Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_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 0.40 2915  86  46 -13 0.00
Thu 02/05 07Z 0.45 2917  85  45 -13 0.00
Thu 02/05 08Z 0.50 2919  87  48 -13 0.00
Thu 02/05 09Z 0.50 2919  86  62 -14 0.00
Thu 02/05 10Z 0.49 2920  85  68 -14 0.00
Thu 02/05 11Z 0.47 2920  86  75 -14 0.00
Thu 02/05 12Z 0.44 3021  85  36 -14 0.00
Thu 02/05 13Z 0.41 3022  83   8 -14 0.00
Thu 02/05 14Z 0.36 3121  80   5 -15 0.00
Thu 02/05 15Z 0.32 3118  77   5 -15 0.00
Thu 02/05 16Z 0.29 3117  75   5 -14 0.00
Thu 02/05 17Z 0.26 3115  71   5 -14 0.00
Thu 02/05 18Z 0.23 3113  66   6 -14 0.00
Thu 02/05 19Z 0.20 3212  61   6 -13 0.00
Thu 02/05 20Z 0.17 3212  56   7 -13 0.00
Thu 02/05 21Z 0.15 3211  51   9 -12 0.00
Thu 02/05 22Z 0.12 3311  44  12 -11 0.00
Thu 02/05 23Z 0.10 3310  41  15 -11 0.00
Fri 02/06 00Z 0.07 3309  37  18 -10 0.00
Fri 02/06 01Z 0.05 3307  35  21 -10 0.00
Fri 02/06 02Z 0.02 3405  32  23 -10 0.00
Fri 02/06 03Z 0.02 3402  31  23 -10 0.00
Fri 02/06 04Z 0.02 3301  30  22  -9 0.00
Fri 02/06 05Z 0.02 2600  29  20  -9 0.00
Fri 02/06 06Z 0.03 2601  29  17  -9 0.00
Fri 02/06 07Z 0.04 2302  29  14  -9 0.00
Fri 02/06 08Z 0.03 2003  28  10 -10 0.00
Fri 02/06 09Z 0.02 1805  27   9  -9 0.00
Fri 02/06 10Z 0.02 1807  25  10  -9 0.00
Fri 02/06 11Z 0.03 1808  24  12  -9 0.00
Fri 02/06 12Z 0.05 2008  23  37  -9 0.00
Fri 02/06 13Z 0.07 2008  25  59  -9 0.00
Fri 02/06 14Z 0.08 2008  28  72  -9 0.00
Fri 02/06 15Z 0.08 2008  32  80 -10 0.00
Fri 02/06 16Z 0.07 2007  38  84 -10 0.00
Fri 02/06 17Z 0.06 2006  43  85 -10 0.00
Fri 02/06 18Z 0.05 1906  46  78 -10 0.00
Fri 02/06 19Z 0.06 1906  50  78  -9 0.00
Fri 02/06 20Z 0.07 2006  54  84  -9 0.00
Fri 02/06 21Z 0.09 2007  58  86 -10 0.00
Fri 02/06 22Z 0.11 2008  59  86 -10 0.00
Fri 02/06 23Z 0.13 2109  58  87 -10 0.00
Sat 02/07 00Z 0.15 2110  58  90 -10 0.00
Sat 02/07 01Z 0.14 2010  61  87 -10 0.00
Sat 02/07 02Z 0.15 2010  74  86 -11 0.00
Sat 02/07 03Z 0.22 2112  84  87 -12 0.00
Sat 02/07 04Z 0.27 2112  88  88 -12 0.00
Sat 02/07 05Z 0.24 2112  89  88 -12 0.00
Sat 02/07 06Z 0.21 2210  90  87 -11 0.01
Sat 02/07 07Z 0.18 2208  91  86 -11 0.01
Sat 02/07 08Z 0.15 2306  92  86 -11 0.01
Sat 02/07 09Z 0.13 2305  92  86 -11 0.01
Sat 02/07 10Z 0.12 2404  92  85 -11 0.01
Sat 02/07 11Z 0.14 2704  92  84 -12 0.01
Sat 02/07 12Z 0.20 3108  92  83 -13 0.02
Sat 02/07 13Z 0.14 3313  91  82 -13 0.02
Sat 02/07 14Z 0.10 3414  89  83 -13 0.01
Sat 02/07 15Z 0.08 3416  86  83 -14 0.01
Sat 02/07 16Z 0.10 3317  85  83 -15 0.01
Sat 02/07 17Z 0.13 3319  84  83 -15 0.01

Run total areal avg QPF for the greens is: 0.14 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