NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20251107_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)
=== ===== === ==== ==== === === === ====
Fri 11/07 12Z 0.34 2612  77  71  -4 0.00
Fri 11/07 13Z 0.33 2413  73  82  -3 0.00
Fri 11/07 14Z 0.28 2313  72  79  -2 0.00
Fri 11/07 15Z 0.29 2216  71  81  -2 0.00
Fri 11/07 16Z 0.28 2120  65  85  -1 0.00
Fri 11/07 17Z 0.25 2022  60  79  -1 0.00
Fri 11/07 18Z 0.26 1925  55  28   0 0.00
Fri 11/07 19Z 0.18 1929  56  13   0 0.00
Fri 11/07 20Z 0.16 1832  60  20   0 0.00
Fri 11/07 21Z 0.15 1835  63  84   1 0.00
Fri 11/07 22Z 0.29 1938  78  87   0 0.02
Fri 11/07 23Z 0.39 2037  82  95   1 0.02
Sat 11/08 00Z 0.44 2038  90  95   1 0.03
Sat 11/08 01Z 0.50 2139  95  94   2 0.06
Sat 11/08 02Z 0.56 2141  96  95   2 0.07
Sat 11/08 03Z 0.58 2138  96  93   3 0.06
Sat 11/08 04Z 0.59 2236  96  90   3 0.04
Sat 11/08 05Z 0.62 2234  96  86   3 0.03
Sat 11/08 06Z 0.63 2331  96  60   3 0.02
Sat 11/08 07Z 0.67 2430  96  35   3 0.02
Sat 11/08 08Z 0.73 2430  94  30   2 0.01
Sat 11/08 09Z 0.77 2431  94  30   3 0.01
Sat 11/08 10Z 0.87 2530  95  35   2 0.01
Sat 11/08 11Z 0.97 2530  96  34   1 0.01
Sat 11/08 12Z 1.03 2628  95  34   0 0.01
Sat 11/08 13Z 0.92 2724  94  32   0 0.01
Sat 11/08 14Z 0.94 2723  94  27   0 0.01
Sat 11/08 15Z 0.94 2821  95  27   0 0.01
Sat 11/08 16Z 0.89 2820  93  23  -1 0.01
Sat 11/08 17Z 0.85 2819  93  15  -1 0.01
Sat 11/08 18Z 0.72 2818  92   3  -1 0.01
Sat 11/08 19Z 0.66 2917  88   0  -2 0.01
Sat 11/08 20Z 0.58 2918  87   1  -2 0.00
Sat 11/08 21Z 0.49 3017  83   2  -3 0.00
Sat 11/08 22Z 0.36 3016  77   3  -2 0.00
Sat 11/08 23Z 0.31 3015  71   3  -3 0.00
Sun 11/09 00Z 0.23 3113  64   3  -2 0.00
Sun 11/09 01Z 0.19 3111  59   3  -2 0.00
Sun 11/09 02Z 0.15 3009  53  10  -2 0.00
Sun 11/09 03Z 0.13 2906  49  17  -2 0.00
Sun 11/09 04Z 0.09 2704  47  23  -2 0.00
Sun 11/09 05Z 0.10 2405  48  33  -2 0.00
Sun 11/09 06Z 0.12 2307  47  50  -1 0.00
Sun 11/09 07Z 0.12 2307  47  67  -1 0.00
Sun 11/09 08Z 0.11 2207  48  81  -1 0.00
Sun 11/09 09Z 0.11 2208  52  93  -1 0.00
Sun 11/09 10Z 0.09 2108  58  89  -1 0.00
Sun 11/09 11Z 0.05 1910  69  89   0 0.00
Sun 11/09 12Z 0.02 1812  80  89   0 0.00
Sun 11/09 13Z 0.03 1715  90  96   0 0.01
Sun 11/09 14Z 0.05 1719  93  91   0 0.02
Sun 11/09 15Z 0.06 1722  93  92   1 0.02
Sun 11/09 16Z 0.06 1726  95  94   1 0.02
Sun 11/09 17Z 0.06 1727  95  97   2 0.05
Sun 11/09 18Z 0.07 1729  94  95   2 0.04
Sun 11/09 19Z 0.07 1729  92  93   2 0.04
Sun 11/09 20Z 0.10 1628  89  86   3 0.01
Sun 11/09 21Z 0.12 1627  87  77   3 0.01
Sun 11/09 22Z 0.16 1628  88  61   3 0.01
Sun 11/09 23Z 0.21 1529  89  50   3 0.01

Run total areal avg QPF for the greens is: 0.73 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