NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260518_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)
=== ===== === ==== ==== === === === ====
Mon 05/18 00Z 0.36 3213  63  19  11 0.00
Mon 05/18 03Z 0.19 3211  59  29  11 0.00
Mon 05/18 06Z 0.17 2806  60  38  12 0.00
Mon 05/18 09Z 0.18 2308  64  67  13 0.00
Mon 05/18 12Z 0.18 2018  72  73  12 0.00
Mon 05/18 15Z 0.33 2026  82  41  13 0.03
Mon 05/18 18Z 0.65 2223  76  24  14 0.00
Mon 05/18 21Z 1.95 2322  78  19  15 0.00
Tue 05/19 00Z 4.11 2424  67  20  16 0.00
Tue 05/19 03Z 1.68 2528  74  21  17 0.00
Tue 05/19 06Z 1.63 2532  69  16  17 0.00
Tue 05/19 09Z 1.81 2534  76  43  16 0.00
Tue 05/19 12Z 1.31 2532  81  18  15 0.00
Tue 05/19 15Z 1.20 2530  86  73  14 0.03
Tue 05/19 18Z 5.67 2525  83  68  14 0.00
Tue 05/19 21Z 3.21 2622  80  71  14 0.02
Wed 05/20 00Z 1.03 2427  73  74  16 0.01
Wed 05/20 03Z 1.59 2439  83  88  16 0.01
Wed 05/20 06Z 1.53 2639  88  76  15 0.08
Wed 05/20 09Z 1.30 2638  90  81  14 0.02
Wed 05/20 12Z 1.24 2634  94  67  13 0.01
Wed 05/20 15Z 3.15 2726  84  51   8 0.00
Wed 05/20 18Z 10.59 2822  75  15   6 0.00
Wed 05/20 21Z 1.52 2727  84  45   9 0.00
Thu 05/21 00Z 1.48 3125  79   1   1 0.00
Thu 05/21 03Z 1.39 3230  76   1  -1 0.00
Thu 05/21 06Z 0.52 3228  69   1  -2 0.00
Thu 05/21 09Z 0.33 3326  62   2  -3 0.00
Thu 05/21 12Z 0.24 3322  59   8  -2 0.00

Run total areal avg QPF for the greens is: 0.21 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