Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260209_1200
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)
=== ===== === ==== ==== === === === ====
Mon 02/09 12Z 0.18 3328  58  10 -14 0.00
Mon 02/09 13Z 0.18 3330  53  15 -13 0.00
Mon 02/09 14Z 0.16 3431  48  31 -12 0.00
Mon 02/09 15Z 0.15 3430  46  56 -12 0.00
Mon 02/09 16Z 0.14 3428  45  67 -12 0.00
Mon 02/09 17Z 0.16 3327  44  66 -12 0.00
Mon 02/09 18Z 0.16 3325  42  63 -11 0.00
Mon 02/09 19Z 0.17 3324  39  62 -11 0.00
Mon 02/09 20Z 0.19 3323  38  60 -11 0.00
Mon 02/09 21Z 0.21 3223  37  58 -10 0.00
Mon 02/09 22Z 0.21 3223  36  55 -10 0.00
Mon 02/09 23Z 0.21 3223  36  54  -9 0.00
Tue 02/10 00Z 0.19 3322  37  53  -9 0.00
Tue 02/10 01Z 0.18 3321  39  52  -9 0.00
Tue 02/10 02Z 0.18 3320  37  56  -9 0.00
Tue 02/10 03Z 0.14 3318  35  62  -9 0.00
Tue 02/10 04Z 0.14 3317  34  75  -9 0.00
Tue 02/10 05Z 0.14 3316  35  86  -8 0.00
Tue 02/10 06Z 0.16 3216  41  86  -8 0.00
Tue 02/10 07Z 0.16 3214  51  87  -8 0.00
Tue 02/10 08Z 0.16 3113  57  90  -8 0.00
Tue 02/10 09Z 0.14 3111  59  88  -8 0.00
Tue 02/10 10Z 0.15 3110  62  86  -7 0.00
Tue 02/10 11Z 0.14 3110  66  82  -7 0.00
Tue 02/10 12Z 0.13 3008  69  78  -7 0.00
Tue 02/10 13Z 0.11 3006  73  72  -7 0.00
Tue 02/10 14Z 0.09 2704  75  68  -7 0.00
Tue 02/10 15Z 0.07 2304  75  62  -6 0.00
Tue 02/10 16Z 0.11 2207  73  59  -7 0.00
Tue 02/10 17Z 0.10 2108  63  48  -6 0.00
Tue 02/10 18Z 0.09 2010  51  43  -5 0.00
Tue 02/10 19Z 0.09 1913  45  58  -5 0.00
Tue 02/10 20Z 0.10 1916  43  82  -5 0.00
Tue 02/10 21Z 0.02 1720  72  91  -4 0.00
Tue 02/10 22Z 0.05 1724  90  95  -3 0.03
Tue 02/10 23Z 0.05 1728  95  95  -3 0.06
Wed 02/11 00Z 0.09 1829  95  96  -4 0.08
Wed 02/11 01Z 0.20 1929  96  95  -5 0.08
Wed 02/11 02Z 0.31 2027  97  96  -6 0.05
Wed 02/11 03Z 0.42 2126  97  84  -6 0.04
Wed 02/11 04Z 0.51 2325  97  64  -7 0.03
Wed 02/11 05Z 0.57 2422  97  46  -7 0.02
Wed 02/11 06Z 0.61 2521  97  43  -8 0.01
Wed 02/11 07Z 0.68 2521  98  46  -8 0.01
Wed 02/11 08Z 0.81 2621  98  59  -9 0.02
Wed 02/11 09Z 0.91 2821  97  77 -10 0.01
Wed 02/11 10Z 0.90 2821  96  83 -10 0.01
Wed 02/11 12Z 0.76 2820  95  83 -11 0.01
Wed 02/11 13Z 0.76 2921  95  83 -11 0.01
Wed 02/11 15Z 1.20 2920  94  85 -12 0.01
Wed 02/11 16Z 2.78 2918  94  85 -12 0.01
Wed 02/11 17Z 2.94 2918  95  85 -13 0.01
Wed 02/11 18Z 3.11 3018  94  85 -13 0.01
Wed 02/11 19Z 3.26 3118  95  87 -13 0.01
Wed 02/11 20Z 3.01 3120  94  87 -13 0.01
Wed 02/11 21Z 1.52 3222  95  86 -13 0.01
Wed 02/11 22Z 0.77 3325  95  84 -13 0.01
Wed 02/11 23Z 0.52 3326  95  83 -13 0.01

Run total areal avg QPF for the greens is: 0.56 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