NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260225_0000
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)
=== ===== === ==== ==== === === === ====
Wed 02/25 00Z 0.53 2814  79  18 -15 0.00
Wed 02/25 01Z 0.46 2714  78  10 -15 0.00
Wed 02/25 02Z 0.49 2716  77   6 -14 0.00
Wed 02/25 03Z 0.41 2615  72   8 -14 0.00
Wed 02/25 04Z 0.26 2411  66  24 -13 0.00
Wed 02/25 05Z 0.29 2315  67  40 -13 0.00
Wed 02/25 06Z 0.26 2216  62  52 -12 0.00
Wed 02/25 08Z 0.36 2220  58  85 -12 0.00
Wed 02/25 09Z 0.27 2119  61  67 -11 0.00
Wed 02/25 10Z 0.19 2021  66  71 -10 0.00
Wed 02/25 11Z 0.19 1926  74  90 -10 0.00
Wed 02/25 12Z 0.17 1929  88  87  -9 0.00
Wed 02/25 13Z 0.18 1930  91  86  -9 0.01
Wed 02/25 14Z 0.25 2029  92  86  -9 0.03
Wed 02/25 16Z 0.34 2120  94  87  -9 0.03
Wed 02/25 17Z 0.37 2116  94  57  -8 0.03
Wed 02/25 18Z 0.45 2115  93  43  -8 0.02
Wed 02/25 19Z 0.70 2115  93  54  -8 0.01
Wed 02/25 20Z 0.82 2116  93  75  -8 0.00
Wed 02/25 21Z 0.63 2117  94  83  -7 0.00
Wed 02/25 22Z 0.69 2219  95  86  -7 0.00
Thu 02/26 00Z 0.58 2222  94  82  -6 0.00
Thu 02/26 01Z 0.66 2226  97  87  -6 0.01
Thu 02/26 02Z 0.93 2326  96  90  -6 0.00
Thu 02/26 03Z 1.35 2424  93  73  -7 0.00
Thu 02/26 04Z 1.59 2524  91  73  -8 0.02
Thu 02/26 05Z 1.37 2524  91  56  -9 0.03
Thu 02/26 06Z 1.06 2523  90  68  -9 0.02
Thu 02/26 07Z 1.10 2524  94  37 -10 0.01
Thu 02/26 08Z 1.46 2625  95  34 -11 0.00
Thu 02/26 09Z 1.86 2726  93  37 -12 0.00
Thu 02/26 10Z 2.08 2726  87  38 -14 0.00
Thu 02/26 11Z 1.33 2727  77  35 -14 0.01
Thu 02/26 12Z 1.21 2727  73  23 -14 0.01
Thu 02/26 13Z 1.32 2727  75  12 -15 0.00
Thu 02/26 14Z 1.52 2726  76   6 -15 0.00
Thu 02/26 15Z 3.88 2724  77   4 -15 0.00
Thu 02/26 16Z 7.82 2721  79   4 -15 0.00
Thu 02/26 17Z 8.60 2718  80   5 -15 0.00
Thu 02/26 18Z 8.37 2617  80   6 -15 0.00
Thu 02/26 19Z 8.23 2617  81   7 -15 0.00
Thu 02/26 20Z 7.88 2618  81  10 -14 0.00
Thu 02/26 21Z 5.98 2618  79  13 -14 0.00
Thu 02/26 23Z 1.68 2721  77  19 -14 0.00
Fri 02/27 01Z 0.88 2821  74  27 -14 0.00
Fri 02/27 03Z 0.61 2821  68  37 -13 0.00
Fri 02/27 07Z 0.33 2816  54  50 -10 0.00
Fri 02/27 09Z 0.28 2714  47  62  -9 0.00
Fri 02/27 11Z 0.32 2616  47  78  -8 0.00

Run total areal avg QPF for the greens is: 0.24 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