NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20251007_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 10/07 06Z 0.69 2519  63  25  14 0.00
Tue 10/07 07Z 0.63 2518  60  27  14 0.00
Tue 10/07 08Z 0.60 2517  57  28  14 0.00
Tue 10/07 09Z 0.62 2516  56  25  14 0.00
Tue 10/07 10Z 0.69 2514  56  21  14 0.00
Tue 10/07 11Z 0.67 2415  57  19  13 0.00
Tue 10/07 12Z 0.61 2315  58  15  13 0.00
Tue 10/07 13Z 0.59 2217  59  14  13 0.00
Tue 10/07 14Z 0.59 2219  62  23  13 0.00
Tue 10/07 15Z 0.61 2221  68  30  13 0.00
Tue 10/07 16Z 0.73 2124  75  47  13 0.00
Tue 10/07 17Z 1.08 2226  78  72  13 0.00
Tue 10/07 18Z 1.43 2226  77  82  14 0.00
Tue 10/07 19Z 1.34 2225  77  85  14 0.00
Tue 10/07 20Z 1.08 2225  87  89  13 0.00
Tue 10/07 21Z 0.82 2227  94  88  13 0.07
Tue 10/07 22Z 0.76 2227  94  92  12 0.10
Tue 10/07 23Z 0.70 2226  92  84  12 0.10
Wed 10/08 00Z 0.72 2227  94  84  12 0.11
Wed 10/08 01Z 0.71 2231  96  82  12 0.12
Wed 10/08 02Z 0.82 2233  97  84  12 0.14
Wed 10/08 03Z 0.87 2333  96  88  12 0.07
Wed 10/08 04Z 0.79 2233  98  91  12 0.08
Wed 10/08 05Z 0.73 2426  98  94  11 0.28
Wed 10/08 06Z 0.48 2517  98  94  10 0.20
Wed 10/08 07Z 0.32 2711  98  94   9 0.21
Wed 10/08 08Z 0.27 3112  96  94   8 0.11
Wed 10/08 09Z 0.15 3315  96  96   7 0.09
Wed 10/08 10Z 0.12 3416  92  97   6 0.09
Wed 10/08 11Z 0.07 3415  89  88   6 0.06
Wed 10/08 12Z 0.10 3418  88  82   5 0.01
Wed 10/08 13Z 0.16 3419  85  71   4 0.01
Wed 10/08 14Z 0.17 3419  78  54   4 0.00
Wed 10/08 15Z 0.18 3419  70  37   4 0.00
Wed 10/08 16Z 0.20 3419  68  27   3 0.00
Wed 10/08 17Z 0.28 3319  68  13   2 0.00
Wed 10/08 18Z 0.55 3319  70   5   2 0.00
Wed 10/08 19Z 0.96 3319  70   2   1 0.00
Wed 10/08 20Z 1.22 3320  74   2   1 0.00
Wed 10/08 21Z 0.98 3321  80   3   0 0.00
Wed 10/08 22Z 0.60 3324  88  11   0 0.00
Wed 10/08 23Z 0.54 3326  87   3   0 0.01
Thu 10/09 00Z 0.45 3426  93   3   0 0.01
Thu 10/09 01Z 0.38 3425  94   3  -1 0.01
Thu 10/09 02Z 0.28 3424  94   4  -2 0.01
Thu 10/09 03Z 0.23 3422  91   8  -2 0.01
Thu 10/09 04Z 0.24 3422  90   6  -3 0.00
Thu 10/09 05Z 0.20 3421  82   7  -3 0.00
Thu 10/09 06Z 0.17 3421  72   6  -3 0.00
Thu 10/09 07Z 0.17 3422  64   5  -3 0.00
Thu 10/09 08Z 0.16 3422  61   4  -4 0.00
Thu 10/09 09Z 0.15 3423  61   4  -4 0.00
Thu 10/09 10Z 0.15 3425  64   6  -5 0.00
Thu 10/09 11Z 0.13 3427  68   7  -5 0.00
Thu 10/09 12Z 0.13 3527  73   6  -6 0.00
Thu 10/09 13Z 0.12 3527  77   5  -6 0.00
Thu 10/09 14Z 0.10 3524  77   4  -6 0.00
Thu 10/09 15Z 0.12 3422  75   5  -5 0.00
Thu 10/09 16Z 0.16 3420  72   6  -5 0.00
Thu 10/09 17Z 0.26 3419  70   7  -5 0.00

Run total areal avg QPF for the greens is: 1.90 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