Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260313_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)
=== ===== === ==== ==== === === === ====
Fri 03/13 18Z 1.26 2010  74   3  -9 0.00
Fri 03/13 19Z 0.94 1912  66  20  -9 0.00
Fri 03/13 20Z 0.19 1813  58  72  -8 0.00
Fri 03/13 21Z 0.82 1818  54  83  -8 0.00
Fri 03/13 22Z 0.16 1820  64  88  -8 0.00
Fri 03/13 23Z 0.18 1722  81  90  -8 0.01
Sat 03/14 00Z 0.13 1724  88  92  -8 0.01
Sat 03/14 01Z 0.11 1727  90  91  -8 0.01
Sat 03/14 02Z 0.12 1729  91  77  -8 0.01
Sat 03/14 03Z 0.13 1729  94  87  -7 0.02
Sat 03/14 04Z 0.10 1729  95  91  -7 0.05
Sat 03/14 05Z 0.12 1827  95  81  -7 0.05
Sat 03/14 06Z 0.23 1926  96  80  -6 0.02
Sat 03/14 07Z 0.38 2022  97  85  -6 0.02
Sat 03/14 08Z 0.61 2223  97  87  -6 0.02
Sat 03/14 09Z 0.78 2224  97  86  -6 0.02
Sat 03/14 10Z 0.93 2326  96  85  -7 0.02
Sat 03/14 11Z 1.07 2426  96  84  -7 0.03
Sat 03/14 12Z 1.20 2426  95  82  -8 0.02
Sat 03/14 13Z 1.42 2526  95  80  -8 0.03
Sat 03/14 14Z 2.68 2525  94  77  -9 0.02
Sat 03/14 15Z 5.35 2524  93  75  -9 0.01
Sat 03/14 16Z 7.38 2623  90  75  -9 0.01
Sat 03/14 17Z 4.86 2622  91  75  -8 0.01
Sat 03/14 18Z 1.84 2622  93  79  -8 0.01
Sat 03/14 19Z 1.01 2621  95  83  -7 0.02
Sat 03/14 20Z 0.89 2622  96  85  -7 0.03
Sat 03/14 21Z 0.96 2625  97  82  -6 0.04
Sat 03/14 22Z 1.86 2831  94  81  -8 0.05
Sat 03/14 23Z 1.61 2832  94  79  -9 0.03
Sun 03/15 00Z 1.54 2934  94  79  -9 0.03
Sun 03/15 01Z 1.49 2935  95  76 -10 0.03
Sun 03/15 02Z 1.26 3034  95  63 -10 0.02
Sun 03/15 03Z 1.15 3032  95  46 -10 0.01
Sun 03/15 04Z 1.12 3031  96  21 -11 0.02
Sun 03/15 05Z 1.03 3029  95   9 -11 0.01
Sun 03/15 06Z 0.72 3127  90   5 -11 0.01
Sun 03/15 07Z 0.51 3124  80   6 -11 0.00
Sun 03/15 08Z 0.38 3122  67   8 -10 0.00
Sun 03/15 09Z 0.32 3119  58  10  -9 0.00
Sun 03/15 10Z 0.27 3117  48   7  -9 0.00
Sun 03/15 11Z 0.19 3214  37  13  -8 0.00
Sun 03/15 12Z 0.14 3210  31  40  -8 0.00
Sun 03/15 13Z 0.10 3106  27  68  -7 0.00
Sun 03/15 14Z 0.06 2702  26  85  -7 0.00
Sun 03/15 15Z 0.03 1904  27  93  -7 0.00
Sun 03/15 16Z 0.03 1708  29  97  -7 0.00
Sun 03/15 17Z 0.06 1713  36  96  -7 0.00
Sun 03/15 18Z 0.07 1716  51  97  -6 0.00
Sun 03/15 19Z 0.07 1720  55  82  -6 0.00
Sun 03/15 20Z 0.10 1724  53  76  -5 0.00
Sun 03/15 21Z 0.11 1730  43  52  -4 0.00
Sun 03/15 22Z 0.10 1733  41  35  -3 0.00
Sun 03/15 23Z 0.07 1737  41  32  -2 0.00
Mon 03/16 00Z 0.12 1840  42  33  -1 0.00
Mon 03/16 01Z 0.18 1942  37  35   0 0.00
Mon 03/16 02Z 0.22 1945  32  37   1 0.00
Mon 03/16 03Z 0.24 1947  37  36   2 0.00
Mon 03/16 04Z 0.24 1948  43  37   3 0.00
Mon 03/16 05Z 0.22 1949  52  38   4 0.00

Run total areal avg QPF for the greens is: 0.70 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