NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260429_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)
=== ===== === ==== ==== === === === ====
Wed 04/29 12Z 0.09 1911  69  72   6 0.00
Wed 04/29 13Z 0.07 1910  69  63   6 0.00
Wed 04/29 14Z 0.03 1809  70  49   6 0.00
Wed 04/29 15Z 0.04 1708  71  46   6 0.00
Wed 04/29 16Z 0.12 1609  71  41   6 0.00
Wed 04/29 17Z 0.27 1610  72  50   6 0.00
Wed 04/29 18Z 0.72 1612  73  57   5 0.00
Wed 04/29 19Z 1.04 1613  74  66   5 0.00
Wed 04/29 20Z 1.29 1613  75  59   6 0.00
Wed 04/29 21Z 1.26 1514  76  45   6 0.00
Wed 04/29 22Z 0.67 1616  78  47   5 0.00
Wed 04/29 23Z 0.29 1618  79  60   5 0.00
Thu 04/30 00Z 0.19 1619  80  68   6 0.00
Thu 04/30 01Z 0.15 1620  82  74   6 0.00
Thu 04/30 02Z 0.14 1621  83  83   7 0.00
Thu 04/30 03Z 0.13 1622  84  90   7 0.00
Thu 04/30 04Z 0.12 1624  85  95   7 0.00
Thu 04/30 05Z 0.10 1626  90  96   6 0.01
Thu 04/30 06Z 0.08 1727  92  94   5 0.03
Thu 04/30 07Z 0.06 1728  94  99   5 0.07
Thu 04/30 08Z 0.05 1729  95  97   5 0.10
Thu 04/30 09Z 0.05 1727  97  98   4 0.10
Thu 04/30 10Z 0.04 1825  97  99   3 0.09
Thu 04/30 11Z 0.07 1823  97  99   2 0.07
Thu 04/30 12Z 0.08 1821  98  95   1 0.04
Thu 04/30 13Z 0.15 1919  98  92   0 0.02
Thu 04/30 14Z 0.22 1917  99  86   0 0.02
Thu 04/30 15Z 0.25 2014  99  87   0 0.01
Thu 04/30 16Z 0.28 2111  99  81   0 0.01
Thu 04/30 17Z 0.33 2309  99  81   0 0.01
Thu 04/30 18Z 0.44 2509  98  81   0 0.01
Thu 04/30 19Z 0.54 2610  98  77   0 0.01
Thu 04/30 20Z 0.64 2711  97  76   0 0.01
Thu 04/30 21Z 0.67 2811  96  73   0 0.01
Thu 04/30 22Z 0.64 2912  97  70   0 0.02
Thu 04/30 23Z 0.66 3015  96  65   0 0.02
Fri 05/01 00Z 0.60 3016  93  61   0 0.00
Fri 05/01 01Z 0.53 3117  90  64   0 0.00
Fri 05/01 02Z 0.52 3017  92  65  -1 0.00
Fri 05/01 03Z 0.53 3017  93  72  -1 0.01
Fri 05/01 04Z 0.49 3117  95  76  -1 0.01
Fri 05/01 05Z 0.44 3117  95  72  -2 0.01
Fri 05/01 06Z 0.40 3117  93  67  -2 0.00
Fri 05/01 07Z 0.37 3117  91  64  -2 0.00
Fri 05/01 08Z 0.37 3117  89  61  -2 0.00
Fri 05/01 09Z 0.38 3118  85  62  -2 0.00
Fri 05/01 10Z 0.38 3118  78  61  -3 0.00
Fri 05/01 11Z 0.37 3118  76  62  -3 0.00
Fri 05/01 12Z 0.38 3116  78  61  -4 0.00
Fri 05/01 13Z 0.45 3115  82  48  -4 0.00
Fri 05/01 14Z 0.69 3014  82  54  -4 0.00
Fri 05/01 15Z 2.34 3012  78  55  -4 0.00
Fri 05/01 16Z 4.37 3010  76  56  -3 0.00
Fri 05/01 17Z 5.44 2909  76  54  -3 0.00
Fri 05/01 18Z 5.71 2808  76  44  -2 0.00
Fri 05/01 19Z 5.39 2708  77  45  -2 0.00
Fri 05/01 20Z 4.89 2708  78  60  -1 0.00
Fri 05/01 21Z 3.40 2809  82  74  -1 0.02
Fri 05/01 22Z 2.43 2809  79  62  -2 0.01
Fri 05/01 23Z 1.29 2908  79  61  -2 0.00

Run total areal avg QPF for the greens is: 0.72 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