NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260211_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)
=== ===== === ==== ==== === === === ====
Wed 02/11 12Z 0.59 2919  95  77 -10 0.00
Wed 02/11 13Z 0.69 2921  96  83 -10 0.00
Wed 02/11 14Z 0.77 2920  96  81 -10 0.01
Wed 02/11 15Z 0.90 2919  95  78 -10 0.01
Wed 02/11 16Z 1.04 2918  96  80 -10 0.01
Wed 02/11 17Z 1.18 2817  96  85 -10 0.01
Wed 02/11 18Z 1.00 2816  97  87 -10 0.01
Wed 02/11 19Z 1.15 2817  96  87 -10 0.01
Wed 02/11 20Z 1.23 2818  95  85 -10 0.02
Wed 02/11 21Z 1.42 2920  94  84 -11 0.02
Wed 02/11 22Z 1.06 3021  94  83 -11 0.02
Wed 02/11 23Z 0.91 3123  95  85 -12 0.02
Thu 02/12 00Z 0.85 3224  95  85 -13 0.02
Thu 02/12 02Z 0.48 3324  95  84 -13 0.01
Thu 02/12 03Z 0.40 3324  95  83 -12 0.01
Thu 02/12 04Z 0.40 3324  95  82 -13 0.01
Thu 02/12 05Z 0.40 3323  94  81 -13 0.01
Thu 02/12 06Z 0.39 3324  94  79 -13 0.01
Thu 02/12 07Z 0.32 3324  94  79 -13 0.01
Thu 02/12 08Z 0.23 3425  94  79 -13 0.01
Thu 02/12 09Z 0.20 3424  94  78 -12 0.01
Thu 02/12 10Z 0.21 3424  94  75 -12 0.01
Thu 02/12 11Z 0.22 3423  94  75 -12 0.01
Thu 02/12 12Z 0.22 3323  94  72 -12 0.01
Thu 02/12 13Z 0.21 3323  93  68 -12 0.01
Thu 02/12 14Z 0.22 3323  93  65 -12 0.01
Thu 02/12 15Z 0.22 3323  92  63 -12 0.00
Thu 02/12 16Z 0.24 3322  92  57 -12 0.00
Thu 02/12 17Z 0.27 3322  92  42 -12 0.00
Thu 02/12 18Z 0.32 3222  92  27 -12 0.00
Thu 02/12 19Z 0.34 3222  89  16 -12 0.00
Thu 02/12 20Z 0.33 3222  82   8 -12 0.00
Thu 02/12 21Z 0.29 3322  75   8 -11 0.00
Thu 02/12 22Z 0.22 3323  69  12 -11 0.00
Thu 02/12 23Z 0.16 3424  63  12 -10 0.00
Fri 02/13 00Z 0.11 3424  63  14 -10 0.00
Fri 02/13 01Z 0.09 3424  64  16 -10 0.00
Fri 02/13 02Z 0.08 3423  64  17  -9 0.00
Fri 02/13 03Z 0.08 3522  66  19  -9 0.00
Fri 02/13 04Z 0.08 3422  67  21  -9 0.00
Fri 02/13 05Z 0.09 3420  67  23  -9 0.00
Fri 02/13 06Z 0.11 3419  66  25  -9 0.00
Fri 02/13 07Z 0.14 3319  66  27  -9 0.00
Fri 02/13 08Z 0.17 3319  67  26 -10 0.00
Fri 02/13 09Z 0.19 3219  66  22 -10 0.00
Fri 02/13 10Z 0.22 3220  67  22 -10 0.00
Fri 02/13 11Z 0.26 3121  66  19 -10 0.00
Fri 02/13 12Z 0.28 3121  66  19 -10 0.00
Fri 02/13 13Z 0.31 3122  67  20 -10 0.00
Fri 02/13 14Z 0.32 3121  65  21 -10 0.00
Fri 02/13 15Z 0.33 3021  62  21 -10 0.00
Fri 02/13 16Z 0.36 3020  60  26 -11 0.00
Fri 02/13 17Z 0.39 3018  60  37 -11 0.00
Fri 02/13 18Z 0.43 2918  60  47 -11 0.00
Fri 02/13 19Z 0.47 2918  63  56 -11 0.00
Fri 02/13 20Z 0.54 2818  69  61 -11 0.00
Fri 02/13 21Z 0.57 2818  69  47 -11 0.00
Fri 02/13 22Z 0.59 2720  67  45 -10 0.00
Fri 02/13 23Z 0.56 2721  66  49 -10 0.00

Run total areal avg QPF for the greens is: 0.29 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