NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260430_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)
=== ===== === ==== ==== === === === ====
Thu 04/30 06Z 0.04 1726  82  95   6 0.00
Thu 04/30 07Z 0.05 1725  87  95   6 0.02
Thu 04/30 08Z 0.07 1824  95  98   5 0.07
Thu 04/30 09Z 0.14 1922  96  97   4 0.06
Thu 04/30 10Z 0.11 1918  97  99   4 0.03
Thu 04/30 11Z 0.09 1919  98  99   4 0.09
Thu 04/30 12Z 0.11 1919  98  96   3 0.07
Thu 04/30 13Z 0.14 2016  98  92   3 0.05
Thu 04/30 14Z 0.17 2013  97  87   3 0.02
Thu 04/30 15Z 0.23 2212  98  90   3 0.01
Thu 04/30 16Z 0.30 2310  98  91   2 0.01
Thu 04/30 17Z 0.36 2609  99  92   2 0.02
Thu 04/30 18Z 0.35 2909  99  91   1 0.02
Thu 04/30 19Z 0.35 2910  99  91   1 0.02
Thu 04/30 20Z 0.30 3011  98  91   1 0.02
Thu 04/30 21Z 0.31 3011  97  87   1 0.01
Thu 04/30 22Z 0.36 3013  95  78   0 0.01
Thu 04/30 23Z 0.47 3014  96  69   0 0.00
Fri 05/01 00Z 0.56 3015  96  64   0 0.01
Fri 05/01 01Z 0.60 3017  97  63  -1 0.01
Fri 05/01 02Z 0.53 3017  96  63  -1 0.01
Fri 05/01 03Z 0.48 3117  95  63  -1 0.01
Fri 05/01 04Z 0.44 3117  94  69  -1 0.01
Fri 05/01 05Z 0.41 3117  93  75  -2 0.00
Fri 05/01 06Z 0.38 3117  91  72  -2 0.00
Fri 05/01 07Z 0.37 3118  89  67  -2 0.00
Fri 05/01 08Z 0.37 3119  84  67  -2 0.00
Fri 05/01 09Z 0.38 3120  82  59  -3 0.00
Fri 05/01 10Z 0.38 3120  82  47  -3 0.00
Fri 05/01 11Z 0.38 3119  82  41  -3 0.00
Fri 05/01 12Z 0.42 3118  83  35  -3 0.00
Fri 05/01 13Z 0.47 3116  85  43  -4 0.00
Fri 05/01 14Z 0.61 3114  84  49  -4 0.00
Fri 05/01 15Z 2.33 3112  81  47  -4 0.00
Fri 05/01 16Z 3.88 3010  78  46  -3 0.00
Fri 05/01 17Z 4.63 2909  75  39  -2 0.00
Fri 05/01 18Z 5.14 2809  73  37  -2 0.00
Fri 05/01 19Z 5.35 2808  73  36  -1 0.00
Fri 05/01 20Z 4.54 2708  75  35  -1 0.00
Fri 05/01 21Z 3.89 2707  74  52  -1 0.00
Fri 05/01 22Z 2.48 2706  76  52  -1 0.00
Fri 05/01 23Z 0.80 2706  78  53  -1 0.00
Sat 05/02 00Z 0.44 2706  77  54  -1 0.00
Sat 05/02 01Z 0.40 2707  76  58  -1 0.00
Sat 05/02 02Z 0.37 2706  76  68  -1 0.00
Sat 05/02 03Z 0.29 2705  77  74  -1 0.00
Sat 05/02 04Z 0.15 2702  74  81  -1 0.00
Sat 05/02 05Z 0.10 3000  75  88  -1 0.00
Sat 05/02 06Z 0.07 1700  84  93  -2 0.01
Sat 05/02 07Z 0.07 1701  91  94  -2 0.02
Sat 05/02 08Z 0.06 1801  94  95  -2 0.03
Sat 05/02 09Z 0.04 1701  97  93  -2 0.03
Sat 05/02 10Z 0.04 1601  97  90  -2 0.02
Sat 05/02 11Z 0.03 1501  98  89  -2 0.01
Sat 05/02 12Z 0.03 1601  99  90  -2 0.01
Sat 05/02 13Z 0.03 1901 100  89  -2 0.01
Sat 05/02 14Z 0.05 2301 100  85  -2 0.01
Sat 05/02 15Z 0.11 2602 100  80  -3 0.01
Sat 05/02 16Z 0.18 2804  99  68  -3 0.01
Sat 05/02 17Z 0.26 2805  96  53  -3 0.01

Run total areal avg QPF for the greens is: 0.76 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