GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260929_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 09/29 12Z 0.07 3412  99  83   8 0.00
Tue 09/29 15Z 0.06 3411  99  73   8 0.00
Tue 09/29 18Z 0.06 3410  98  74   8 0.00
Tue 09/29 21Z 0.03 3508  97  70   9 0.00
Wed 09/30 00Z 0.02 0009  96  63   9 0.00
Wed 09/30 03Z 0.03 0006  90  45   9 0.00
Wed 09/30 06Z 0.05 3504  85  27   9 0.00
Wed 09/30 09Z 0.06 3205  79  22  10 0.00
Wed 09/30 12Z 0.06 3305  71  23  10 0.00
Wed 09/30 15Z 0.04 2901  65  27  11 0.00
Wed 09/30 18Z 0.10 2403  63  30  11 0.00
Wed 09/30 21Z 0.21 2406  64  26  11 0.00
Thu 10/01 00Z 0.35 2509  70  34  11 0.00
Thu 10/01 03Z 0.54 2614  74  46  11 0.00
Thu 10/01 06Z 0.46 2612  74  43  11 0.00
Thu 10/01 09Z 0.53 2414  80  73  11 0.00
Thu 10/01 12Z 0.40 2314  81  87  11 0.00
Thu 10/01 15Z 0.59 2319  80  95  10 0.00
Thu 10/01 18Z 0.66 2421  90  96  10 0.00
Thu 10/01 21Z 0.65 2623  81  96  12 0.01
Fri 10/02 00Z 0.72 2527  66  94  14 0.00
Fri 10/02 03Z 0.85 2430  71  80  14 0.00
Fri 10/02 06Z 0.89 2432  70  84  14 0.00
Fri 10/02 09Z 0.95 2432  70  87  14 0.00

Run total areal avg QPF for the greens is: 0.01 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