Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260121_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)
=== ===== === ==== ==== === === === ====
Wed 01/21 12Z 0.58 2624  73   2 -17 0.00
Wed 01/21 13Z 0.57 2524  68   2 -16 0.00
Wed 01/21 14Z 0.55 2524  67  24 -16 0.00
Wed 01/21 15Z 0.49 2421  65  56 -15 0.00
Wed 01/21 16Z 0.48 2421  61  70 -15 0.00
Wed 01/21 17Z 0.48 2321  55  82 -14 0.00
Wed 01/21 18Z 0.46 2222  49  87 -14 0.00
Wed 01/21 19Z 0.49 2123  49  87 -14 0.00
Wed 01/21 20Z 0.46 2124  62  88 -13 0.00
Wed 01/21 21Z 0.38 2027  66  88 -11 0.00
Wed 01/21 22Z 0.35 1931  86  89 -11 0.00
Wed 01/21 23Z 0.43 2036  89  90 -10 0.03
Thu 01/22 00Z 0.61 2040  88  93 -11 0.02
Thu 01/22 01Z 0.71 2141  87  87 -11 0.00
Thu 01/22 02Z 0.73 2142  92  91 -10 0.01
Thu 01/22 03Z 0.73 2140  96  89  -9 0.01
Thu 01/22 04Z 0.72 2137  96  92  -8 0.01
Thu 01/22 05Z 0.67 2134  96  91  -7 0.01
Thu 01/22 06Z 0.71 2234  97  88  -7 0.02
Thu 01/22 07Z 0.74 2233  96  93  -7 0.01
Thu 01/22 08Z 0.77 2332  96  70  -7 0.01
Thu 01/22 09Z 0.81 2332  96  50  -7 0.01
Thu 01/22 10Z 0.82 2331  95  59  -7 0.01
Thu 01/22 11Z 0.84 2331  95  75  -7 0.01
Thu 01/22 12Z 0.95 2333  94  81  -8 0.01
Thu 01/22 13Z 1.15 2434  95  86  -9 0.01
Thu 01/22 14Z 1.60 2434  91  76 -10 0.01
Thu 01/22 15Z 2.00 2532  93  49 -11 0.00
Thu 01/22 16Z 2.85 2528  92  49 -11 0.00
Thu 01/22 17Z 9.79 2525  87  46 -12 0.00
Thu 01/22 18Z 13.34 2523  83  41 -12 0.00
Thu 01/22 19Z 10.94 2423  83  45 -11 0.00
Thu 01/22 20Z 3.08 2424  88  37 -11 0.00
Thu 01/22 21Z 2.06 2427  92  53 -11 0.01
Thu 01/22 22Z 4.86 2531  81  53 -12 0.01
Thu 01/22 23Z 2.27 2631  84  41 -12 0.01
Fri 01/23 00Z 1.70 2630  81  38 -13 0.01
Fri 01/23 01Z 1.90 2631  84  37 -14 0.00
Fri 01/23 02Z 1.76 2631  82  41 -15 0.00
Fri 01/23 03Z 1.53 2532  84  45 -15 0.00
Fri 01/23 04Z 1.43 2631  86  43 -16 0.00
Fri 01/23 05Z 1.28 2629  84  39 -16 0.00
Fri 01/23 06Z 1.23 2629  88  40 -17 0.01
Fri 01/23 07Z 1.41 2628  83  39 -17 0.00
Fri 01/23 08Z 1.17 2626  82  38 -17 0.00
Fri 01/23 09Z 1.26 2525  90  40 -18 0.00
Fri 01/23 10Z 1.39 2526  92  47 -18 0.00
Fri 01/23 11Z 1.09 2526  87  52 -17 0.00
Fri 01/23 12Z 0.93 2428  78  41 -16 0.00
Fri 01/23 13Z 1.10 2333  89  31 -16 0.00
Fri 01/23 14Z 1.76 2436  93  23 -17 0.00
Fri 01/23 15Z 8.49 2534  92  19 -17 0.00
Fri 01/23 16Z 14.25 2530  88  19 -17 0.00
Fri 01/23 17Z 17.08 2627  69  17 -18 0.00
Fri 01/23 18Z 16.31 2625  70  23 -18 0.00
Fri 01/23 19Z 15.02 2624  74  30 -19 0.00
Fri 01/23 20Z 12.06 2623  75  39 -19 0.00
Fri 01/23 21Z 9.48 2723  75  49 -19 0.01
Fri 01/23 22Z 5.57 2723  77  58 -20 0.01
Fri 01/23 23Z 3.22 2825  79  60 -20 0.01

Run total areal avg QPF for the Central_Greens is: 0.27 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