Subject: NAM3 EXPERIMENTAL Froude Number for the Northern_Greens

Model run: BTV_GRID_D2D_NAM3_20250403_0000
Using the MB925 to MB850 layer.
Assumes Northern_Greens are orthogonal to 290 degree winds.

925-850 mb Layer Average Wind/RH. Avg QPF over the Northern_Greens.
Date   Time        Wind RH% 700 850T QPF
Day mm/dd hhZ Frd# DDFF (%) RH% (C) (in)
=== ===== === ==== ==== === === === ====
Thu 04/03 00Z 0.26 1720  71  96  -4 0.00
Thu 04/03 01Z 0.29 1624  92  98  -3 0.02
Thu 04/03 02Z 0.33 1630  93  99  -2 0.09
Thu 04/03 03Z 0.34 1732  94  98  -3 0.09
Thu 04/03 04Z 0.35 1737  94  99  -3 0.09
Thu 04/03 05Z 0.29 1739  94  99  -3 0.04
Thu 04/03 06Z 0.23 1841  95  99  -2 0.05
Thu 04/03 07Z 0.18 1845  96  96   0 0.06
Thu 04/03 08Z 0.11 1946  96  96   2 0.09
Thu 04/03 09Z 0.06 1943  96  98   4 0.20
Thu 04/03 12Z 0.13 2047  97  92   7 0.11
Thu 04/03 13Z 0.13 2047  97  93   8 0.04
Thu 04/03 15Z 0.30 2245  92  87  11 0.21
Thu 04/03 16Z 0.39 2248  84  89  12 0.18
Thu 04/03 18Z 0.67 2443  96  46   9 0.10
Thu 04/03 19Z 0.73 2437  98  24   8 0.01
Thu 04/03 20Z 0.91 2436  97  20   7 0.00
Thu 04/03 21Z 1.14 2535  97  28   6 0.03
Thu 04/03 22Z 1.30 2536  85  23   6 0.01
Thu 04/03 23Z 2.09 2539  70  11   6 0.02
Fri 04/04 00Z 3.11 2543  69   1   5 0.02
Fri 04/04 01Z 3.04 2643  82   1   3 0.02
Fri 04/04 02Z 2.54 2640  92   3   2 0.01
Fri 04/04 03Z 1.86 2736  95   6   1 0.00
Fri 04/04 04Z 1.60 2835  90   8   0 0.00
Fri 04/04 05Z 1.72 2836  83  11  -1 0.00
Fri 04/04 06Z 1.27 2933  84  14  -2 0.01
Fri 04/04 07Z 1.12 2931  88  17  -3 0.02
Fri 04/04 08Z 0.93 2929  89  18  -3 0.02
Fri 04/04 09Z 0.82 2930  88  15  -3 0.01
Fri 04/04 10Z 0.74 2930  86   8  -3 0.00
Fri 04/04 11Z 0.65 2929  84   1  -2 0.00
Fri 04/04 12Z 0.61 2930  81   2  -1 0.01
Fri 04/04 13Z 0.60 3031  78   1  -1 0.01
Fri 04/04 14Z 0.56 3030  73   1  -1 0.01
Fri 04/04 15Z 0.52 3029  68   1   0 0.01
Fri 04/04 16Z 0.49 3028  65   1   0 0.01
Fri 04/04 17Z 0.49 3128  64   1   0 0.01
Fri 04/04 18Z 0.51 3128  66   1   0 0.01
Fri 04/04 19Z 0.53 3128  65   1   0 0.01
Fri 04/04 20Z 0.53 3128  63   1   0 0.00
Fri 04/04 21Z 0.50 3228  62   1   0 0.00
Fri 04/04 22Z 0.43 3227  61   2   0 0.00
Fri 04/04 23Z 0.36 3326  61   4   1 0.00
Sat 04/05 00Z 0.28 3424  58   6   2 0.00
Sat 04/05 01Z 0.22 3422  57   7   2 0.00
Sat 04/05 02Z 0.17 3520  54   7   3 0.00
Sat 04/05 03Z 0.10 0018  49   5   3 0.00
Sat 04/05 04Z 0.02 0114  43   7   4 0.00
Sat 04/05 05Z 0.02 0112  41   7   4 0.00
Sat 04/05 06Z 0.03 0307  41   7   4 0.00
Sat 04/05 07Z 0.07 0606  35   6   4 0.00
Sat 04/05 08Z 0.11 0906  29   5   5 0.00
Sat 04/05 09Z 0.13 1207  26   6   5 0.00
Sat 04/05 10Z 0.14 1410  27  13   4 0.00
Sat 04/05 11Z 0.14 1614  30  36   4 0.00

Run total areal avg QPF for the Northern_Greens is: 1.63 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