Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260126_1200
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 01/26 12Z 0.23 1317  85  84 -12 0.00
Mon 01/26 13Z 0.23 1317  86  90 -12 0.00
Mon 01/26 14Z 0.17 1414  87  92 -12 0.00
Mon 01/26 15Z 0.14 1412  89  91 -12 0.01
Mon 01/26 16Z 0.14 1512  89  92 -12 0.01
Mon 01/26 17Z 0.08 1508  90  93 -12 0.01
Mon 01/26 18Z 0.05 1606  91  92 -12 0.02
Mon 01/26 19Z 0.05 1805  91  93 -12 0.02
Mon 01/26 20Z 0.12 2205  91  93 -13 0.02
Mon 01/26 21Z 0.22 2407  90  93 -14 0.02
Mon 01/26 22Z 0.36 2810  88  92 -15 0.02
Mon 01/26 23Z 0.47 2914  88  86 -16 0.02
Tue 01/27 00Z 0.68 3018  88  85 -17 0.02
Tue 01/27 01Z 0.65 3020  87  81 -18 0.01
Tue 01/27 02Z 0.71 3122  90  76 -19 0.01
Tue 01/27 03Z 0.76 3124  91  75 -20 0.01
Tue 01/27 04Z 0.76 3124  92  75 -20 0.01
Tue 01/27 05Z 0.79 3026  91  71 -20 0.01
Tue 01/27 06Z 0.83 3027  90  67 -21 0.01
Tue 01/27 07Z 0.79 3028  90  59 -21 0.00
Tue 01/27 08Z 0.73 3028  89  46 -21 0.00
Tue 01/27 09Z 0.67 3028  83  32 -20 0.00
Tue 01/27 10Z 0.59 3028  73  21 -20 0.00
Tue 01/27 11Z 0.54 3027  65  23 -20 0.00
Tue 01/27 12Z 0.48 3025  63  34 -19 0.00
Tue 01/27 13Z 0.43 3022  64  35 -19 0.00
Tue 01/27 14Z 0.40 2918  68  55 -19 0.00
Tue 01/27 15Z 0.42 2815  75  73 -19 0.00
Tue 01/27 16Z 0.41 2613  79  79 -19 0.00
Tue 01/27 17Z 0.47 2512  79  81 -19 0.00
Tue 01/27 18Z 0.46 2412  76  79 -19 0.00
Tue 01/27 19Z 0.49 2411  76  80 -19 0.00
Tue 01/27 20Z 1.35 2412  82  81 -19 0.01
Tue 01/27 21Z 1.19 2313  87  80 -19 0.00
Tue 01/27 22Z 0.90 2315  88  77 -19 0.00
Tue 01/27 23Z 1.61 2517  88  68 -19 0.01
Wed 01/28 00Z 1.56 2619  87  68 -20 0.01
Wed 01/28 01Z 1.25 2719  81  70 -20 0.00
Wed 01/28 02Z 0.89 2619  78  72 -20 0.00
Wed 01/28 03Z 0.87 2620  83  71 -20 0.00
Wed 01/28 04Z 0.90 2721  87  72 -20 0.00
Wed 01/28 05Z 0.83 2820  88  71 -20 0.01
Wed 01/28 06Z 0.79 2819  87  68 -20 0.01
Wed 01/28 07Z 0.75 2918  89  66 -20 0.00
Wed 01/28 08Z 0.70 2919  89  65 -20 0.00
Wed 01/28 09Z 0.67 2919  90  64 -20 0.00
Wed 01/28 10Z 0.64 2919  90  62 -20 0.00
Wed 01/28 11Z 0.63 2919  91  58 -20 0.00
Wed 01/28 12Z 0.58 2918  92  53 -20 0.00
Wed 01/28 13Z 0.57 2918  92  48 -21 0.00
Wed 01/28 14Z 0.57 2917  89  44 -21 0.00
Wed 01/28 15Z 0.63 2916  88  41 -21 0.00
Wed 01/28 16Z 0.87 2915  85  37 -20 0.00
Wed 01/28 17Z 0.92 2913  82  35 -20 0.00
Wed 01/28 18Z 1.31 2912  78  40 -20 0.00
Wed 01/28 19Z 1.53 2911  74  48 -19 0.00
Wed 01/28 20Z 1.10 2810  71  55 -19 0.00
Wed 01/28 21Z 0.68 2810  72  57 -19 0.00
Wed 01/28 22Z 0.67 2712  77  57 -19 0.00
Wed 01/28 23Z 0.79 2614  81  65 -19 0.00

Run total areal avg QPF for the greens is: 0.28 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