  94  38  -8 0.01
Mon 04/06 06Z 1.83 2732  94  45  -9 0.01
Mon 04/06 07Z 1.34 2831  94  54  -8 0.01
Mon 04/06 08Z 1.30 2832  95  60  -9 0.01
Mon 04/06 09Z 1.42 2831  95  62  -9 0.01
Mon 04/06 10Z 1.65 2931  96  63 -10 0.01
Mon 04/06 11Z 1.67 2929  97  62 -10 0.01
Mon 04/06 12Z 1.92 2927  97  64 -11 0.01
Mon 04/06 13Z 2.60 2926  96  70 -11 0.01
Mon 04/06 14Z 3.16 2825  96  73 -11 0.01
Mon 04/06 15Z 3.73 2824  94  75 -10 0.01
Mon 04/06 16Z 6.45 2823  92  74 -10 0.00
Mon 04/06 17Z 10.76 2822  85  75 -10 0.00
Mon 04/06 18Z 14.38 2821  78  75  -9 0.00
Mon 04/06 19Z 14.59 2821  73  76  -9 0.00
Mon 04/06 20Z 14.27 2821  71  81  -8 0.00
Mon 04/06 21Z 13.72 2821  70  81  -8 0.00
Mon 04/06 22Z 11.71 2822  73  80  -9 0.00
Mon 04/06 23Z 5.98 2822  76  78  -9 0.00
Tue 04/07 00Z 3.20 2921  79  76  -9 0.00
Tue 04/07 01Z 1.70 2920  83  76 -10 0.00
Tue 04/07 02Z 1.20 2919  87  77 -10 0.01
Tue 04/07 03Z 1.10 2919  88  79 -10 0.01
Tue 04/07 04Z 0.85 3018  86  79 -11 0.01
Tue 04/07 05Z 0.72 3016  84  82 -11 0.01
Tue 04/07 06Z 0.61 3015  83  82 -12 0.01
Tue 04/07 07Z 0.52 3114  81  81 -12 0.00
Tue 04/07 08Z 0.55 3014  83  84 -12 0.00
Tue 04/07 09Z 0.58 3014  86  85 -13 0.00
Tue 04/07 10Z 0.58 3014  90  78 -13 0.00
Tue 04/07 11Z 0.57 3014  91  73 -14 0.01
Tue 04/07 12Z 0.61 3014  93  68 -14 0.01
Tue 04/07 13Z 0.73 3014  91  61 -15 0.00
Tue 04/07 14Z 1.19 3013  86  51 -15 0.00
Tue 04/07 15Z 2.95 3013  82  38 -15 0.00
Tue 04/07 16Z 3.95 3014  79  21 -15 0.00
Tue 04/07 17Z 5.34 3014  76  19 -15 0.00
Tue 04/07 18Z 7.08 3014  71  11 -15 0.00
Tue 04/07 19Z 7.18 3014  64  11 -14 0.00
Tue 04/07 20Z 7.09 3014  56  15 -14 0.00
Tue 04/07 21Z 6.11 2914  52  24 -14 0.00
Tue 04/07 22Z 3.29 2914  49  30 -14 0.00
Tue 04/07 23Z 1.28 2915  46  33 -14 0.00

Run total areal avg QPF for the greens is: 0.63 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