Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20241231_0000
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 12/31 00Z 1.55 2328  83  86  -2 0.00
Tue 12/31 03Z 1.38 2431  90  92  -3 0.00
Tue 12/31 06Z 1.29 2533  93  86  -4 0.00
Tue 12/31 09Z 1.17 2630  95  59  -4 0.00
Tue 12/31 12Z 0.67 2724  92  18  -2 0.00
Tue 12/31 15Z 0.38 2617  83  21  -1 0.00
Tue 12/31 18Z 0.21 2411  67  20   0 0.00
Tue 12/31 21Z 0.13 2112  52  37   1 0.00
Wed 01/01 00Z 0.06 1914  48  60   2 0.00
Wed 01/01 03Z 0.10 1618  50  55   2 0.00
Wed 01/01 06Z 0.31 1426  58  37   1 0.00
Wed 01/01 09Z 0.67 1233  82  87   0 0.05
Wed 01/01 12Z 0.88 1041  96  95   0 0.18
Wed 01/01 15Z 0.64 1232  97  95   0 0.11
Wed 01/01 18Z 0.35 1317  97  92  -1 0.10
Wed 01/01 21Z 0.19 2110  97  93  -3 0.04
Thu 01/02 00Z 0.61 2618  98  91  -4 0.04
Thu 01/02 03Z 0.87 2723  97  91  -5 0.03
Thu 01/02 06Z 1.03 2828  96  87  -6 0.06
Thu 01/02 09Z 1.34 2833  95  86  -7 0.06
Thu 01/02 12Z 1.71 2838  92  84  -8 0.06
Thu 01/02 15Z 2.30 2840  90  84 -10 0.05
Thu 01/02 18Z 2.95 2839  89  85 -10 0.03
Thu 01/02 21Z 2.15 2839  90  84  -9 0.03
Fri 01/03 00Z 2.42 2941  89  82 -11 0.04
Fri 01/03 03Z 2.50 2839  90  80 -12 0.03
Fri 01/03 06Z 2.62 2837  90  72 -13 0.02
Fri 01/03 09Z 2.39 2835  91  63 -13 0.02
Fri 01/03 12Z 2.87 2833  89  67 -14 0.01

Run total areal avg QPF for the greens is: 0.96 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