Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260309_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 03/09 12Z 0.76 2429  57  53   3 0.00
Mon 03/09 13Z 0.77 2432  48  37   4 0.00
Mon 03/09 14Z 0.74 2433  39  34   5 0.00
Mon 03/09 15Z 0.72 2334  38  34   6 0.00
Mon 03/09 16Z 0.74 2335  35  34   6 0.00
Mon 03/09 17Z 0.78 2235  30  35   6 0.00
Mon 03/09 18Z 0.83 2236  27  34   6 0.00
Mon 03/09 19Z 0.83 2234  26  35   7 0.00
Mon 03/09 21Z 0.79 2333  32  34   7 0.00
Mon 03/09 23Z 0.88 2435  39  40   7 0.00
Tue 03/10 00Z 0.92 2436  41  43   7 0.00
Tue 03/10 01Z 0.95 2436  42  44   7 0.00
Tue 03/10 02Z 0.94 2436  43  44   8 0.00
Tue 03/10 04Z 1.02 2536  44  49   7 0.00
Tue 03/10 05Z 1.08 2635  46  49   7 0.00
Tue 03/10 06Z 1.14 2633  50  45   6 0.00
Tue 03/10 07Z 1.04 2731  56  44   6 0.00
Tue 03/10 09Z 0.75 2725  65  47   6 0.00
Tue 03/10 10Z 0.64 2722  68  53   6 0.00
Tue 03/10 11Z 0.54 2619  69  55   6 0.00
Tue 03/10 12Z 0.46 2617  69  50   7 0.00
Tue 03/10 13Z 0.46 2518  73  39   6 0.00
Tue 03/10 14Z 0.46 2419  76  43   6 0.00
Tue 03/10 15Z 0.47 2420  78  35   6 0.00
Tue 03/10 16Z 0.53 2421  82  38   6 0.00
Tue 03/10 17Z 0.57 2420  85  45   6 0.00
Tue 03/10 18Z 0.63 2420  89  48   6 0.00
Tue 03/10 19Z 0.60 2519  91  48   6 0.00
Tue 03/10 20Z 0.61 2518  93  48   6 0.00
Tue 03/10 21Z 0.64 2619  96  46   6 0.00
Tue 03/10 22Z 0.62 2719  97  46   6 0.00
Tue 03/10 23Z 0.53 2718  98  47   5 0.00
Wed 03/11 00Z 0.42 2916  98  48   5 0.00
Wed 03/11 01Z 0.28 3012  98  48   4 0.01
Wed 03/11 02Z 0.18 3109  97  45   4 0.02
Wed 03/11 03Z 0.13 3107  95  43   4 0.03
Wed 03/11 04Z 0.09 3105  94  47   4 0.02
Wed 03/11 05Z 0.09 3105  92  48   3 0.01
Wed 03/11 06Z 0.07 3003  90  54   4 0.01
Wed 03/11 07Z 0.05 2401  90  52   4 0.01
Wed 03/11 08Z 0.04 1603  90  56   4 0.01
Wed 03/11 09Z 0.07 1508  91  74   4 0.00
Wed 03/11 10Z 0.08 1613  93  82   5 0.00
Wed 03/11 11Z 0.12 1619  96  85   6 0.00
Wed 03/11 12Z 0.14 1624  97  82   6 0.00
Wed 03/11 13Z 0.14 1632  97  76   7 0.00
Wed 03/11 14Z 0.11 1636  96  62   7 0.01
Wed 03/11 15Z 0.06 1739  96  51   7 0.02
Wed 03/11 16Z 0.04 1741  96  48   8 0.03
Wed 03/11 17Z 0.06 1842  96  55   8 0.03
Wed 03/11 18Z 0.10 1841  97  67  10 0.02
Wed 03/11 19Z 0.15 1941  97  71  10 0.02
Wed 03/11 20Z 0.21 2041  97  71  10 0.02
Wed 03/11 21Z 0.21 2040  96  60  11 0.02
Wed 03/11 22Z 0.22 2042  95  84  11 0.02
Wed 03/11 23Z 0.25 2043  95  83  11 0.02

Run total areal avg QPF for the greens is: 0.33 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