Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260310_1800
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)
=== ===== === ==== ==== === === === ====
Tue 03/10 18Z 1.95 2316  56  43   8 0.00
Tue 03/10 19Z 1.51 2418  55  41   8 0.00
Tue 03/10 20Z 1.43 2418  57  42   8 0.00
Tue 03/10 21Z 1.33 2518  66  45   8 0.00
Tue 03/10 22Z 1.15 2619  75  43   7 0.00
Tue 03/10 23Z 0.83 2618  85  50   7 0.01
Wed 03/11 00Z 0.63 2719  92  40   7 0.01
Wed 03/11 01Z 0.53 2717  91  32   7 0.00
Wed 03/11 02Z 0.42 2816  90  33   6 0.00
Wed 03/11 03Z 0.32 2813  91  33   6 0.00
Wed 03/11 04Z 0.22 2909  92  33   6 0.00
Wed 03/11 05Z 0.15 2807  93  32   7 0.00
Wed 03/11 06Z 0.11 2605  93  59   7 0.00
Wed 03/11 07Z 0.07 2204  94  58   7 0.00
Wed 03/11 08Z 0.03 1707  96  72   7 0.00
Wed 03/11 09Z 0.06 1613  97  72   8 0.01
Wed 03/11 10Z 0.10 1621  97  68   8 0.01
Wed 03/11 11Z 0.12 1627  97  50   8 0.02
Wed 03/11 12Z 0.07 1732  97  46   8 0.03
Wed 03/11 13Z 0.04 1734  97  49   9 0.03
Wed 03/11 14Z 0.05 1836  97  63   9 0.02
Wed 03/11 15Z 0.06 1837  96  87  10 0.02
Wed 03/11 16Z 0.07 1837  95  90  10 0.02
Wed 03/11 17Z 0.15 1940  96  90   9 0.04
Wed 03/11 18Z 0.22 1942  94  46  10 0.03
Wed 03/11 19Z 0.26 2042  93  50  11 0.01
Wed 03/11 20Z 0.27 2039  90  58  11 0.00
Wed 03/11 21Z 0.26 2037  95  92  10 0.02
Wed 03/11 22Z 0.24 2039  95  95  10 0.04
Wed 03/11 23Z 0.32 2041  96  93  10 0.03
Thu 03/12 01Z 0.52 2145  98  94  11 0.03
Thu 03/12 02Z 0.65 2144  97  92  11 0.06
Thu 03/12 03Z 0.69 2239  98  88  11 0.04
Thu 03/12 04Z 0.78 2335  97  72  11 0.05
Thu 03/12 05Z 0.73 2330  98  80  10 0.13
Thu 03/12 06Z 0.79 2329  97  78   9 0.07
Thu 03/12 07Z 0.96 2429  98  81   8 0.06
Thu 03/12 08Z 1.14 2530  98  91   6 0.06
Thu 03/12 09Z 0.96 2632  96  90   1 0.08
Thu 03/12 10Z 1.17 2731  96  69  -3 0.09
Thu 03/12 11Z 1.28 2730  96  41  -6 0.04
Thu 03/12 12Z 1.31 2731  96  37  -8 0.02
Thu 03/12 13Z 1.63 2830  96  21 -10 0.01
Thu 03/12 14Z 2.10 2830  95  11 -12 0.01
Thu 03/12 15Z 3.08 2829  93   4 -13 0.01
Thu 03/12 16Z 4.78 2927  92   4 -14 0.01
Thu 03/12 17Z 5.35 2926  91  11 -14 0.00
Thu 03/12 18Z 6.48 2925  90  18 -14 0.00
Thu 03/12 19Z 6.71 2925  91  23 -14 0.00
Thu 03/12 20Z 6.27 3025  89  25 -13 0.00
Thu 03/12 21Z 4.28 3024  89  30 -13 0.00
Thu 03/12 22Z 2.29 3023  90  36 -13 0.00
Thu 03/12 23Z 1.48 3023  90  39 -13 0.00
Fri 03/13 01Z 1.15 3024  91  41 -14 0.00
Fri 03/13 02Z 1.13 3024  93  40 -14 0.00
Fri 03/13 03Z 1.07 3124  94  40 -14 0.00
Fri 03/13 04Z 1.01 3123  95  41 -14 0.00
Fri 03/13 05Z 0.86 3121  94  42 -14 0.00

Run total areal avg QPF for the greens is: 1.12 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