GFS EXPERIMENTAL Froude Number for the Green Mountains

Subject: GFS EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_GFS_20241121_0600
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 11/21 06Z 0.22 1416  84  50   2 0.00
Thu 11/21 09Z 0.33 1316  92  51   1 0.00
Thu 11/21 12Z 0.32 1313  97  71   0 0.00
Thu 11/21 15Z 0.39 1114  98  83   0 0.01
Thu 11/21 18Z 0.43 0914  99  91   0 0.04
Thu 11/21 21Z 0.42 0913  99  94   0 0.07
Fri 11/22 00Z 0.48 0816  99  94   0 0.13
Fri 11/22 03Z 0.61 0819  99  95  -1 0.13
Fri 11/22 06Z 0.75 0823  99  96  -1 0.14
Fri 11/22 09Z 0.85 0726  98  95  -1 0.09
Fri 11/22 12Z 0.84 0629  99  96  -1 0.14
Fri 11/22 15Z 1.08 1230  97  37  -3 0.06
Fri 11/22 18Z 2.50 1226  84  25  -3 0.01
Fri 11/22 21Z 1.56 1124  75  45  -2 0.00
Sat 11/23 00Z 0.76 1020  84  74  -2 0.01
Sat 11/23 03Z 0.64 0717  87  81  -2 0.04
Sat 11/23 06Z 0.28 0313  96  92  -2 0.05
Sat 11/23 09Z 0.10 3518  99  96  -2 0.16
Sat 11/23 12Z 0.13 3520  99  94  -2 0.20
Sat 11/23 15Z 0.14 3419  99  89  -2 0.21
Sat 11/23 18Z 0.28 3321  99  87  -3 0.04
Sat 11/23 21Z 0.53 3124  98  80  -3 0.02
Sun 11/24 00Z 0.50 3019  98  64  -3 0.01
Sun 11/24 03Z 0.65 2920  93  53  -4 0.01

Run total areal avg QPF for the greens is: 1.57 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