Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260121_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)
=== ===== === ==== ==== === === === ====
Wed 01/21 18Z 0.41 2218  76  72 -14 0.00
Wed 01/21 19Z 0.41 2120  68  84 -14 0.00
Wed 01/21 20Z 0.37 2022  77  88 -13 0.00
Wed 01/21 21Z 0.35 1926  79  87 -12 0.00
Wed 01/21 22Z 0.30 1928  88  89 -10 0.01
Wed 01/21 23Z 0.43 2033  91  94 -10 0.03
Thu 01/22 00Z 0.59 2038  85  91 -11 0.01
Thu 01/22 01Z 0.64 2139  81  91 -10 0.00
Thu 01/22 02Z 0.69 2140  89  94  -9 0.00
Thu 01/22 03Z 0.73 2140  92  93  -9 0.01
Thu 01/22 04Z 0.74 2138  88  93  -8 0.00
Thu 01/22 05Z 0.69 2136  87  89  -7 0.00
Thu 01/22 06Z 0.65 2234  90  84  -6 0.01
Thu 01/22 07Z 0.65 2233  95  82  -6 0.01
Thu 01/22 08Z 0.68 2232  96  72  -6 0.01
Thu 01/22 09Z 0.69 2231  96  59  -6 0.01
Thu 01/22 10Z 0.79 2332  96  72  -7 0.01
Thu 01/22 11Z 0.95 2433  95  88  -7 0.01
Thu 01/22 12Z 1.33 2432  93  90  -9 0.02
Thu 01/22 13Z 1.52 2433  93  79 -10 0.01
Thu 01/22 14Z 1.85 2532  94  51 -11 0.01
Thu 01/22 15Z 1.88 2530  93  72 -11 0.00
Thu 01/22 16Z 2.90 2528  92  69 -12 0.00
Thu 01/22 17Z 4.02 2527  90  61 -12 0.00
Thu 01/22 18Z 6.21 2525  84  62 -11 0.00
Thu 01/22 19Z 8.22 2524  83  56 -11 0.00
Thu 01/22 20Z 6.14 2525  85  51 -11 0.00
Thu 01/22 21Z 2.55 2526  86  39 -11 0.00
Thu 01/22 22Z 4.14 2529  86  53 -12 0.01
Thu 01/22 23Z 2.24 2528  83  42 -12 0.01
Fri 01/23 00Z 2.00 2527  87  50 -13 0.00
Fri 01/23 01Z 2.40 2529  82  52 -14 0.01
Fri 01/23 02Z 2.35 2630  80  44 -14 0.01
Fri 01/23 03Z 1.45 2529  82  43 -14 0.00
Fri 01/23 04Z 1.27 2530  82  47 -15 0.01
Fri 01/23 05Z 1.31 2630  79  45 -15 0.01
Fri 01/23 06Z 1.34 2629  81  40 -16 0.01
Fri 01/23 07Z 1.33 2628  86  37 -16 0.00
Fri 01/23 08Z 1.43 2627  85  36 -17 0.00
Fri 01/23 09Z 1.38 2625  86  31 -17 0.00
Fri 01/23 10Z 1.28 2524  89  28 -17 0.00
Fri 01/23 11Z 1.24 2525  92  47 -17 0.00
Fri 01/23 12Z 0.93 2426  83  55 -16 0.00
Fri 01/23 13Z 1.06 2429  86  42 -16 0.00
Fri 01/23 14Z 1.26 2333  91  14 -16 0.00
Fri 01/23 15Z 2.11 2431  89  22 -16 0.00
Fri 01/23 16Z 8.24 2428  91  28 -16 0.00
Fri 01/23 17Z 5.61 2525  89  43 -16 0.01
Fri 01/23 18Z 14.50 2625  79  45 -17 0.01
Fri 01/23 19Z 13.90 2626  74  49 -18 0.01
Fri 01/23 20Z 14.30 2726  72  53 -18 0.01
Fri 01/23 21Z 12.68 2626  72  54 -19 0.00
Fri 01/23 22Z 5.11 2723  77  68 -20 0.01
Fri 01/23 23Z 2.13 2922  82  70 -20 0.01
Sat 01/24 00Z 2.53 2921  77  65 -21 0.01
Sat 01/24 01Z 2.22 3022  72  65 -22 0.01
Sat 01/24 02Z 1.49 3124  62  65 -23 0.01
Sat 01/24 03Z 1.11 3025  60  57 -23 0.00
Sat 01/24 04Z 1.23 3025  73  57 -24 0.00
Sat 01/24 05Z 3.21 3026  82  40 -25 0.00

Run total areal avg QPF for the Central_Greens is: 0.32 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