Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260331_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)
=== ===== === ==== ==== === === === ====
Tue 03/31 06Z 0.63 2516  81  82   5 0.00
Tue 03/31 07Z 0.50 2516  83  82   5 0.01
Tue 03/31 08Z 0.43 2414  85  72   5 0.01
Tue 03/31 09Z 0.32 2213  86  46   5 0.00
Tue 03/31 10Z 0.29 2215  88  45   6 0.00
Tue 03/31 11Z 0.27 2118  89  77   6 0.00
Tue 03/31 12Z 0.30 2121  94  96   7 0.04
Tue 03/31 13Z 0.35 2118  95  95   6 0.13
Tue 03/31 14Z 0.37 2122  97  91   6 0.08
Tue 03/31 15Z 0.47 2224  97  90   7 0.04
Tue 03/31 16Z 0.56 2324  96  85   7 0.02
Tue 03/31 17Z 0.68 2428  98  91   7 0.07
Tue 03/31 18Z 0.75 2428  99  79   7 0.06
Tue 03/31 19Z 0.71 2525  96  63   7 0.02
Tue 03/31 20Z 0.67 2524  97  46   7 0.02
Tue 03/31 21Z 0.75 2525  98  58   8 0.02
Tue 03/31 22Z 0.64 2424  98  63   8 0.02
Tue 03/31 23Z 0.51 2323  99  80   9 0.03
Wed 04/01 00Z 0.45 2223  99  81   9 0.08
Wed 04/01 01Z 0.56 2227  97  82  10 0.13
Wed 04/01 02Z 0.64 2232  97  86  10 0.06
Wed 04/01 03Z 0.71 2235  98  83  10 0.07
Wed 04/01 04Z 0.72 2234  98  86  10 0.13
Wed 04/01 05Z 0.86 2336  98  79   9 0.18
Wed 04/01 06Z 0.90 2336  98  92   9 0.22
Wed 04/01 07Z 1.01 2537  98  88   9 0.05
Wed 04/01 08Z 0.84 2533  98  92   9 0.05
Wed 04/01 09Z 0.74 2630  98  94   8 0.12
Wed 04/01 10Z 0.63 2728  98  81   7 0.09
Wed 04/01 11Z 0.42 2924  98  66   6 0.02
Wed 04/01 12Z 0.35 3023  97  40   5 0.02
Wed 04/01 13Z 0.30 3022  95  21   3 0.01
Wed 04/01 14Z 0.27 3122  92  16   2 0.01
Wed 04/01 15Z 0.27 3122  88  11   1 0.01
Wed 04/01 16Z 0.27 3121  85   7   0 0.01
Wed 04/01 17Z 0.27 3221  84  10   0 0.00
Wed 04/01 18Z 0.24 3220  82  11  -1 0.00
Wed 04/01 19Z 0.19 3318  81  13  -2 0.00
Wed 04/01 20Z 0.14 3316  80  15  -2 0.00
Wed 04/01 21Z 0.07 3413  77  17  -2 0.00
Wed 04/01 22Z 0.06 3411  76  17  -3 0.00
Wed 04/01 23Z 0.03 3509  74  19  -3 0.00
Thu 04/02 00Z 0.04 0008  71  21  -2 0.00
Thu 04/02 01Z 0.05 0107  66  21  -2 0.00
Thu 04/02 02Z 0.08 0306  56  22  -1 0.00
Thu 04/02 03Z 0.08 0604  52  23  -1 0.00
Thu 04/02 04Z 0.10 0905  52  26   0 0.00
Thu 04/02 05Z 0.09 1205  55  30  -1 0.00
Thu 04/02 06Z 0.09 1408  56  36   0 0.00
Thu 04/02 07Z 0.07 1510  60  40   0 0.00
Thu 04/02 08Z 0.07 1611  64  40   0 0.00
Thu 04/02 09Z 0.08 1612  66  40   0 0.01
Thu 04/02 10Z 0.07 1613  64  42   0 0.01
Thu 04/02 11Z 0.06 1612  62  36   0 0.01
Thu 04/02 12Z 0.06 1613  59  38   1 0.01
Thu 04/02 13Z 0.06 1613  58  33   1 0.00
Thu 04/02 14Z 0.07 1612  61  36   1 0.00
Thu 04/02 15Z 0.05 1612  66  62   0 0.00
Thu 04/02 16Z 0.07 1613  74  91   1 0.00
Thu 04/02 17Z 0.08 1615  80  97   2 0.00

Run total areal avg QPF for the greens is: 1.87 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