GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260929_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)
=== ===== === ==== ==== === === === ====
Tue 09/29 00Z 0.12 0505  99  98   8 0.00
Tue 09/29 03Z 0.03 0105  99  98   8 0.03
Tue 09/29 06Z 0.06 3407 100  98   8 0.01
Tue 09/29 09Z 0.07 3409  99  92   8 0.00
Tue 09/29 12Z 0.09 3412  99  83   8 0.00
Tue 09/29 15Z 0.08 3411  99  73   8 0.00
Tue 09/29 18Z 0.07 3410  98  74   8 0.00
Tue 09/29 21Z 0.02 3509  97  69   9 0.00
Wed 09/30 00Z 0.02 0009  95  67   9 0.00
Wed 09/30 03Z 0.03 0105  89  55   9 0.00
Wed 09/30 06Z 0.05 3203  86  40   9 0.00
Wed 09/30 09Z 0.06 3305  83  24   9 0.00
Wed 09/30 12Z 0.08 3205  77  23   9 0.00
Wed 09/30 15Z 0.08 2903  73  25  10 0.00
Wed 09/30 18Z 0.17 2605  71  32  10 0.00
Wed 09/30 21Z 0.25 2607  71  29  11 0.00
Thu 10/01 00Z 0.42 2610  75  54  11 0.00
Thu 10/01 03Z 0.50 2613  73  73  11 0.00
Thu 10/01 06Z 0.53 2514  76  69  11 0.00
Thu 10/01 09Z 0.54 2514  78  78  11 0.00
Thu 10/01 12Z 0.47 2415  72  71  12 0.00
Thu 10/01 15Z 0.59 2415  74  62  11 0.00
Thu 10/01 18Z 0.67 2419  79  97  11 0.00
Thu 10/01 21Z 0.71 2420  72  98  12 0.00

Run total areal avg QPF for the greens is: 0.04 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