NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260419_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)
=== ===== === ==== ==== === === === ====
Sun 04/19 18Z 0.43 3017  97  90  -5 0.00
Sun 04/19 19Z 0.66 3019  93  88  -6 0.00
Sun 04/19 20Z 0.83 2920  93  87  -6 0.01
Sun 04/19 21Z 0.81 3018  93  78  -6 0.02
Sun 04/19 22Z 0.90 2917  92  29  -6 0.00
Sun 04/19 23Z 0.93 2818  94  35  -6 0.00
Mon 04/20 00Z 1.15 2821  93  46  -6 0.01
Mon 04/20 01Z 1.47 2823  91  53  -6 0.01
Mon 04/20 02Z 1.56 2925  90  59  -6 0.02
Mon 04/20 03Z 1.43 2925  84  25  -7 0.00
Mon 04/20 04Z 1.42 2825  74  32  -7 0.00
Mon 04/20 05Z 1.33 2824  69  46  -7 0.00
Mon 04/20 06Z 1.16 2822  72  42  -7 0.00
Mon 04/20 07Z 1.16 2821  76  38  -7 0.00
Mon 04/20 08Z 0.99 2819  82  46  -8 0.00
Mon 04/20 09Z 0.80 2817  86  54  -8 0.00
Mon 04/20 10Z 0.72 2816  87  55  -8 0.00
Mon 04/20 11Z 0.63 2914  91  53  -9 0.00
Mon 04/20 12Z 0.55 2912  93  61  -9 0.01
Mon 04/20 13Z 0.50 3011  94  53  -9 0.01
Mon 04/20 14Z 0.85 3111  92  48 -10 0.01
Mon 04/20 15Z 1.47 3111  88  52 -10 0.00
Mon 04/20 16Z 3.24 3110  83  52 -10 0.00
Mon 04/20 17Z 4.39 3010  74  53  -9 0.00
Mon 04/20 18Z 5.30 3011  67  52  -9 0.00
Mon 04/20 19Z 6.09 3011  62  48  -9 0.00
Mon 04/20 20Z 6.27 3011  59  39  -9 0.00
Mon 04/20 21Z 5.67 3111  59  39  -9 0.00
Mon 04/20 22Z 4.34 3111  59  33  -9 0.00
Mon 04/20 23Z 2.60 3111  58  26  -9 0.00
Tue 04/21 00Z 1.35 3112  57  22  -9 0.00
Tue 04/21 01Z 1.16 3212  56  16  -9 0.00
Tue 04/21 02Z 1.18 3213  55  13  -9 0.00
Tue 04/21 03Z 1.50 3214  53  11 -10 0.00
Tue 04/21 04Z 1.29 3213  51  10 -10 0.00
Tue 04/21 05Z 1.15 3213  50  11 -10 0.00
Tue 04/21 06Z 0.93 3212  49  13 -10 0.00
Tue 04/21 07Z 0.71 3111  48  13 -10 0.00
Tue 04/21 08Z 0.58 3110  48  12 -10 0.00
Tue 04/21 09Z 0.52 3108  48  13 -10 0.00
Tue 04/21 10Z 0.44 3007  47  14 -10 0.00
Tue 04/21 11Z 0.38 2906  47  13 -10 0.00
Tue 04/21 12Z 0.51 2704  46  12 -10 0.00
Tue 04/21 13Z 1.49 2404  46  11 -10 0.00
Tue 04/21 14Z 2.04 2304  44  11  -9 0.00
Tue 04/21 15Z 2.70 2205  42  11  -9 0.00
Tue 04/21 16Z 2.86 2206  40  12  -8 0.00
Tue 04/21 17Z 3.51 2207  39  18  -7 0.00
Tue 04/21 18Z 3.85 2307  38  44  -6 0.00
Tue 04/21 19Z 3.34 2306  37  73  -6 0.00
Tue 04/21 20Z 2.96 2306  37  88  -5 0.00
Tue 04/21 21Z 2.05 2206  37  85  -5 0.00
Tue 04/21 22Z 1.09 2206  37  76  -5 0.00
Tue 04/21 23Z 0.30 2107  37  66  -4 0.00
Wed 04/22 00Z 0.27 2009  38  62  -4 0.00
Wed 04/22 01Z 0.30 2011  40  71  -4 0.00
Wed 04/22 02Z 0.39 2113  42  82  -4 0.00
Wed 04/22 03Z 0.49 2115  44  90  -4 0.00
Wed 04/22 04Z 0.51 2117  47  92  -4 0.00
Wed 04/22 05Z 0.52 2117  50  89  -4 0.00

Run total areal avg QPF for the greens is: 0.10 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