Subject: NAM3 EXPERIMENTAL Froude Number for the Northern_Greens

Model run: BTV_GRID_D2D_NAM3_20250401_1800
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)
=== ===== === ==== ==== === === === ====
Tue 04/01 18Z 13.90 3022  83   3 -11 0.00
Tue 04/01 19Z 16.95 3026  79   4 -11 0.00
Tue 04/01 20Z 15.69 3027  76   5 -11 0.00
Tue 04/01 21Z 15.08 3028  75   5 -11 0.00
Tue 04/01 22Z 13.93 3028  75   6 -12 0.00
Tue 04/01 23Z 8.97 3030  74   7 -12 0.00
Wed 04/02 00Z 5.15 3132  73   7 -13 0.00
Wed 04/02 01Z 3.44 3131  68   8 -13 0.00
Wed 04/02 02Z 1.79 3229  61  10 -13 0.00
Wed 04/02 03Z 1.23 3227  61  10 -14 0.00
Wed 04/02 04Z 0.79 3225  59  11 -14 0.00
Wed 04/02 05Z 0.55 3322  55  10 -13 0.00
Wed 04/02 06Z 0.44 3320  50   9 -13 0.00
Wed 04/02 07Z 0.36 3319  42   9 -12 0.00
Wed 04/02 08Z 0.28 3418  34  11 -12 0.00
Wed 04/02 09Z 0.22 3417  28  11 -12 0.00
Wed 04/02 10Z 0.17 3516  24  14 -11 0.00
Wed 04/02 11Z 0.13 3514  23  17 -11 0.00
Wed 04/02 12Z 0.09 3511  24  18 -11 0.00
Wed 04/02 13Z 0.05 0009  25  22 -10 0.00
Wed 04/02 14Z 0.04 0006  27  24 -10 0.00
Wed 04/02 15Z 0.02 0202  28  25  -9 0.00
Wed 04/02 16Z 0.03 0900  28  26  -9 0.00
Wed 04/02 17Z 0.05 1402  27  24  -8 0.00
Wed 04/02 18Z 0.06 1603  23  22  -6 0.00
Wed 04/02 19Z 0.09 1606  19  24  -5 0.00
Wed 04/02 20Z 0.10 1708  18  36  -4 0.00
Wed 04/02 21Z 0.13 1610  18  53  -3 0.00
Wed 04/02 22Z 0.14 1715  19  82  -3 0.00
Wed 04/02 23Z 0.14 1818  19  93  -2 0.00
Thu 04/03 00Z 0.16 1822  39  95  -3 0.00
Thu 04/03 01Z 0.26 1725  47  97  -2 0.00
Thu 04/03 02Z 0.41 1628  67  98  -4 0.00
Thu 04/03 03Z 0.42 1634  91  99  -4 0.06
Thu 04/03 04Z 0.42 1638  94  99  -3 0.09
Thu 04/03 05Z 0.41 1741  94  98  -3 0.19
Thu 04/03 06Z 0.37 1742  96  97  -3 0.22
Thu 04/03 07Z 0.30 1845  97  98  -2 0.20
Thu 04/03 08Z 0.14 1948  97  97   0 0.10
Thu 04/03 09Z 0.04 1950  97  95   3 0.09
Thu 04/03 10Z 0.08 2051  96  83   7 0.08
Thu 04/03 11Z 0.18 2153  96  65   9 0.06
Thu 04/03 12Z 0.28 2247  97  83   9 0.04
Thu 04/03 13Z 0.23 2145  96  94  10 0.04
Thu 04/03 14Z 0.28 2247  96  84  10 0.13
Thu 04/03 15Z 0.34 2249  95  71  10 0.02
Thu 04/03 16Z 0.51 2350  97  86  10 0.01
Thu 04/03 17Z 0.62 2445  93  93  10 0.03
Thu 04/03 18Z 0.57 2441  71  90  11 0.01
Thu 04/03 19Z 0.71 2437  77  20   8 0.00
Thu 04/03 20Z 0.75 2434  86  10   7 0.00
Thu 04/03 21Z 0.98 2534  93   3   6 0.00
Thu 04/03 22Z 1.15 2635  96   4   6 0.01
Thu 04/03 23Z 1.50 2538  92   3   5 0.01
Fri 04/04 00Z 2.14 2640  88   1   4 0.00
Fri 04/04 01Z 2.36 2643  89   1   3 0.00
Fri 04/04 02Z 1.90 2741  91   5   2 0.00
Fri 04/04 03Z 1.98 2737  94   9   0 0.01
Fri 04/04 04Z 1.72 2737  89  11  -1 0.01
Fri 04/04 05Z 1.26 2836  79  11  -1 0.00

Run total areal avg QPF for the Northern_Greens is: 1.41 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