Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260123_1800
Using the MB925 to MB850 layer.
Assumes Central_Greens are orthogonal to 260 degree winds.

925-850 mb Layer Average Wind/RH. Avg QPF over the Central_Greens.
Date   Time        Wind RH% 700 850T QPF
Day mm/dd hhZ Frd# DDFF (%) RH% (C) (in)
=== ===== === ==== ==== === === === ====
Fri 01/23 18Z 7.20 2618  82  57 -17 0.00
Fri 01/23 19Z 12.08 2621  80  65 -18 0.00
Fri 01/23 20Z 10.20 2823  75  76 -18 0.01
Fri 01/23 21Z 7.99 2923  75  78 -19 0.02
Fri 01/23 22Z 5.58 2924  77  79 -20 0.01
Fri 01/23 23Z 8.38 2924  71  77 -21 0.00
Sat 01/24 00Z 4.27 2825  72  68 -22 0.00
Sat 01/24 01Z 8.95 2931  69  62 -23 0.01
Sat 01/24 02Z 3.77 2832  57  33 -24 0.00
Sat 01/24 03Z 12.41 2831  66  31 -25 0.00
Sat 01/24 04Z 8.94 2930  73  29 -26 0.00
Sat 01/24 05Z 5.74 3029  79  23 -27 0.00
Sat 01/24 06Z 3.98 3029  72  19 -28 0.00
Sat 01/24 07Z 0.84 3129  57  20 -29 0.00
Sat 01/24 08Z 0.58 3129  56  18 -28 0.00
Sat 01/24 09Z 0.46 3128  58  18 -27 0.00
Sat 01/24 10Z 0.40 3128  59  16 -27 0.00
Sat 01/24 11Z 0.36 3128  58  16 -26 0.00
Sat 01/24 12Z 0.33 3128  55  15 -25 0.00
Sat 01/24 13Z 0.31 3127  52  15 -24 0.00
Sat 01/24 14Z 0.29 3126  50  16 -24 0.00
Sat 01/24 15Z 0.29 3124  47  15 -24 0.00
Sat 01/24 16Z 0.30 3123  43  16 -23 0.00
Sat 01/24 17Z 0.32 3123  43  16 -23 0.00
Sat 01/24 18Z 0.33 3123  46  16 -23 0.00
Sat 01/24 19Z 0.35 3021  51  14 -23 0.00
Sat 01/24 21Z 0.43 2919  56  10 -24 0.00
Sat 01/24 22Z 0.42 2920  53  10 -23 0.00
Sat 01/24 23Z 0.38 3021  50  10 -22 0.00
Sun 01/25 00Z 0.35 3021  48  11 -21 0.00
Sun 01/25 01Z 0.31 3022  45  10 -21 0.00
Sun 01/25 02Z 0.28 3021  43  11 -20 0.00
Sun 01/25 03Z 0.24 3120  39   8 -20 0.00
Sun 01/25 04Z 0.21 3118  36   7 -19 0.00
Sun 01/25 05Z 0.19 3117  33   6 -18 0.00
Sun 01/25 06Z 0.17 3115  30   6 -18 0.00
Sun 01/25 07Z 0.15 3114  30   6 -18 0.00
Sun 01/25 08Z 0.14 3112  31   9 -18 0.00
Sun 01/25 09Z 0.13 3010  32  11 -18 0.00
Sun 01/25 10Z 0.13 3009  31  17 -17 0.00
Sun 01/25 11Z 0.11 2907  29  28 -17 0.00
Sun 01/25 12Z 0.10 2806  27  41 -16 0.00
Sun 01/25 13Z 0.08 2604  25  57 -16 0.00
Sun 01/25 14Z 0.04 2102  24  75 -15 0.00
Sun 01/25 15Z 0.03 1904  26  86 -15 0.00
Sun 01/25 16Z 0.02 1706  30  88 -16 0.00
Sun 01/25 17Z 0.02 1707  45  87 -17 0.00
Sun 01/25 18Z 0.03 1608  73  88 -17 0.00
Sun 01/25 19Z 0.07 1409  84  88 -15 0.00
Sun 01/25 20Z 0.10 1411  85  87 -15 0.01
Sun 01/25 21Z 0.15 1416  85  89 -15 0.02
Sun 01/25 22Z 0.19 1319  85  91 -14 0.03
Sun 01/25 23Z 0.18 1422  85  94 -14 0.03
Mon 01/26 00Z 0.19 1425  85  94 -14 0.04
Mon 01/26 01Z 0.31 1329  86  95 -14 0.03
Mon 01/26 02Z 0.39 1332  86  94 -14 0.03
Mon 01/26 03Z 0.42 1233  86  92 -13 0.04
Mon 01/26 04Z 0.38 1231  86  92 -11 0.04
Mon 01/26 05Z 0.40 1231  86  92 -11 0.04

Run total areal avg QPF for the Central_Greens is: 0.36 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