NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260203_1200
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 02/03 12Z 0.10 3207  65  46  -9 0.00
Tue 02/03 15Z 0.11 3005  62  29 -10 0.00
Tue 02/03 18Z 0.15 2505  58  44 -10 0.00
Tue 02/03 21Z 0.23 2407  54  33 -11 0.00
Wed 02/04 00Z 0.36 2609  71  39 -12 0.00
Wed 02/04 03Z 0.56 2709  80  67 -13 0.00
Wed 02/04 06Z 0.51 2610  85  72 -13 0.00
Wed 02/04 09Z 0.53 2713  91  84 -13 0.01
Wed 02/04 12Z 0.45 2916  88  86 -14 0.01
Wed 02/04 15Z 0.42 3121  88  84 -13 0.01
Wed 02/04 18Z 0.63 3017  86  62 -14 0.00
Wed 02/04 21Z 0.60 3014  90  37 -14 0.00
Thu 02/05 00Z 0.34 3015  83  33 -12 0.00
Thu 02/05 03Z 0.27 3014  78  29 -12 0.00
Thu 02/05 06Z 0.27 2812  77  37 -12 0.00
Thu 02/05 09Z 0.33 2814  81  37 -13 0.00
Thu 02/05 12Z 0.39 2917  86  48 -14 0.00
Thu 02/05 15Z 0.35 3016  91   5 -15 0.00
Thu 02/05 18Z 0.35 3114  87   4 -16 0.00
Thu 02/05 21Z 0.30 3113  80   2 -16 0.00
Fri 02/06 00Z 0.17 3213  61   3 -14 0.00
Fri 02/06 03Z 0.08 3311  50   6 -12 0.00
Fri 02/06 06Z 0.05 3307  42  12 -11 0.00
Fri 02/06 09Z 0.09 3310  51   7 -12 0.00
Fri 02/06 12Z 0.03 2501  33  24 -10 0.00
Fri 02/06 15Z 0.01 1707  31  27 -10 0.00
Fri 02/06 18Z 0.04 1809  38  18 -10 0.00
Fri 02/06 21Z 0.03 1810  45  18 -10 0.00
Sat 02/07 00Z 0.11 1912  58  75 -12 0.00

Run total areal avg QPF for the greens is: 0.03 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