NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260405_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)
=== ===== === ==== ==== === === === ====
Sun 04/05 18Z 0.62 2522  94  33   2 0.00
Sun 04/05 21Z 1.25 2626  94  19  -1 0.00
Mon 04/06 00Z 2.03 2728  85  17  -3 0.00
Mon 04/06 03Z 1.89 2728  92  38  -5 0.00
Mon 04/06 06Z 1.95 2728  92  68  -7 0.01
Mon 04/06 09Z 1.24 2930  96  83  -8 0.01
Mon 04/06 12Z 1.66 2928  94  64 -10 0.01
Mon 04/06 15Z 8.83 2823  90  65 -10 0.00
Mon 04/06 18Z 14.22 2820  72  76  -9 0.00
Mon 04/06 21Z 13.22 2819  62  82  -8 0.00
Tue 04/07 00Z 4.49 2921  72  84  -9 0.00
Tue 04/07 03Z 1.43 3019  80  87 -10 0.00
Tue 04/07 06Z 0.59 3015  82  80 -11 0.00
Tue 04/07 09Z 0.49 2910  86  93 -12 0.00
Tue 04/07 12Z 0.31 3010  93  92 -12 0.01
Tue 04/07 15Z 0.48 3114  91  73 -14 0.02
Tue 04/07 18Z 8.60 3014  73   9 -14 0.00
Tue 04/07 21Z 9.29 2915  55  19 -14 0.00
Wed 04/08 00Z 1.19 2916  47  32 -14 0.00
Wed 04/08 03Z 1.08 2920  47  35 -14 0.00
Wed 04/08 06Z 0.74 2918  52  29 -14 0.00
Wed 04/08 09Z 0.58 2814  58  16 -13 0.00
Wed 04/08 12Z 0.46 2714  50  14 -12 0.00
Wed 04/08 15Z 0.57 2815  53  19 -13 0.00
Wed 04/08 18Z 0.41 2310  31   5  -7 0.00
Wed 04/08 21Z 0.42 2213  28   2  -5 0.00
Thu 04/09 00Z 0.43 2219  25   6  -3 0.00
Thu 04/09 03Z 0.50 2325  27  42  -1 0.00
Thu 04/09 06Z 0.51 2327  28  77   0 0.00

Run total areal avg QPF for the greens is: 0.06 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