NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20250904_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)
=== ===== === ==== ==== === === === ====
Thu 09/04 00Z 0.37 2108  60  65  12 0.00
Thu 09/04 03Z 0.39 2111  62  46  12 0.00
Thu 09/04 06Z 0.45 2215  64  38  11 0.00
Thu 09/04 09Z 0.41 2114  68  33  11 0.00
Thu 09/04 12Z 0.36 2115  68  35  11 0.00
Thu 09/04 15Z 0.27 1917  65  22  11 0.00
Thu 09/04 18Z 0.72 1816  61  38  12 0.00
Thu 09/04 21Z 0.71 1720  65  68  13 0.00
Fri 09/05 00Z 0.13 1730  79  51  12 0.00
Fri 09/05 03Z 0.28 1937  70  66  12 0.00
Fri 09/05 06Z 0.37 2032  97  96  10 0.07
Fri 09/05 09Z 0.49 2319  98  75   9 0.08
Fri 09/05 12Z 0.24 2211  91  22   9 0.01
Fri 09/05 15Z 0.21 2015  87  28  10 0.00
Fri 09/05 18Z 1.43 1917  86  57  10 0.00
Fri 09/05 21Z 0.44 1922  81  44  13 0.00
Sat 09/06 00Z 0.87 2032  86  63  13 0.00
Sat 09/06 03Z 0.88 2136  91  63  14 0.00
Sat 09/06 06Z 1.09 2337  82  60  14 0.01
Sat 09/06 09Z 0.83 2326  82  50  14 0.00
Sat 09/06 12Z 0.51 2320  97  86  13 0.04
Sat 09/06 15Z 0.42 2416  99  95  12 0.27
Sat 09/06 18Z 0.52 2418  87  87  10 0.09
Sat 09/06 21Z 0.47 2418  94  90  12 0.00
Sun 09/07 00Z 0.32 2411  75  95  10 0.12
Sun 09/07 03Z 0.41 2710  94  98   8 0.06
Sun 09/07 06Z 0.08 3206  98  98   8 0.17
Sun 09/07 09Z 0.14 3208  98  98   6 0.41
Sun 09/07 12Z 0.15 3107  92  95   6 0.03

Run total areal avg QPF for the greens is: 1.36 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