NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260501_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)
=== ===== === ==== ==== === === === ====
Fri 05/01 18Z 6.23 2909  68  58  -1 0.00
Fri 05/01 19Z 6.15 2909  67  66  -1 0.00
Fri 05/01 20Z 4.95 2908  66  70   0 0.00
Fri 05/01 21Z 4.58 2808  66  72   0 0.00
Fri 05/01 22Z 3.59 2807  67  74   0 0.00
Fri 05/01 23Z 1.41 2806  68  74   0 0.00
Sat 05/02 00Z 0.71 2806  68  70   0 0.00
Sat 05/02 01Z 0.37 2804  67  69   0 0.00
Sat 05/02 02Z 0.17 2902  67  66   0 0.00
Sat 05/02 03Z 0.08 2001  67  75   0 0.00
Sat 05/02 04Z 0.09 1903  69  84   0 0.00
Sat 05/02 05Z 0.17 1704  75  91  -1 0.00
Sat 05/02 06Z 0.10 1606  85  95  -1 0.01
Sat 05/02 07Z 0.08 1607  93  96  -1 0.02
Sat 05/02 08Z 0.07 1607  97  93  -1 0.04
Sat 05/02 09Z 0.11 1806  99  88  -1 0.04
Sat 05/02 10Z 0.18 2105  99  87  -1 0.03
Sat 05/02 11Z 0.23 2406  99  84  -2 0.03
Sat 05/02 12Z 0.25 2707  99  79  -2 0.02
Sat 05/02 13Z 0.26 2808  98  82  -2 0.01
Sat 05/02 14Z 0.29 3010  98  79  -3 0.01
Sat 05/02 15Z 0.30 3110  97  76  -3 0.01
Sat 05/02 16Z 0.35 3009  96  74  -3 0.00
Sat 05/02 17Z 0.78 3009  92  64  -3 0.00
Sat 05/02 18Z 1.52 3007  89  43  -2 0.00
Sat 05/02 19Z 2.14 2906  86  30  -2 0.00
Sat 05/02 20Z 2.36 2906  82  37  -2 0.00
Sat 05/02 21Z 1.63 2806  85  53  -1 0.00
Sat 05/02 22Z 0.66 2907  90  58  -2 0.01
Sat 05/02 23Z 0.41 2907  87  51  -2 0.01
Sun 05/03 00Z 0.41 3008  89  53  -2 0.00
Sun 05/03 01Z 0.42 3109  89  50  -2 0.00
Sun 05/03 02Z 0.36 3209  89  44  -3 0.00
Sun 05/03 03Z 0.37 3211  89  40  -3 0.01
Sun 05/03 04Z 0.44 3213  91  36  -3 0.00
Sun 05/03 05Z 0.45 3214  92  34  -3 0.00
Sun 05/03 06Z 0.39 3214  90  37  -4 0.00
Sun 05/03 07Z 0.36 3215  87  43  -4 0.00
Sun 05/03 08Z 0.32 3316  86  46  -4 0.00
Sun 05/03 09Z 0.29 3316  84  48  -4 0.00
Sun 05/03 10Z 0.29 3316  83  48  -5 0.00
Sun 05/03 11Z 0.29 3315  81  52  -5 0.00
Sun 05/03 12Z 0.34 3314  79  53  -5 0.00
Sun 05/03 13Z 1.30 3314  77  44  -5 0.00
Sun 05/03 14Z 3.88 3214  73  40  -5 0.00
Sun 05/03 15Z 5.98 3214  69  43  -4 0.00
Sun 05/03 16Z 6.92 3114  66  47  -4 0.00
Sun 05/03 17Z 7.77 3014  68  53  -3 0.00
Sun 05/03 18Z 8.17 3014  70  54  -3 0.00
Sun 05/03 19Z 8.04 2914  70  51  -3 0.00
Sun 05/03 20Z 8.22 2914  66  54  -2 0.00
Sun 05/03 21Z 7.80 2913  64  49  -2 0.00
Sun 05/03 22Z 6.82 2914  62  47  -2 0.00
Sun 05/03 23Z 3.50 2816  60  44  -1 0.00
Mon 05/04 00Z 1.95 2819  59  39  -1 0.00
Mon 05/04 01Z 1.99 2820  59  38  -1 0.00
Mon 05/04 02Z 1.57 2722  63  22  -1 0.00
Mon 05/04 03Z 1.15 2724  72  85  -1 0.00
Mon 05/04 04Z 1.14 2725  77  87  -1 0.00
Mon 05/04 05Z 0.94 2626  87  91   0 0.01

Run total areal avg QPF for the greens is: 0.26 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