NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260101_1800
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)
=== ===== === ==== ==== === === === ====
Thu 01/01 18Z 5.25 2818  81  56 -19 0.00
Thu 01/01 21Z 8.27 2821  76  49 -20 0.00
Fri 01/02 00Z 1.46 2828  60  11 -21 0.00
Fri 01/02 03Z 1.01 2930  57  20 -20 0.00
Fri 01/02 06Z 0.65 2821  72  78 -18 0.00
Fri 01/02 09Z 0.67 2521  87  82 -17 0.01
Fri 01/02 12Z 0.74 2621  90  74 -17 0.01
Fri 01/02 15Z 1.01 2719  87  57 -18 0.00
Fri 01/02 18Z 10.44 2718  83  29 -18 0.00
Fri 01/02 21Z 5.23 2720  76  19 -17 0.00
Sat 01/03 00Z 2.14 2826  80  39 -18 0.00
Sat 01/03 03Z 1.88 2827  85  82 -18 0.00
Sat 01/03 06Z 1.70 2827  82  90 -18 0.00
Sat 01/03 09Z 0.95 2725  91  92 -16 0.00
Sat 01/03 12Z 0.92 2825  94  68 -17 0.01
Sat 01/03 15Z 1.22 2924  90  34 -20 0.01
Sat 01/03 18Z 1.01 2922  78  24 -19 0.00
Sat 01/03 21Z 0.45 2821  65  32 -15 0.00
Sun 01/04 00Z 0.40 2922  54  18 -13 0.00
Sun 01/04 03Z 0.33 2818  51  18 -12 0.00
Sun 01/04 06Z 0.30 2615  59  86 -12 0.00
Sun 01/04 09Z 0.46 2719  81  91 -13 0.00
Sun 01/04 12Z 0.52 2719  91  90 -14 0.03
Sun 01/04 15Z 0.41 2718  77  89 -13 0.04
Sun 01/04 18Z 0.41 2712  91  83 -13 0.02
Sun 01/04 21Z 0.54 2714  93  61 -12 0.00
Mon 01/05 00Z 1.05 2920  94  40 -15 0.02
Mon 01/05 03Z 0.93 3123  92  30 -19 0.01
Mon 01/05 06Z 0.39 3223  80  15 -18 0.00

Run total areal avg QPF for the greens is: 0.16 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