NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260120_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)
=== ===== === ==== ==== === === === ====
Tue 01/20 06Z 1.21 2426  84  56 -17 0.00
Tue 01/20 07Z 1.44 2531  86  45 -17 0.00
Tue 01/20 08Z 1.56 2630  88  41 -18 0.00
Tue 01/20 09Z 1.91 2729  86  48 -18 0.00
Tue 01/20 10Z 2.02 2727  85  52 -19 0.00
Tue 01/20 11Z 1.78 2625  85  52 -19 0.00
Tue 01/20 13Z 1.76 2722  83  59 -20 0.00
Tue 01/20 14Z 2.58 2720  78  60 -20 0.00
Tue 01/20 15Z 6.12 2618  79  66 -21 0.00
Tue 01/20 16Z 8.10 2617  81  70 -21 0.00
Tue 01/20 17Z 8.24 2517  83  71 -21 0.00
Tue 01/20 18Z 7.71 2517  84  69 -20 0.00
Tue 01/20 19Z 8.27 2518  82  64 -20 0.00
Tue 01/20 20Z 8.84 2519  79  60 -20 0.00
Tue 01/20 21Z 7.95 2522  79  54 -20 0.00
Tue 01/20 22Z 3.64 2624  79  45 -20 0.00
Tue 01/20 23Z 2.34 2624  82  44 -21 0.00
Wed 01/21 00Z 2.11 2626  82  43 -21 0.00
Wed 01/21 02Z 1.39 2627  77  43 -21 0.00
Wed 01/21 03Z 1.14 2627  74  42 -21 0.00
Wed 01/21 04Z 1.06 2628  76  39 -20 0.00
Wed 01/21 05Z 1.00 2628  77  34 -20 0.00
Wed 01/21 06Z 0.93 2628  79  34 -20 0.00
Wed 01/21 07Z 0.81 2627  82  38 -19 0.00
Wed 01/21 08Z 0.78 2627  81  32 -19 0.00
Wed 01/21 09Z 0.76 2527  77  27 -18 0.00
Wed 01/21 10Z 0.70 2527  72  17 -18 0.00
Wed 01/21 11Z 0.65 2527  67  14 -17 0.00
Wed 01/21 12Z 0.59 2526  61  25 -16 0.00
Wed 01/21 13Z 0.54 2525  61  25 -16 0.00
Wed 01/21 14Z 0.51 2425  57  19 -15 0.00
Wed 01/21 15Z 0.45 2423  51  47 -14 0.00
Wed 01/21 16Z 0.40 2321  47  84 -14 0.00
Wed 01/21 17Z 0.39 2222  47  86 -14 0.00
Wed 01/21 18Z 0.39 2124  43  87 -14 0.00
Wed 01/21 19Z 0.35 2025  50  81 -14 0.00
Wed 01/21 20Z 0.25 2026  57  89 -12 0.00
Wed 01/21 21Z 0.18 1929  65  90 -11 0.00
Wed 01/21 22Z 0.17 1932  87  88 -10 0.00
Wed 01/21 23Z 0.33 2038  90  91 -11 0.00
Thu 01/22 00Z 0.51 2040  91  87 -12 0.01
Thu 01/22 01Z 0.54 2140  91  89 -11 0.04
Thu 01/22 02Z 0.58 2139  93  81 -10 0.02
Thu 01/22 03Z 0.63 2238  94  73  -9 0.01
Thu 01/22 04Z 0.66 2236  94  69  -9 0.00
Thu 01/22 05Z 0.67 2334  93  65  -9 0.01
Thu 01/22 06Z 0.66 2332  92  59  -9 0.01
Thu 01/22 07Z 0.65 2331  92  54  -9 0.00
Thu 01/22 08Z 0.67 2331  92  57  -9 0.00
Thu 01/22 09Z 0.71 2331  91  66  -9 0.00
Thu 01/22 10Z 0.77 2331  92  78  -9 0.00
Thu 01/22 11Z 0.84 2431  92  84  -9 0.00
Thu 01/22 12Z 0.93 2432  92  81 -10 0.01
Thu 01/22 13Z 1.00 2432  92  64 -10 0.01
Thu 01/22 14Z 1.10 2432  90  50 -10 0.00
Thu 01/22 15Z 1.41 2431  89  35 -11 0.00
Thu 01/22 16Z 2.65 2429  90  61 -11 0.00
Thu 01/22 17Z 2.63 2427  92  56 -11 0.00

Run total areal avg QPF for the greens is: 0.12 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