Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260316_1800
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)
=== ===== === ==== ==== === === === ====
Mon 03/16 18Z 0.04 1832  92  87   8 0.00
Mon 03/16 19Z 0.06 1835  95  79   9 0.02
Mon 03/16 20Z 0.07 1840  97  79  10 0.05
Mon 03/16 21Z 0.11 1844  96  91  10 0.04
Mon 03/16 22Z 0.09 1846  95  86  10 0.02
Mon 03/16 23Z 0.10 1848  94  85  11 0.01
Tue 03/17 00Z 0.06 1845  95  82  11 0.01
Tue 03/17 01Z 0.04 1741  95  89  10 0.05
Tue 03/17 02Z 0.05 1741  92  80  10 0.03
Tue 03/17 03Z 0.06 1744  95  82  10 0.01
Tue 03/17 07Z 0.97 2530  97  97  -3 0.01
Tue 03/17 08Z 1.22 2532  94  90  -5 0.02
Tue 03/17 10Z 5.36 2433  91   4 -10 0.11
Tue 03/17 11Z 8.85 2535  83  11 -12 0.06
Tue 03/17 12Z 6.32 2437  65  16 -13 0.05
Tue 03/17 14Z 20.49 2433  75  14 -15 0.01
Tue 03/17 15Z 18.96 2431  71  13 -16 0.01
Tue 03/17 16Z 20.28 2531  70  15 -16 0.00
Tue 03/17 17Z 19.53 2529  71  17 -16 0.00
Tue 03/17 18Z 18.12 2428  73  21 -16 0.00
Tue 03/17 19Z 17.93 2528  74  36 -16 0.00
Tue 03/17 20Z 18.29 2528  73  47 -16 0.00
Tue 03/17 21Z 18.45 2628  75  48 -17 0.00
Tue 03/17 22Z 15.51 2627  80  51 -17 0.00
Tue 03/17 23Z 11.09 2726  84  50 -18 0.00
Wed 03/18 00Z 8.60 2726  87  47 -19 0.00
Wed 03/18 01Z 5.84 2724  86  52 -19 0.00
Wed 03/18 02Z 4.11 2724  86  52 -19 0.00
Wed 03/18 03Z 3.33 2823  82  54 -18 0.00
Wed 03/18 04Z 2.77 2722  82  57 -18 0.00
Wed 03/18 05Z 2.36 2722  82  60 -18 0.00
Wed 03/18 06Z 2.23 2722  82  42 -18 0.00
Wed 03/18 07Z 1.89 2723  81  28 -18 0.00
Wed 03/18 08Z 1.78 2723  78  16 -19 0.00
Wed 03/18 09Z 1.51 2823  76  10 -19 0.00
Wed 03/18 10Z 1.10 2822  76   7 -19 0.00
Wed 03/18 11Z 0.80 2920  75   5 -19 0.00
Wed 03/18 12Z 0.60 2917  73   4 -19 0.00
Wed 03/18 13Z 0.50 2915  68   5 -19 0.00
Wed 03/18 14Z 0.48 2812  63   5 -18 0.00
Wed 03/18 15Z 0.46 2710  57   6 -18 0.00
Wed 03/18 16Z 0.63 2608  51   7 -17 0.00
Wed 03/18 17Z 1.07 2506  47   8 -16 0.00
Wed 03/18 18Z 1.19 2505  43   8 -16 0.00
Wed 03/18 19Z 1.06 2405  41   8 -15 0.00
Wed 03/18 20Z 0.68 2204  40   7 -14 0.00
Wed 03/18 21Z 0.45 2106  41   8 -13 0.00
Wed 03/18 22Z 0.18 1907  42   7 -13 0.00
Wed 03/18 23Z 0.11 1911  42   9 -12 0.00
Thu 03/19 00Z 0.13 1914  42  15 -12 0.00
Thu 03/19 01Z 0.18 1916  39  25 -12 0.00
Thu 03/19 02Z 0.27 1919  41  51 -11 0.00
Thu 03/19 03Z 0.36 2022  46  84 -11 0.00
Thu 03/19 04Z 0.51 2023  55  90 -11 0.00
Thu 03/19 05Z 0.54 2123  59  86 -11 0.00

Run total areal avg QPF for the greens is: 0.51 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