NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260506_1800
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)
=== ===== === ==== ==== === === === ====
Wed 05/06 18Z 0.18 2015  99  99   6 0.00
Wed 05/06 19Z 0.22 2114  99  99   6 0.10
Wed 05/06 20Z 0.24 2213  98  98   6 0.06
Wed 05/06 21Z 0.25 2511  98  98   6 0.05
Wed 05/06 22Z 0.19 2709  96  95   6 0.05
Wed 05/06 23Z 0.21 2910  92  94   5 0.03
Thu 05/07 00Z 0.23 3011  90  95   4 0.01
Thu 05/07 01Z 0.29 3015  89  96   3 0.04
Thu 05/07 02Z 0.31 3015  87  95   2 0.03
Thu 05/07 03Z 0.29 3014  85  89   2 0.01
Thu 05/07 04Z 0.35 3015  86  76   1 0.00
Thu 05/07 05Z 0.44 2916  88  55   1 0.01
Thu 05/07 06Z 0.46 2916  89  61   0 0.01
Thu 05/07 07Z 0.47 2916  88  57   0 0.01
Thu 05/07 08Z 0.49 2916  88  54   0 0.00
Thu 05/07 09Z 0.60 2819  92  58   0 0.00
Thu 05/07 10Z 0.74 2820  94  54  -1 0.01
Thu 05/07 11Z 0.72 2819  83  33  -1 0.01
Thu 05/07 12Z 0.64 2718  69  21  -1 0.00
Thu 05/07 13Z 0.71 2818  71  19  -2 0.00
Thu 05/07 14Z 0.79 2817  75  17  -2 0.00
Thu 05/07 15Z 4.29 2816  80  24  -3 0.00
Thu 05/07 16Z 9.98 2815  79  27  -2 0.00
Thu 05/07 17Z 10.34 2814  75  25  -2 0.00
Thu 05/07 18Z 10.21 2814  70  23  -1 0.00
Thu 05/07 19Z 10.16 2714  66  20   0 0.00
Thu 05/07 20Z 10.65 2815  62  22   0 0.00
Thu 05/07 21Z 10.64 2816  59  24   0 0.00
Thu 05/07 22Z 10.32 2816  57  23   0 0.00
Thu 05/07 23Z 7.97 2917  60  25   0 0.00
Fri 05/08 00Z 3.32 2918  62  25  -1 0.00
Fri 05/08 01Z 2.87 2919  64  26  -1 0.00
Fri 05/08 02Z 2.24 2920  67  30  -2 0.00
Fri 05/08 03Z 1.56 2920  71  34  -2 0.00
Fri 05/08 04Z 1.28 2920  74  44  -2 0.00
Fri 05/08 05Z 1.07 2921  74  53  -2 0.00
Fri 05/08 06Z 0.95 2920  75  59  -2 0.00
Fri 05/08 07Z 0.89 2920  74  57  -2 0.00
Fri 05/08 08Z 0.83 2919  74  59  -3 0.00
Fri 05/08 09Z 0.77 2919  75  57  -3 0.00
Fri 05/08 10Z 0.72 2919  75  54  -3 0.00
Fri 05/08 11Z 0.75 2918  78  52  -3 0.00
Fri 05/08 12Z 0.91 2916  78  50  -3 0.00
Fri 05/08 13Z 3.71 2914  78  47  -4 0.00
Fri 05/08 14Z 8.05 2813  77  50  -4 0.00
Fri 05/08 15Z 9.00 2813  76  52  -3 0.00
Fri 05/08 16Z 7.42 2813  77  51  -3 0.00
Fri 05/08 17Z 7.22 2813  77  45  -2 0.00
Fri 05/08 18Z 7.73 2713  76  45  -2 0.00
Fri 05/08 19Z 8.33 2713  74  43  -2 0.00
Fri 05/08 20Z 7.24 2711  72  39  -2 0.00
Fri 05/08 21Z 6.82 2711  71  37  -1 0.00
Fri 05/08 22Z 6.90 2612  71  40  -1 0.00
Fri 05/08 23Z 4.65 2613  70  38  -1 0.00
Sat 05/09 00Z 2.03 2614  70  26  -1 0.00
Sat 05/09 01Z 1.77 2615  68  19  -1 0.00
Sat 05/09 02Z 1.62 2716  65  16  -1 0.00
Sat 05/09 03Z 1.52 2716  62  19  -2 0.00
Sat 05/09 04Z 1.17 2716  58  23  -2 0.00
Sat 05/09 05Z 0.85 2715  55  20  -1 0.00

Run total areal avg QPF for the greens is: 0.43 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