NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260419_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)
=== ===== === ==== ==== === === === ====
Sun 04/19 12Z 0.66 2918  98  99   1 0.00
Sun 04/19 13Z 0.56 3020  98  99   0 0.06
Sun 04/19 14Z 0.60 3120  98  99  -1 0.11
Sun 04/19 15Z 0.51 3120  97  99  -2 0.07
Sun 04/19 16Z 0.53 3021  97  99  -3 0.08
Sun 04/19 17Z 0.54 3021  96  96  -4 0.07
Sun 04/19 18Z 0.59 3122  96  94  -6 0.05
Sun 04/19 19Z 0.65 3022  96  91  -6 0.04
Sun 04/19 20Z 0.64 3122  83  88  -7 0.04
Sun 04/19 21Z 0.61 3121  89  77  -7 0.01
Sun 04/19 22Z 0.79 3020  90  18  -6 0.00
Sun 04/19 23Z 0.80 2920  91  20  -6 0.00
Mon 04/20 00Z 0.98 2922  94  27  -6 0.00
Mon 04/20 01Z 1.24 2825  94  39  -6 0.01
Mon 04/20 02Z 1.14 2924  86  31  -6 0.01
Mon 04/20 03Z 1.24 2924  78  45  -6 0.00
Mon 04/20 04Z 1.28 2822  75  49  -6 0.00
Mon 04/20 05Z 1.34 2721  76  56  -6 0.00
Mon 04/20 06Z 1.02 2720  83  39  -7 0.00
Mon 04/20 07Z 0.85 2720  85  53  -7 0.00
Mon 04/20 08Z 0.70 2819  80  44  -7 0.01
Mon 04/20 09Z 0.60 2918  83  56  -8 0.02
Mon 04/20 10Z 0.59 3018  86  62  -9 0.01
Mon 04/20 11Z 0.53 3117  86  54  -9 0.01
Mon 04/20 12Z 0.50 3115  87  51 -10 0.01
Mon 04/20 13Z 0.85 3213  91  42 -11 0.00
Mon 04/20 14Z 2.21 3212  89  40 -11 0.00
Mon 04/20 15Z 2.97 3111  83  38 -11 0.00
Mon 04/20 16Z 4.45 3110  77  39 -11 0.00
Mon 04/20 17Z 5.31 3010  71  39 -10 0.00
Mon 04/20 18Z 5.81 3011  67  36 -10 0.00
Mon 04/20 19Z 5.84 3010  64  31 -10 0.00
Mon 04/20 20Z 5.31 3011  64  28 -10 0.00
Mon 04/20 21Z 4.97 3011  62  24  -9 0.00
Mon 04/20 22Z 4.21 3111  60  18  -9 0.00
Mon 04/20 23Z 2.18 3111  59  13 -10 0.00
Tue 04/21 00Z 1.08 3211  56  13 -10 0.00
Tue 04/21 01Z 1.11 3212  54  12 -10 0.00
Tue 04/21 02Z 1.43 3213  53  11 -10 0.00
Tue 04/21 03Z 1.48 3213  52  10 -10 0.00
Tue 04/21 04Z 1.25 3212  52  11 -11 0.00
Tue 04/21 05Z 1.01 3212  52  12 -11 0.00
Tue 04/21 06Z 0.74 3111  52  11 -11 0.00
Tue 04/21 07Z 0.56 3109  50  10 -11 0.00
Tue 04/21 08Z 0.56 3109  50  11 -11 0.00
Tue 04/21 09Z 0.49 3009  49  11 -11 0.00
Tue 04/21 10Z 0.42 2907  48  11 -11 0.00
Tue 04/21 11Z 0.38 2807  47  11 -11 0.00
Tue 04/21 12Z 0.44 2706  47  11 -10 0.00
Tue 04/21 13Z 1.43 2506  48  10 -10 0.00
Tue 04/21 14Z 2.75 2405  47   8 -10 0.00
Tue 04/21 15Z 3.33 2306  45   8  -9 0.00
Tue 04/21 16Z 3.24 2206  42   9  -8 0.00
Tue 04/21 17Z 3.52 2207  40  17  -7 0.00
Tue 04/21 18Z 3.78 2307  39  40  -6 0.00
Tue 04/21 19Z 3.49 2307  38  80  -6 0.00
Tue 04/21 20Z 3.40 2307  39  85  -5 0.00
Tue 04/21 21Z 2.29 2307  39  81  -5 0.00
Tue 04/21 22Z 0.99 2207  38  75  -5 0.00
Tue 04/21 23Z 0.39 2208  39  72  -4 0.00

Run total areal avg QPF for the greens is: 0.61 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