NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20251012_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 10/12 12Z 0.64 1019  71   5   4 0.00
Sun 10/12 13Z 0.67 0923  68   6   5 0.00
Sun 10/12 14Z 0.67 0924  67   6   5 0.00
Sun 10/12 15Z 0.66 1024  64   6   5 0.00
Sun 10/12 16Z 0.64 1023  62   7   5 0.00
Sun 10/12 17Z 0.68 0921  66   7   5 0.00
Sun 10/12 18Z 0.88 1020  69   6   4 0.00
Sun 10/12 19Z 1.05 1019  73   6   3 0.00
Sun 10/12 20Z 1.34 1019  76   6   3 0.00
Sun 10/12 21Z 0.90 1020  80   6   4 0.00
Sun 10/12 22Z 0.75 1023  84   5   4 0.00
Sun 10/12 23Z 0.69 1025  85   4   4 0.01
Mon 10/13 00Z 0.62 1025  82   4   5 0.01
Mon 10/13 01Z 0.64 1027  78   4   5 0.00
Mon 10/13 02Z 0.59 1128  76   5   6 0.00
Mon 10/13 03Z 0.55 1127  74  13   7 0.00
Mon 10/13 04Z 0.52 1126  73  15   7 0.00
Mon 10/13 05Z 0.48 1125  69  25   7 0.01
Mon 10/13 06Z 0.48 1025  65  32   8 0.01
Mon 10/13 07Z 0.46 1025  60  31   8 0.01
Mon 10/13 08Z 0.44 1023  53  29   8 0.00
Mon 10/13 09Z 0.45 1023  49  29   8 0.00
Mon 10/13 10Z 0.46 1023  49  58   7 0.00
Mon 10/13 11Z 0.47 0922  44  65   7 0.00
Mon 10/13 12Z 0.46 0921  42  72   7 0.00
Mon 10/13 13Z 0.46 0921  43  83   7 0.00
Mon 10/13 14Z 0.50 0922  45  93   6 0.00
Mon 10/13 15Z 0.50 0923  52  92   6 0.00
Mon 10/13 16Z 0.52 0924  56  92   6 0.00
Mon 10/13 17Z 0.52 0924  61  93   6 0.00
Mon 10/13 18Z 0.50 1024  70  95   6 0.00
Mon 10/13 19Z 0.49 1024  74  95   7 0.01
Mon 10/13 20Z 0.50 1025  79  95   7 0.02
Mon 10/13 21Z 0.46 1023  88  95   7 0.03
Mon 10/13 22Z 0.44 1023  95  96   8 0.05
Mon 10/13 23Z 0.43 1023  96  95   8 0.06
Tue 10/14 00Z 0.42 1023  95  96   8 0.05
Tue 10/14 01Z 0.42 1124  95  96   8 0.04
Tue 10/14 02Z 0.41 1124  94  96   9 0.03
Tue 10/14 03Z 0.39 1124  94  95   9 0.02
Tue 10/14 04Z 0.37 1223  94  95   9 0.02
Tue 10/14 05Z 0.36 1222  92  94   9 0.02
Tue 10/14 06Z 0.32 1120  90  95   9 0.01
Tue 10/14 07Z 0.30 1118  91  95   9 0.01
Tue 10/14 08Z 0.28 1116  90  94   9 0.01
Tue 10/14 09Z 0.29 1115  88  89   9 0.00
Tue 10/14 10Z 0.27 1114  88  87   9 0.00
Tue 10/14 11Z 0.23 1112  91  88   9 0.00
Tue 10/14 12Z 0.18 1009  92  93   9 0.00
Tue 10/14 13Z 0.14 1006  91  90   9 0.00
Tue 10/14 14Z 0.13 1106  91  84   9 0.00
Tue 10/14 15Z 0.13 1204  91  79   9 0.00
Tue 10/14 16Z 0.10 1101  90  76   9 0.00
Tue 10/14 17Z 0.09 3102  91  76   9 0.00
Tue 10/14 18Z 0.10 3104  93  77   9 0.00
Tue 10/14 19Z 0.11 3106  95  80   9 0.00
Tue 10/14 20Z 0.12 3208  97  83   9 0.00
Tue 10/14 21Z 0.14 3210  98  85   8 0.01
Tue 10/14 22Z 0.14 3311  97  81   8 0.01
Tue 10/14 23Z 0.16 3313  85  68   8 0.00

Run total areal avg QPF for the greens is: 0.45 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