Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20250430_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)
=== ===== === ==== ==== === === === ====
Wed 04/30 18Z 1.00 3115  64   4   0 0.00
Wed 04/30 19Z 0.70 3118  59   5   0 0.00
Wed 04/30 20Z 0.59 3117  59   6   0 0.00
Wed 04/30 21Z 0.55 3118  59   7   0 0.00
Wed 04/30 22Z 0.45 3118  55   7   1 0.00
Wed 04/30 23Z 0.35 3218  47   8   1 0.00
Thu 05/01 00Z 0.29 3219  40   9   2 0.00
Thu 05/01 01Z 0.28 3219  40  10   2 0.00
Thu 05/01 02Z 0.25 3217  42  10   1 0.00
Thu 05/01 03Z 0.18 3315  41  11   1 0.00
Thu 05/01 04Z 0.13 3312  37  12   2 0.00
Thu 05/01 05Z 0.16 3211  35  12   2 0.00
Thu 05/01 06Z 0.16 3109  29  13   2 0.00
Thu 05/01 07Z 0.15 3009  25  14   3 0.00
Thu 05/01 08Z 0.14 3008  21  15   4 0.00
Thu 05/01 09Z 0.15 2908  19  15   4 0.00
Thu 05/01 10Z 0.17 2908  18  15   4 0.00
Thu 05/01 11Z 0.15 2807  17  14   4 0.00
Thu 05/01 12Z 0.16 2808  17  13   4 0.00
Thu 05/01 13Z 0.14 2906  19  11   4 0.00
Thu 05/01 14Z 0.14 2805  21   9   4 0.00
Thu 05/01 15Z 0.12 2603  25   8   4 0.00
Thu 05/01 16Z 0.16 2503  29   6   3 0.00
Thu 05/01 17Z 0.34 2304  31   5   4 0.00
Thu 05/01 18Z 0.38 2104  30   5   4 0.00
Thu 05/01 19Z 0.46 2105  29   7   5 0.00
Thu 05/01 20Z 0.42 2106  28  14   6 0.00
Thu 05/01 21Z 0.32 2107  28  18   6 0.00
Thu 05/01 22Z 0.10 1908  28  15   7 0.00
Thu 05/01 23Z 0.06 1810  36  40   7 0.00
Fri 05/02 00Z 0.09 1813  44  74   7 0.00
Fri 05/02 01Z 0.07 1817  50  93   8 0.00
Fri 05/02 02Z 0.07 1819  58  94   8 0.00
Fri 05/02 03Z 0.11 1922  68  96   8 0.00
Fri 05/02 04Z 0.17 1927  82  96   8 0.02
Fri 05/02 05Z 0.27 2032  91  85   9 0.04
Fri 05/02 06Z 0.34 2036  90  85  10 0.02
Fri 05/02 07Z 0.36 2137  87  81  11 0.01
Fri 05/02 08Z 0.37 2139  90  86  11 0.02
Fri 05/02 09Z 0.36 2138  92  79  11 0.03
Fri 05/02 10Z 0.39 2137  93  86  11 0.01
Fri 05/02 11Z 0.43 2135  95  74  10 0.01
Fri 05/02 12Z 0.46 2133  96  89  10 0.01
Fri 05/02 13Z 0.52 2231  97  77  10 0.01
Fri 05/02 14Z 0.56 2327  97  54  10 0.02
Fri 05/02 15Z 0.61 2324  95  43   9 0.01
Fri 05/02 16Z 0.72 2323  90  38   9 0.00
Fri 05/02 17Z 1.66 2322  89  36   9 0.00
Fri 05/02 18Z 2.68 2321  88  37   9 0.00
Fri 05/02 19Z 2.51 2320  88  35  10 0.00
Fri 05/02 20Z 2.93 2320  89  35  10 0.00
Fri 05/02 21Z 4.05 2420  87  37  10 0.01
Fri 05/02 22Z 2.44 2422  90  47  10 0.01
Fri 05/02 23Z 1.39 2520  83  49  10 0.00
Sat 05/03 00Z 1.06 2520  85  66  10 0.00
Sat 05/03 01Z 0.98 2518  90  76   9 0.01
Sat 05/03 02Z 0.74 2517  91  75   9 0.00
Sat 05/03 03Z 0.62 2516  91  75   9 0.01
Sat 05/03 04Z 0.56 2516  90  78   9 0.01
Sat 05/03 05Z 0.50 2515  91  72   9 0.00

Run total areal avg QPF for the greens is: 0.26 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