NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260101_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 01/01 06Z 0.18 1921  90  82 -12 0.00
Thu 01/01 07Z 0.25 2022  92  81 -11 0.02
Thu 01/01 08Z 0.33 2023  93  82 -11 0.02
Thu 01/01 09Z 0.61 2124  95  80 -11 0.01
Thu 01/01 10Z 0.96 2324  95  82 -12 0.01
Thu 01/01 11Z 1.37 2424  95  80 -13 0.01
Thu 01/01 12Z 2.62 2724  90  63 -15 0.02
Thu 01/01 13Z 2.32 2824  88  54 -17 0.01
Thu 01/01 14Z 2.75 2924  87  49 -18 0.00
Thu 01/01 15Z 5.08 2922  87  48 -19 0.00
Thu 01/01 16Z 8.31 2921  88  49 -20 0.00
Thu 01/01 17Z 7.84 2921  84  45 -21 0.00
Thu 01/01 18Z 7.32 2920  79  44 -21 0.00
Thu 01/01 19Z 4.20 2919  73  42 -20 0.00
Thu 01/01 20Z 3.85 2819  73  41 -21 0.00
Thu 01/01 21Z 2.39 2820  73  49 -21 0.00
Thu 01/01 22Z 1.38 2821  67  43 -20 0.00
Thu 01/01 23Z 1.41 2823  63  25 -20 0.00
Fri 01/02 00Z 0.91 2825  50  11 -20 0.00
Fri 01/02 01Z 0.73 2927  43  10 -19 0.00
Fri 01/02 02Z 0.71 2926  50  15 -19 0.00
Fri 01/02 03Z 0.70 2824  58  16 -19 0.00
Fri 01/02 04Z 0.73 2822  70  39 -19 0.00
Fri 01/02 05Z 0.75 2721  81  41 -19 0.00
Fri 01/02 06Z 0.77 2620  87  62 -19 0.00
Fri 01/02 07Z 0.84 2619  89  60 -19 0.00
Fri 01/02 08Z 1.00 2619  91  60 -20 0.00
Fri 01/02 09Z 1.07 2719  91  50 -20 0.00
Fri 01/02 10Z 1.07 2720  88  44 -20 0.00
Fri 01/02 11Z 0.97 2821  80  38 -21 0.00
Fri 01/02 12Z 0.87 2821  76  32 -21 0.00
Fri 01/02 13Z 0.85 2821  75  29 -21 0.00
Fri 01/02 14Z 0.98 2821  77  20 -21 0.00
Fri 01/02 15Z 1.90 2720  79  17 -21 0.00
Fri 01/02 16Z 4.28 2819  80  13 -20 0.00
Fri 01/02 17Z 6.48 2718  82  12 -20 0.00
Fri 01/02 18Z 7.98 2718  82  15 -20 0.00
Fri 01/02 19Z 8.20 2718  81  15 -19 0.00
Fri 01/02 20Z 7.19 2719  77  24 -19 0.00
Fri 01/02 21Z 2.77 2822  72  15 -19 0.00
Fri 01/02 22Z 1.77 2824  71  16 -19 0.00
Fri 01/02 23Z 1.71 2825  73  30 -19 0.00
Sat 01/03 00Z 1.44 2825  76  68 -19 0.00
Sat 01/03 01Z 1.43 2826  81  78 -18 0.00
Sat 01/03 02Z 1.50 2827  84  73 -18 0.00
Sat 01/03 03Z 1.64 2828  83  65 -19 0.00
Sat 01/03 04Z 1.89 2829  83  53 -19 0.00
Sat 01/03 05Z 1.54 2829  84  43 -19 0.00
Sat 01/03 06Z 1.53 2830  83  46 -19 0.00
Sat 01/03 07Z 1.19 2931  82  59 -19 0.00
Sat 01/03 08Z 1.01 2931  82  68 -18 0.00
Sat 01/03 09Z 0.89 2929  84  66 -18 0.00
Sat 01/03 10Z 0.85 2928  88  58 -18 0.00
Sat 01/03 11Z 0.80 2827  92  53 -17 0.00
Sat 01/03 12Z 0.85 2826  94  54 -17 0.00
Sat 01/03 13Z 0.99 2826  96  36 -17 0.00
Sat 01/03 14Z 1.15 2725  96  63 -17 0.01
Sat 01/03 15Z 1.30 2724  96  67 -17 0.00
Sat 01/03 16Z 1.40 2723  96  56 -17 0.01
Sat 01/03 17Z 1.40 2722  95  39 -17 0.01

Run total areal avg QPF for the greens is: 0.13 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