NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260501_0600
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 06Z 0.48 3216  95  76  -3 0.00
Fri 05/01 07Z 0.47 3218  90  74  -2 0.00
Fri 05/01 08Z 0.48 3219  87  72  -3 0.00
Fri 05/01 09Z 0.49 3119  86  72  -3 0.00
Fri 05/01 10Z 0.52 3120  86  75  -3 0.00
Fri 05/01 11Z 0.55 3120  88  73  -3 0.00
Fri 05/01 12Z 0.57 3119  89  61  -4 0.00
Fri 05/01 13Z 0.75 3118  91  46  -4 0.00
Fri 05/01 14Z 2.27 3116  90  44  -4 0.00
Fri 05/01 15Z 4.01 3014  88  47  -4 0.00
Fri 05/01 16Z 3.87 3013  85  46  -4 0.00
Fri 05/01 17Z 4.83 2912  82  46  -3 0.00
Fri 05/01 18Z 5.86 2912  76  46  -2 0.00
Fri 05/01 19Z 6.46 2911  72  41  -2 0.00
Fri 05/01 20Z 6.24 2910  69  38  -1 0.00
Fri 05/01 21Z 4.92 2908  65  38  -1 0.00
Fri 05/01 22Z 2.92 2906  62  35  -1 0.00
Fri 05/01 23Z 0.79 2904  60  42   0 0.00
Sat 05/02 00Z 0.26 2702  60  47   0 0.00
Sat 05/02 01Z 0.21 2602  60  59   0 0.00
Sat 05/02 02Z 0.18 2302  61  66   0 0.00
Sat 05/02 03Z 0.14 1802  62  76   0 0.00
Sat 05/02 04Z 0.16 1604  63  84   0 0.00
Sat 05/02 05Z 0.19 1506  70  93  -1 0.01
Sat 05/02 06Z 0.26 1409  83  94  -1 0.04
Sat 05/02 07Z 0.24 1410  92  94  -1 0.06
Sat 05/02 08Z 0.22 1509  96  91  -1 0.05
Sat 05/02 09Z 0.23 1707  98  87  -1 0.03
Sat 05/02 10Z 0.24 1806  99  86  -2 0.03
Sat 05/02 11Z 0.23 1805  99  84  -2 0.02
Sat 05/02 12Z 0.24 1905  99  84  -2 0.02
Sat 05/02 13Z 0.23 2205  99  75  -2 0.01
Sat 05/02 14Z 0.24 2506  99  71  -3 0.01
Sat 05/02 15Z 0.26 2807  98  66  -3 0.01
Sat 05/02 16Z 0.31 2909  97  60  -3 0.00
Sat 05/02 17Z 0.41 3010  96  52  -3 0.00
Sat 05/02 18Z 0.94 3010  94  44  -3 0.00
Sat 05/02 19Z 1.54 3109  92  38  -3 0.00
Sat 05/02 20Z 1.34 3108  89  41  -2 0.00
Sat 05/02 21Z 0.96 3007  86  36  -2 0.00
Sat 05/02 22Z 0.48 3106  80  39  -2 0.00
Sat 05/02 23Z 0.25 3206  79  48  -2 0.00
Sun 05/03 00Z 0.23 3106  85  65  -2 0.00
Sun 05/03 01Z 0.20 3207  89  65  -2 0.01
Sun 05/03 02Z 0.19 3309  90  64  -3 0.01
Sun 05/03 03Z 0.20 3310  89  67  -3 0.01
Sun 05/03 04Z 0.27 3312  90  70  -3 0.00
Sun 05/03 05Z 0.32 3314  88  66  -3 0.00
Sun 05/03 06Z 0.31 3315  87  60  -4 0.00
Sun 05/03 07Z 0.28 3317  90  49  -4 0.00
Sun 05/03 08Z 0.26 3418  89  44  -4 0.00
Sun 05/03 09Z 0.25 3418  89  44  -4 0.00
Sun 05/03 10Z 0.26 3419  85  48  -4 0.00
Sun 05/03 11Z 0.30 3319  81  52  -5 0.00
Sun 05/03 12Z 0.39 3319  79  49  -5 0.00
Sun 05/03 13Z 1.19 3218  75  47  -5 0.00
Sun 05/03 14Z 4.44 3217  74  45  -5 0.00
Sun 05/03 15Z 7.05 3217  70  46  -5 0.00
Sun 05/03 16Z 8.15 3117  67  48  -5 0.00
Sun 05/03 17Z 9.21 3118  62  51  -4 0.00

Run total areal avg QPF for the greens is: 0.32 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