NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_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 1417  88  67   3 0.00
Thu 11/21 09Z 0.37 1319  92  74   2 0.01
Thu 11/21 12Z 0.38 1220  96  78   1 0.01
Thu 11/21 15Z 0.47 1120  97  64   1 0.02
Thu 11/21 18Z 0.49 1020  97  82   0 0.03
Thu 11/21 21Z 0.48 0919  97  90   0 0.09
Fri 11/22 00Z 0.60 0924  97  93   0 0.13
Fri 11/22 03Z 0.64 0928  97  95   0 0.12
Fri 11/22 06Z 0.71 0931  96  96   0 0.09
Fri 11/22 09Z 0.83 0835  97  94   0 0.12
Fri 11/22 12Z 0.97 0940  97  86   0 0.09
Fri 11/22 15Z 0.92 1131  96  39  -1 0.06
Fri 11/22 18Z 1.13 1120  89  28  -2 0.01
Fri 11/22 21Z 0.63 1117  87  28  -2 0.00
Sat 11/23 00Z 0.50 0914  89  35  -2 0.00
Sat 11/23 03Z 0.35 0811  90  58  -2 0.00
Sat 11/23 06Z 0.07 0207  91  62  -1 0.00
Sat 11/23 09Z 0.11 3520  97  91  -2 0.02
Sat 11/23 12Z 0.14 3522  96  90  -2 0.07
Sat 11/23 15Z 0.12 3421  98  77  -3 0.06
Sat 11/23 18Z 0.38 3329  97  65  -4 0.02
Sat 11/23 21Z 0.44 3233  97  70  -2 0.03
Sun 11/24 00Z 0.52 3232  96  64  -2 0.02
Sun 11/24 03Z 0.45 3231  97  67  -3 0.00
Sun 11/24 06Z 0.91 3035  97  11  -3 0.02
Sun 11/24 09Z 1.24 3036  96   8  -6 0.01
Sun 11/24 12Z 1.53 3036  97  28  -7 0.00
Sun 11/24 15Z 1.78 3033  90  28  -8 0.00
Sun 11/24 18Z 10.21 2928  85   8  -8 0.00

Run total areal avg QPF for the greens is: 1.03 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