NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260113_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 01/13 06Z 0.96 2723  95  75  -9 0.00
Tue 01/13 09Z 0.80 2623  91  86  -7 0.00
Tue 01/13 12Z 0.71 2622  93  49  -6 0.00
Tue 01/13 15Z 0.67 2618  94  19  -7 0.00
Tue 01/13 18Z 0.37 2315  84  11  -5 0.00
Tue 01/13 21Z 0.17 2018  74  35  -1 0.00
Wed 01/14 00Z 0.25 2032  69  82   0 0.00
Wed 01/14 03Z 0.35 2037  68  90   0 0.00
Wed 01/14 06Z 0.60 2138  96  77  -2 0.08
Wed 01/14 09Z 0.62 2233  91  70  -1 0.00
Wed 01/14 12Z 0.67 2332  92  59  -2 0.00
Wed 01/14 15Z 0.73 2429  94  57  -3 0.00
Wed 01/14 18Z 0.86 2424  96  35  -4 0.00
Wed 01/14 21Z 0.69 2318  95  55  -4 0.00
Thu 01/15 00Z 0.33 2113  93  89  -4 0.00
Thu 01/15 03Z 0.14 1912  96  90  -4 0.02
Thu 01/15 06Z 0.10 1912  97  89  -4 0.05
Thu 01/15 09Z 0.12 2704  95  89  -4 0.09
Thu 01/15 12Z 0.26 3217  88  89 -10 0.12
Thu 01/15 15Z 4.75 2822  86  86 -17 0.06
Thu 01/15 18Z 11.91 2723  84  77 -21 0.03
Thu 01/15 21Z 18.00 2728  80  72 -23 0.01
Fri 01/16 00Z 3.25 2829  77  71 -25 0.00
Fri 01/16 03Z 8.85 2727  80  74 -23 0.00
Fri 01/16 06Z 0.50 3026  70  23 -20 0.01
Fri 01/16 09Z 0.47 2823  67  20 -18 0.00
Fri 01/16 12Z 0.55 2724  82  24 -16 0.00
Fri 01/16 15Z 0.47 2324  69  14 -14 0.00
Fri 01/16 18Z 0.40 2125  44  68 -12 0.00

Run total areal avg QPF for the greens is: 0.47 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