Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260119_1800
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)
=== ===== === ==== ==== === === === ====
Mon 01/19 18Z 4.34 2212  88  74 -14 0.00
Mon 01/19 19Z 4.01 2214  88  72 -14 0.00
Mon 01/19 20Z 3.50 2318  90  83 -14 0.00
Mon 01/19 21Z 1.66 2322  91  79 -14 0.00
Mon 01/19 22Z 2.28 2425  90  74 -14 0.01
Mon 01/19 23Z 1.83 2426  85  57 -14 0.01
Tue 01/20 00Z 1.57 2427  85  62 -14 0.00
Tue 01/20 01Z 2.11 2430  85  53 -15 0.00
Tue 01/20 02Z 2.36 2431  83  50 -15 0.00
Tue 01/20 03Z 3.66 2431  81  49 -16 0.00
Tue 01/20 04Z 3.25 2531  79  53 -17 0.00
Tue 01/20 05Z 2.26 2530  80  48 -17 0.00
Tue 01/20 06Z 1.25 2530  82  51 -17 0.00
Tue 01/20 07Z 1.36 2531  88  40 -17 0.00
Tue 01/20 08Z 1.46 2630  89  48 -18 0.00
Tue 01/20 09Z 1.89 2628  89  49 -18 0.00
Tue 01/20 10Z 3.04 2627  88  61 -19 0.00
Tue 01/20 11Z 2.98 2625  84  57 -19 0.00
Tue 01/20 12Z 2.16 2625  80  55 -20 0.00
Tue 01/20 13Z 1.77 2724  74  56 -20 0.00
Tue 01/20 14Z 2.55 2723  72  56 -21 0.00
Tue 01/20 15Z 4.72 2722  72  61 -21 0.00
Tue 01/20 16Z 8.80 2721  72  65 -21 0.00
Tue 01/20 17Z 11.04 2619  75  66 -21 0.00
Tue 01/20 18Z 10.55 2618  77  68 -21 0.00
Tue 01/20 19Z 10.01 2519  75  69 -21 0.00
Tue 01/20 20Z 8.56 2520  75  68 -20 0.00
Tue 01/20 21Z 8.23 2521  77  59 -21 0.00
Tue 01/20 22Z 3.31 2622  81  55 -21 0.00
Tue 01/20 23Z 3.48 2623  82  51 -21 0.00
Wed 01/21 00Z 3.04 2625  80  50 -21 0.00
Wed 01/21 01Z 1.90 2627  75  49 -21 0.00
Wed 01/21 02Z 1.45 2628  75  45 -21 0.00
Wed 01/21 03Z 1.25 2628  77  43 -21 0.00
Wed 01/21 04Z 1.10 2627  79  44 -20 0.00
Wed 01/21 05Z 1.02 2627  80  43 -20 0.00
Wed 01/21 06Z 0.95 2627  79  41 -20 0.00
Wed 01/21 07Z 0.81 2626  78  37 -19 0.00
Wed 01/21 08Z 0.85 2628  84  28 -19 0.00
Wed 01/21 09Z 0.76 2527  81  16 -18 0.00
Wed 01/21 10Z 0.69 2526  75  13 -18 0.00
Wed 01/21 11Z 0.68 2527  69  11 -17 0.00
Wed 01/21 12Z 0.66 2527  69  13 -17 0.00
Wed 01/21 13Z 0.62 2527  67  16 -16 0.00
Wed 01/21 14Z 0.55 2525  61  24 -15 0.00
Wed 01/21 15Z 0.47 2423  56  35 -15 0.00
Wed 01/21 16Z 0.42 2322  52  78 -14 0.00
Wed 01/21 17Z 0.36 2222  42  90 -14 0.00
Wed 01/21 18Z 0.29 2022  36  92 -13 0.00
Wed 01/21 19Z 0.32 2025  41  89 -14 0.00
Wed 01/21 20Z 0.25 1926  57  89 -12 0.00
Wed 01/21 21Z 0.17 1928  74  88 -11 0.01
Wed 01/21 22Z 0.16 1935  88  90 -10 0.03
Wed 01/21 23Z 0.33 2040  91  91 -11 0.04
Thu 01/22 00Z 0.43 2041  93  91 -11 0.02
Thu 01/22 01Z 0.50 2042  94  91 -10 0.01
Thu 01/22 02Z 0.58 2141  94  92  -9 0.01
Thu 01/22 03Z 0.64 2140  94  92  -9 0.02
Thu 01/22 04Z 0.64 2238  94  86  -8 0.01
Thu 01/22 05Z 0.64 2236  92  73  -8 0.01

Run total areal avg QPF for the greens is: 0.18 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