NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260510_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)
=== ===== === ==== ==== === === === ====
Sun 05/10 06Z 0.38 2026  88  44   3 0.00
Sun 05/10 09Z 0.68 2326  96  90   4 0.03
Sun 05/10 12Z 0.52 2419  90  55   4 0.00
Sun 05/10 15Z 0.57 2416  95  84   3 0.02
Sun 05/10 18Z 0.98 2511  91  90   3 0.07
Sun 05/10 21Z 3.82 2613  79  74   2 0.04
Mon 05/11 00Z 2.65 2717  65  30   1 0.00
Mon 05/11 03Z 1.03 2920  62  29   0 0.00
Mon 05/11 06Z 0.87 3019  72  36  -1 0.00
Mon 05/11 09Z 0.53 3213  71  36  -2 0.00
Mon 05/11 12Z 0.49 3109  73  44  -3 0.00
Mon 05/11 15Z 7.21 2810  69  46  -2 0.00
Mon 05/11 18Z 8.12 2812  59  75  -1 0.00
Mon 05/11 21Z 6.32 2913  59  90  -1 0.01
Tue 05/12 00Z 2.64 3115  70  89  -2 0.00
Tue 05/12 03Z 0.49 3216  90  90  -3 0.01
Tue 05/12 06Z 0.44 3320  76  78  -4 0.01
Tue 05/12 09Z 0.60 3318  66  52  -5 0.00
Tue 05/12 12Z 0.75 3216  64  42  -5 0.00
Tue 05/12 15Z 7.42 3013  67  32  -4 0.00
Tue 05/12 18Z 8.05 2912  64  41  -3 0.00
Tue 05/12 21Z 7.69 2912  62  25  -1 0.00
Wed 05/13 00Z 0.86 2908  62  13  -1 0.00
Wed 05/13 03Z 4.11 2911  63  26  -2 0.00
Wed 05/13 06Z 0.43 2208  59  15   0 0.00
Wed 05/13 09Z 0.34 2014  58  25   0 0.00
Wed 05/13 12Z 0.38 2021  63  59   1 0.00
Wed 05/13 15Z 0.11 1726  84  95   1 0.00
Wed 05/13 18Z 0.12 1834  97  96   3 0.10

Run total areal avg QPF for the greens is: 0.29 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