NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260331_1800
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)
=== ===== === ==== ==== === === === ====
Tue 03/31 18Z 0.56 2321  98  88   6 0.00
Tue 03/31 19Z 0.53 2321  96  89   7 0.02
Tue 03/31 20Z 0.52 2323  98  86   8 0.02
Tue 03/31 21Z 0.55 2323  97  73   8 0.02
Tue 03/31 22Z 0.57 2323  96  59   8 0.02
Tue 03/31 23Z 0.48 2223  98  87   9 0.06
Wed 04/01 00Z 0.47 2126  97  97   9 0.20
Wed 04/01 01Z 0.60 2228  93  93   9 0.31
Wed 04/01 02Z 0.47 2224  94  93   9 0.17
Wed 04/01 03Z 0.46 2223  94  96   9 0.19
Wed 04/01 04Z 0.59 2330  98  92  10 0.20
Wed 04/01 05Z 0.86 2336  98  89  10 0.22
Wed 04/01 06Z 1.02 2435  99  95   9 0.16
Wed 04/01 07Z 0.91 2532  99  85   9 0.06
Wed 04/01 10Z 0.58 2829  98  44   6 0.04
Wed 04/01 11Z 0.45 2826  96  42   5 0.03
Wed 04/01 12Z 0.36 2923  94  32   4 0.03
Wed 04/01 13Z 0.32 3023  92  25   3 0.02
Wed 04/01 14Z 0.27 3122  86  18   2 0.02
Wed 04/01 15Z 0.24 3221  82  16   2 0.01
Wed 04/01 16Z 0.23 3220  79  14   1 0.01
Wed 04/01 17Z 0.22 3219  77  14   0 0.01
Wed 04/01 18Z 0.21 3217  76  13   0 0.00
Wed 04/01 20Z 0.17 3314  70  15  -1 0.00
Wed 04/01 21Z 0.10 3413  66  17  -1 0.00
Wed 04/01 22Z 0.06 3411  63  20  -1 0.00
Wed 04/01 23Z 0.04 3511  61  21  -1 0.00
Thu 04/02 00Z 0.02 3509  59  22  -1 0.00
Thu 04/02 01Z 0.02 0008  56  22   0 0.00
Thu 04/02 02Z 0.07 0207  51  25   0 0.00
Thu 04/02 03Z 0.07 0405  49  26   0 0.00
Thu 04/02 04Z 0.10 0805  47  29   0 0.00
Thu 04/02 05Z 0.11 1106  45  34   0 0.00
Thu 04/02 06Z 0.09 1407  49  36   0 0.00
Thu 04/02 07Z 0.08 1508  54  39  -1 0.00
Thu 04/02 08Z 0.07 1610  61  47  -1 0.00
Thu 04/02 09Z 0.06 1710  66  52  -1 0.00
Thu 04/02 10Z 0.09 1611  69  71   0 0.00
Thu 04/02 11Z 0.09 1613  68  91   0 0.00
Thu 04/02 12Z 0.08 1612  70  97   0 0.00
Thu 04/02 13Z 0.10 1512  73  97   0 0.00
Thu 04/02 14Z 0.11 1412  76  98   0 0.00
Thu 04/02 15Z 0.13 1413  80  97   0 0.01
Thu 04/02 16Z 0.12 1412  85  98   0 0.01
Thu 04/02 17Z 0.14 1413  90  98   0 0.01
Thu 04/02 18Z 0.15 1413  92  99   0 0.01
Thu 04/02 19Z 0.17 1415  92  99   0 0.01
Thu 04/02 20Z 0.15 1415  92  99   0 0.02
Thu 04/02 21Z 0.15 1516  93  98   0 0.01
Thu 04/02 22Z 0.12 1518  94  98   0 0.01
Thu 04/02 23Z 0.09 1620  96  97   1 0.02
Fri 04/03 00Z 0.04 1722  96  95   2 0.02
Fri 04/03 01Z 0.04 1824  97  84   4 0.02
Fri 04/03 04Z 0.15 2026  96  64   8 0.02
Fri 04/03 05Z 0.17 2027  95  54   9 0.02

Run total areal avg QPF for the greens is: 2.01 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