NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260415_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)
=== ===== === ==== ==== === === === ====
Wed 04/15 06Z 0.33 2712  85  33   8 0.00
Wed 04/15 07Z 0.32 2714  80  23   9 0.00
Wed 04/15 08Z 0.28 2814  75  19  10 0.00
Wed 04/15 09Z 0.25 2914  72  19  11 0.00
Wed 04/15 10Z 0.21 2912  68  19  12 0.00
Wed 04/15 11Z 0.15 2909  61  18  12 0.00
Wed 04/15 12Z 0.10 2805  56  18  13 0.00
Wed 04/15 13Z 0.13 2507  55  19  12 0.00
Wed 04/15 14Z 0.14 2309  51  18  13 0.00
Wed 04/15 15Z 0.11 2012  53  15  13 0.00
Wed 04/15 16Z 0.23 2118  66  55  11 0.00
Wed 04/15 17Z 0.24 2021  86  94  10 0.05
Wed 04/15 18Z 0.24 2022  94  97  10 0.06
Wed 04/15 19Z 0.44 2225  99  76  11 0.06
Wed 04/15 20Z 0.60 2325  98  56  11 0.03
Wed 04/15 21Z 0.82 2426  99  40  10 0.03
Wed 04/15 22Z 0.90 2526  99  39  10 0.02
Wed 04/15 23Z 0.88 2525  98  51  10 0.02
Thu 04/16 00Z 0.77 2524  97  59  11 0.01
Thu 04/16 01Z 0.80 2625  95  67  11 0.01
Thu 04/16 02Z 0.64 2623  90  77  12 0.01
Thu 04/16 03Z 0.55 2621  87  81  12 0.01
Thu 04/16 04Z 0.53 2620  87  88  12 0.01
Thu 04/16 05Z 0.45 2617  85  83  12 0.01
Thu 04/16 06Z 0.49 2617  90  87  11 0.02
Thu 04/16 07Z 0.30 2611  90  87  11 0.15
Thu 04/16 08Z 0.25 2709  90  89  11 0.03
Thu 04/16 09Z 0.27 2810  92  84  11 0.02
Thu 04/16 10Z 0.35 2813  94  72  10 0.00
Thu 04/16 11Z 0.38 2814  96  72  10 0.01
Thu 04/16 12Z 0.37 2914  95  74  10 0.01
Thu 04/16 13Z 0.34 2813  95  71  10 0.00
Thu 04/16 14Z 0.31 2811  95  71  10 0.00
Thu 04/16 15Z 0.31 2810  95  68  10 0.00
Thu 04/16 16Z 0.33 2609  95  79  10 0.00
Thu 04/16 17Z 0.32 2509  96  83  10 0.00
Thu 04/16 18Z 0.30 2409  97  86  10 0.01
Thu 04/16 19Z 0.31 2309  97  89  10 0.00
Thu 04/16 20Z 0.35 2211  97  88  10 0.01
Thu 04/16 21Z 0.42 2214  97  88  10 0.01
Thu 04/16 22Z 0.49 2216  96  95  11 0.02
Thu 04/16 23Z 0.40 2216  94  82  11 0.03
Fri 04/17 00Z 0.39 2119  94  86  12 0.01
Fri 04/17 01Z 0.39 2121  95  83  12 0.07
Fri 04/17 02Z 0.44 2120  93  86  12 0.06
Fri 04/17 03Z 0.39 2218  92  89  12 0.18
Fri 04/17 04Z 0.51 2418  97  93  11 0.38
Fri 04/17 05Z 0.78 2726  88  92  10 0.33
Fri 04/17 06Z 0.50 2418  63  60  12 0.01
Fri 04/17 07Z 0.69 2423  80  36  11 0.00
Fri 04/17 08Z 0.72 2525  87  41  10 0.00
Fri 04/17 09Z 0.76 2525  93  43   9 0.01
Fri 04/17 10Z 0.70 2623  95  42   9 0.01
Fri 04/17 11Z 0.65 2721  96  48   8 0.01
Fri 04/17 12Z 0.67 2821  97  65   8 0.02
Fri 04/17 13Z 0.64 2820  97  77   8 0.03
Fri 04/17 14Z 0.60 2820  97  81   8 0.03
Fri 04/17 15Z 0.56 2919  97  82   7 0.03
Fri 04/17 16Z 0.47 3119  97  81   7 0.03
Fri 04/17 17Z 0.36 3219  96  82   7 0.02

Run total areal avg QPF for the greens is: 1.88 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