Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260205_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 02/05 06Z 0.42 2916  84  47 -13 0.00
Thu 02/05 09Z 0.53 2919  84  61 -14 0.00
Thu 02/05 12Z 0.49 3021  84  39 -15 0.00
Thu 02/05 15Z 0.35 3119  77   6 -15 0.00
Thu 02/05 18Z 0.25 3114  66   6 -14 0.00
Thu 02/05 21Z 0.16 3212  54  11 -13 0.00
Fri 02/06 00Z 0.09 3310  37  23 -10 0.00
Fri 02/06 03Z 0.03 3404  32  26 -10 0.00
Fri 02/06 06Z 0.03 2202  32  19 -10 0.00
Fri 02/06 09Z 0.05 2005  29  11  -9 0.00
Fri 02/06 12Z 0.07 2007  30  32  -9 0.00
Fri 02/06 15Z 0.09 2010  40  85 -10 0.00
Fri 02/06 18Z 0.05 1909  52  86 -10 0.00
Fri 02/06 21Z 0.11 1911  63  79 -10 0.00
Sat 02/07 00Z 0.19 2013  70  87 -11 0.00
Sat 02/07 03Z 0.29 2114  86  85 -12 0.00
Sat 02/07 06Z 0.19 2111  89  84 -12 0.02
Sat 02/07 09Z 0.12 2107  90  84 -11 0.02
Sat 02/07 12Z 0.17 3006  90  81 -12 0.04
Sat 02/07 15Z 0.11 3419  87  80 -15 0.04
Sat 02/07 18Z 0.12 3319  83  81 -15 0.02
Sat 02/07 21Z 0.22 3224  83  78 -18 0.01
Sun 02/08 00Z 0.29 3229  84  73 -22 0.00
Sun 02/08 03Z 0.21 3324  83  77 -18 0.00
Sun 02/08 06Z 0.25 3329  74  21 -23 0.01
Sun 02/08 09Z 0.28 3231  72  24 -23 0.00
Sun 02/08 12Z 0.28 3233  70  16 -22 0.00
Sun 02/08 15Z 0.28 3231  67  11 -22 0.00
Sun 02/08 18Z 0.36 3226  61  17 -22 0.00

Run total areal avg QPF for the greens is: 0.16 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