Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260322_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)
=== ===== === ==== ==== === === === ====
Sun 03/22 18Z 0.32 2119  99  98   0 0.00
Sun 03/22 19Z 0.41 2219  99  94   0 0.07
Sun 03/22 20Z 0.46 2419  99  87   0 0.06
Sun 03/22 21Z 0.55 2619  99  69   0 0.06
Sun 03/22 22Z 0.52 2917  99  52  -1 0.03
Sun 03/22 23Z 0.41 3018  98  35  -2 0.01
Mon 03/23 00Z 0.19 3219  94  32  -2 0.01
Mon 03/23 01Z 0.11 3318  89  25  -3 0.00
Mon 03/23 02Z 0.04 3415  85  21  -3 0.00
Mon 03/23 03Z 0.06 3513  86  16  -4 0.00
Mon 03/23 04Z 0.09 0012  86  14  -4 0.00
Mon 03/23 05Z 0.15 0112  88  10  -5 0.00
Mon 03/23 06Z 0.15 0112  87   7  -5 0.00
Mon 03/23 07Z 0.14 0211  87   7  -5 0.00
Mon 03/23 08Z 0.17 0211  86   7  -5 0.00
Mon 03/23 09Z 0.22 0312  89   8  -6 0.00
Mon 03/23 10Z 0.23 0311  92   8  -7 0.00
Mon 03/23 11Z 0.25 0410  93  10  -7 0.00
Mon 03/23 12Z 0.27 0509  95  11  -8 0.00
Mon 03/23 13Z 0.28 0509  96  15  -8 0.01
Mon 03/23 14Z 0.25 0407  96  11  -9 0.00
Mon 03/23 15Z 0.19 0405  96  10  -9 0.00
Mon 03/23 16Z 0.19 0304  95  14  -9 0.00
Mon 03/23 17Z 0.15 0203  96  10  -9 0.00
Mon 03/23 18Z 0.07 3503  97  10  -9 0.00
Mon 03/23 19Z 0.07 3306  97  11  -8 0.00
Mon 03/23 20Z 0.09 3309  96  14  -9 0.00
Mon 03/23 21Z 0.12 3313  96  21  -9 0.01
Mon 03/23 22Z 0.12 3416  96  31  -9 0.01
Mon 03/23 23Z 0.10 3419  95  37  -9 0.01
Tue 03/24 00Z 0.08 3421  94  48  -9 0.01
Tue 03/24 01Z 0.10 3423  92  55 -10 0.01
Tue 03/24 02Z 0.10 3425  85  59 -10 0.00
Tue 03/24 03Z 0.08 3428  76  65 -10 0.00
Tue 03/24 04Z 0.07 3428  72  66 -11 0.00
Tue 03/24 05Z 0.08 3427  76  55 -11 0.00
Tue 03/24 06Z 0.08 3425  80  38 -11 0.00
Tue 03/24 07Z 0.10 3423  82  23 -11 0.00
Tue 03/24 08Z 0.11 3421  81  10 -12 0.00
Tue 03/24 09Z 0.08 3419  79   3 -12 0.00
Tue 03/24 10Z 0.11 3417  78   5 -12 0.00
Tue 03/24 11Z 0.10 3416  75   9 -12 0.00
Tue 03/24 12Z 0.14 3315  74   9 -12 0.00
Tue 03/24 13Z 0.20 3314  73  11 -12 0.00
Tue 03/24 14Z 0.22 3211  67  10 -11 0.00
Tue 03/24 15Z 0.28 3009  62   7 -11 0.00
Tue 03/24 16Z 0.38 2809  54   5 -10 0.00
Tue 03/24 17Z 0.47 2609  50   4  -9 0.00
Tue 03/24 18Z 0.43 2409  42  11  -8 0.00
Tue 03/24 19Z 0.45 2411  36  15  -7 0.00
Tue 03/24 20Z 0.53 2314  33  12  -6 0.00
Tue 03/24 21Z 0.83 2418  40   9  -7 0.00
Tue 03/24 22Z 0.96 2422  42   8  -7 0.00
Tue 03/24 23Z 1.08 2426  43  12  -6 0.00
Wed 03/25 00Z 1.22 2531  36  18  -6 0.00
Wed 03/25 01Z 1.32 2534  32  15  -6 0.00
Wed 03/25 02Z 1.68 2635  47  88  -7 0.00
Wed 03/25 03Z 1.90 2633  66  85  -8 0.00
Wed 03/25 04Z 1.46 2731  73  77  -8 0.00
Wed 03/25 05Z 1.04 2828  85  38  -8 0.00

Run total areal avg QPF for the Central_Greens is: 0.30 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