Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20251229_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)
=== ===== === ==== ==== === === === ====
Mon 12/29 06Z 0.09 1828  94  99   2 0.00
Mon 12/29 07Z 0.12 1927  96  99   1 0.17
Mon 12/29 08Z 0.14 1926  97  98   2 0.16
Mon 12/29 09Z 0.19 2026  97  95   3 0.14
Mon 12/29 10Z 0.21 2124  97  92   5 0.08
Mon 12/29 11Z 0.21 2123  97  95   6 0.08
Mon 12/29 12Z 0.20 2123  97  96   7 0.08
Mon 12/29 13Z 0.21 2124  97  93   7 0.11
Mon 12/29 14Z 0.30 2227  97  98   7 0.14
Mon 12/29 15Z 0.24 2126  95  81   8 0.07
Mon 12/29 16Z 0.29 2031  96  84   7 0.05
Mon 12/29 17Z 0.41 2130  96  89   4 0.05
Mon 12/29 18Z 0.48 2226  97  68   2 0.07
Mon 12/29 19Z 0.63 2322  97  55   0 0.04
Mon 12/29 20Z 0.79 2420  97  47  -2 0.03
Mon 12/29 21Z 1.05 2522  96  72  -4 0.04
Mon 12/29 22Z 1.28 2521  94  73  -5 0.03
Mon 12/29 23Z 1.54 2522  92  51  -7 0.01
Tue 12/30 00Z 2.93 2524  86  72  -9 0.01
Tue 12/30 01Z 2.53 2425  87  87 -10 0.00
Tue 12/30 02Z 2.17 2325  84  77 -11 0.00
Tue 12/30 03Z 2.06 2326  86  75 -12 0.00
Tue 12/30 04Z 2.00 2428  89  71 -13 0.00
Tue 12/30 05Z 4.06 2528  86  74 -13 0.00
Tue 12/30 06Z 5.45 2529  85  76 -15 0.00
Tue 12/30 07Z 8.35 2628  84  79 -16 0.00
Tue 12/30 08Z 7.17 2628  83  79 -17 0.00
Tue 12/30 09Z 6.62 2727  81  80 -19 0.01
Tue 12/30 10Z 6.74 2727  81  79 -20 0.01
Tue 12/30 11Z 8.08 2728  80  78 -21 0.01
Tue 12/30 12Z 9.34 2728  78  77 -22 0.00
Tue 12/30 13Z 9.28 2728  78  77 -22 0.00
Tue 12/30 14Z 11.06 2727  79  73 -23 0.00
Tue 12/30 15Z 14.29 2726  79  71 -23 0.00
Tue 12/30 16Z 15.17 2726  78  75 -23 0.00
Tue 12/30 17Z 14.26 2726  77  72 -23 0.00
Tue 12/30 18Z 15.16 2726  77  72 -22 0.00
Tue 12/30 19Z 14.04 2727  77  68 -22 0.00
Tue 12/30 20Z 10.26 2728  79  70 -22 0.00
Tue 12/30 21Z 4.25 2727  81  76 -21 0.00
Tue 12/30 22Z 3.26 2726  83  77 -21 0.01
Tue 12/30 23Z 3.83 2828  85  73 -21 0.01
Wed 12/31 00Z 4.11 2830  86  77 -21 0.00
Wed 12/31 01Z 2.18 2830  84  79 -21 0.01
Wed 12/31 02Z 2.08 2830  85  79 -20 0.00
Wed 12/31 03Z 2.07 2729  86  76 -20 0.00
Wed 12/31 04Z 1.85 2729  86  75 -20 0.00
Wed 12/31 05Z 1.73 2729  81  50 -19 0.00
Wed 12/31 06Z 1.80 2731  69  44 -19 0.00
Wed 12/31 07Z 2.49 2731  63  44 -19 0.00
Wed 12/31 08Z 1.55 2730  63  31 -19 0.00
Wed 12/31 09Z 1.18 2729  65  25 -18 0.00
Wed 12/31 10Z 1.13 2629  67  31 -18 0.00
Wed 12/31 11Z 1.09 2630  69  45 -17 0.00
Wed 12/31 12Z 1.05 2629  72  52 -17 0.00
Wed 12/31 13Z 0.98 2628  73  55 -16 0.00
Wed 12/31 14Z 1.13 2626  76  59 -16 0.00
Wed 12/31 15Z 1.92 2524  79  66 -16 0.00
Wed 12/31 16Z 2.86 2522  80  74 -16 0.00
Wed 12/31 17Z 4.55 2520  83  77 -15 0.00

Run total areal avg QPF for the greens is: 1.42 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