GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20260817_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)
=== ===== === ==== ==== === === === ====
Mon 08/17 00Z 0.41 2020  90  99  12 0.00
Mon 08/17 03Z 0.39 2026  95  95  13 0.09
Mon 08/17 06Z 0.40 2123  98  92  13 0.01
Mon 08/17 09Z 0.36 2318  99  97  13 0.00
Mon 08/17 12Z 0.30 2412  99  97  14 0.01
Mon 08/17 15Z 0.22 2409  98  92  14 0.01
Mon 08/17 18Z 0.21 2407  98  87  14 0.01
Mon 08/17 21Z 0.15 2504  97  77  14 0.04
Tue 08/18 00Z 0.10 2903  93  77  14 0.08
Tue 08/18 03Z 0.03 3402  94  79  13 0.03
Tue 08/18 06Z 0.04 0102  95  51  13 0.01
Tue 08/18 09Z 0.04 0002  95  28  13 0.01
Tue 08/18 12Z 0.06 3203  92  20  12 0.02
Tue 08/18 15Z 0.11 3306  87  17  12 0.02
Tue 08/18 18Z 0.74 3008  74  16  12 0.01
Tue 08/18 21Z 0.57 3107  65   8  13 0.00
Wed 08/19 00Z 0.21 3207  65  11  13 0.00
Wed 08/19 03Z 0.22 3005  64  13  12 0.00
Wed 08/19 06Z 0.22 2805  62  19  13 0.00
Wed 08/19 09Z 0.23 2505  62  31  13 0.00
Wed 08/19 12Z 0.22 2207  67  47  13 0.00
Wed 08/19 15Z 0.51 2209  73  70  12 0.00
Wed 08/19 18Z 0.85 2513  72  89  13 0.01
Wed 08/19 21Z 0.53 2713  78  86  12 0.02

Run total areal avg QPF for the greens is: 0.38 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