NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20250419_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)
=== ===== === ==== ==== === === === ====
Sat 04/19 00Z 0.90 2133  47  91   8 0.00
Sat 04/19 03Z 0.95 2338  57  93   9 0.00
Sat 04/19 06Z 0.90 2440  57  97  11 0.00
Sat 04/19 09Z 0.69 2433  63  72  13 0.00
Sat 04/19 12Z 0.90 2537  75  69  12 0.00
Sat 04/19 15Z 0.83 2534  95  91  12 0.02
Sat 04/19 18Z 1.15 2534  85  73  12 0.01
Sat 04/19 21Z 1.96 2532  91  52  12 0.04
Sun 04/20 00Z 1.17 2828  94  97   8 0.04
Sun 04/20 03Z 1.55 2834  96  84   5 0.11
Sun 04/20 06Z 1.54 3034  95  29  -1 0.04
Sun 04/20 09Z 1.07 3033  86  11  -6 0.00
Sun 04/20 12Z 0.78 3132  84  13  -8 0.00
Sun 04/20 15Z 0.71 3126  68   9  -7 0.00
Sun 04/20 18Z 1.69 3125  59   6  -6 0.00
Sun 04/20 21Z 1.33 3125  53   4  -4 0.00
Mon 04/21 00Z 0.55 3227  47   2  -2 0.00
Mon 04/21 03Z 0.20 3323  38   3   0 0.00
Mon 04/21 06Z 0.03 3515  29  12   1 0.00
Mon 04/21 09Z 0.06 3307  22  13   2 0.00
Mon 04/21 12Z 0.11 2906  15  12   2 0.00
Mon 04/21 15Z 0.09 2304  16   8   2 0.00
Mon 04/21 18Z 0.10 1907  25  29   3 0.00
Mon 04/21 21Z 0.10 2404  19  16   2 0.00
Tue 04/22 00Z 0.10 1817  62  94   3 0.00
Tue 04/22 03Z 0.17 1925  84  83   6 0.04
Tue 04/22 06Z 0.37 2134  77  89   7 0.00
Tue 04/22 09Z 0.51 2334  94  79   7 0.06
Tue 04/22 12Z 0.62 2429  93  74   5 0.01

Run total areal avg QPF for the greens is: 0.37 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