Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260320_1200
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 03/20 12Z 0.37 2712  80  21  -5 0.00
Fri 03/20 13Z 0.25 2509  72  22  -4 0.00
Fri 03/20 14Z 0.28 2310  74   8  -4 0.00
Fri 03/20 15Z 0.28 2112  71  39  -4 0.00
Fri 03/20 16Z 0.30 2015  70  86  -3 0.00
Fri 03/20 17Z 0.14 1818  67  94  -2 0.00
Fri 03/20 18Z 0.11 1820  77  89  -1 0.00
Fri 03/20 19Z 0.16 1820  88  96  -1 0.04
Fri 03/20 20Z 0.21 1921  98  97  -1 0.17
Fri 03/20 21Z 0.40 2024  99  95   0 0.17
Fri 03/20 22Z 0.55 2227  99  94   0 0.09
Fri 03/20 23Z 0.61 2326  99  95   1 0.07
Sat 03/21 00Z 0.60 2424  99  84   1 0.08
Sat 03/21 01Z 0.55 2521  99  31   0 0.03
Sat 03/21 02Z 0.50 2519  98   7   0 0.02
Sat 03/21 03Z 0.49 2518  98   6   0 0.02
Sat 03/21 04Z 0.40 2716  98   8   0 0.02
Sat 03/21 05Z 0.27 2913  99   8   0 0.01
Sat 03/21 06Z 0.17 3113  99   7  -1 0.01
Sat 03/21 07Z 0.11 3213  98   6  -2 0.01
Sat 03/21 08Z 0.08 3315  97   8  -2 0.01
Sat 03/21 09Z 0.07 3316  96  36  -3 0.00
Sat 03/21 10Z 0.09 3318  95  53  -4 0.00
Sat 03/21 11Z 0.09 3320  94  68  -5 0.00
Sat 03/21 12Z 0.09 3421  91  55  -6 0.00
Sat 03/21 13Z 0.08 3421  86  50  -6 0.00
Sat 03/21 14Z 0.09 3421  82  29  -6 0.00
Sat 03/21 15Z 0.10 3320  80  21  -6 0.00
Sat 03/21 16Z 0.12 3319  78  18  -6 0.00
Sat 03/21 17Z 0.19 3319  78  18  -7 0.00
Sat 03/21 18Z 0.28 3218  75  20  -7 0.00
Sat 03/21 19Z 0.31 3217  70  20  -7 0.00
Sat 03/21 20Z 0.26 3316  67  18  -6 0.00
Sat 03/21 21Z 0.23 3214  65  14  -6 0.00
Sat 03/21 22Z 0.18 3211  62  18  -6 0.00
Sat 03/21 23Z 0.16 3109  62  35  -6 0.00
Sun 03/22 00Z 0.22 3010  67  63  -6 0.00
Sun 03/22 01Z 0.22 2908  71  89  -5 0.00
Sun 03/22 02Z 0.19 2706  72  93  -5 0.00
Sun 03/22 03Z 0.24 2507  76  95  -5 0.00
Sun 03/22 04Z 0.35 2409  86  94  -6 0.00
Sun 03/22 05Z 0.31 2310  89  92  -6 0.01
Sun 03/22 06Z 0.36 2213  90  92  -5 0.01
Sun 03/22 07Z 0.40 2215  88  92  -5 0.01
Sun 03/22 08Z 0.51 2218  94  91  -6 0.02
Sun 03/22 09Z 0.44 2120  97  93  -4 0.04
Sun 03/22 10Z 0.43 2121  98  92  -3 0.06
Sun 03/22 11Z 0.40 2021  98  93  -3 0.08
Sun 03/22 12Z 0.34 2022  98  95  -3 0.10
Sun 03/22 13Z 0.31 2022  98  98  -1 0.07
Sun 03/22 14Z 0.28 2021  98  99   0 0.05
Sun 03/22 15Z 0.27 2019  99  99   0 0.09
Sun 03/22 16Z 0.30 2117  99  99   0 0.11
Sun 03/22 17Z 0.27 2214  99  98   0 0.09
Sun 03/22 18Z 0.26 2212  99  98   0 0.08
Sun 03/22 19Z 0.27 2311  99  97   0 0.09
Sun 03/22 20Z 0.26 2410  99  97   0 0.07
Sun 03/22 21Z 0.28 2510  99  94   0 0.07
Sun 03/22 22Z 0.27 2710  99  72   0 0.05
Sun 03/22 23Z 0.22 2910  99  52  -1 0.03

Run total areal avg QPF for the Central_Greens is: 1.88 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