NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260510_0600
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)
=== ===== === ==== ==== === === === ====
Sun 05/10 06Z 0.32 2024  90  47   4 0.00
Sun 05/10 07Z 0.35 2124  91  60   5 0.00
Sun 05/10 08Z 0.49 2128  98  69   4 0.04
Sun 05/10 09Z 0.59 2325  97  92   4 0.03
Sun 05/10 10Z 0.62 2323  95  67   4 0.01
Sun 05/10 11Z 0.58 2420  94  51   4 0.01
Sun 05/10 12Z 0.48 2417  92  49   4 0.01
Sun 05/10 13Z 0.44 2316  93  63   4 0.00
Sun 05/10 14Z 0.48 2316  94  75   4 0.00
Sun 05/10 15Z 0.46 2315  93  81   4 0.01
Sun 05/10 16Z 0.50 2313  96  89   4 0.02
Sun 05/10 17Z 0.68 2514  91  91   3 0.04
Sun 05/10 18Z 0.53 2717  89  86   3 0.05
Sun 05/10 19Z 0.63 2720  77  88   3 0.03
Sun 05/10 20Z 0.87 2719  66  47   3 0.00
Sun 05/10 21Z 1.58 2818  67  41   2 0.00
Sun 05/10 22Z 1.35 2816  73  45   2 0.00
Sun 05/10 23Z 0.96 2817  76  39   1 0.00
Mon 05/11 00Z 1.09 2919  76  22   1 0.00
Mon 05/11 01Z 1.27 3021  71  25   0 0.00
Mon 05/11 02Z 1.38 3021  73  30   0 0.00
Mon 05/11 03Z 1.15 3020  77  30  -1 0.00
Mon 05/11 04Z 1.01 3019  76  27  -1 0.00
Mon 05/11 05Z 0.74 3118  79  24  -2 0.00
Mon 05/11 06Z 0.64 3117  81  22  -2 0.00
Mon 05/11 07Z 0.60 3116  80  22  -2 0.00
Mon 05/11 08Z 0.54 3115  82  22  -3 0.00
Mon 05/11 09Z 0.47 3113  82  19  -3 0.00
Mon 05/11 10Z 0.44 3111  83  20  -3 0.00
Mon 05/11 11Z 0.40 3010  85  27  -3 0.00
Mon 05/11 12Z 0.45 2909  85  30  -3 0.00
Mon 05/11 13Z 0.99 2808  87  33  -4 0.00
Mon 05/11 14Z 2.03 2708  86  37  -3 0.00
Mon 05/11 15Z 3.29 2708  78  39  -3 0.00
Mon 05/11 16Z 4.85 2709  77  43  -2 0.00
Mon 05/11 17Z 4.68 2710  79  49  -2 0.00
Mon 05/11 18Z 5.32 2711  75  63  -2 0.00
Mon 05/11 19Z 5.14 2811  72  71  -2 0.00
Mon 05/11 20Z 5.48 2811  71  72  -2 0.00
Mon 05/11 21Z 5.40 2911  69  73  -2 0.00
Mon 05/11 22Z 4.68 2911  69  69  -2 0.00
Mon 05/11 23Z 2.70 3012  71  66  -2 0.00
Tue 05/12 00Z 1.06 3013  76  67  -2 0.00
Tue 05/12 01Z 0.65 3113  80  66  -3 0.00
Tue 05/12 02Z 0.50 3115  83  65  -3 0.00
Tue 05/12 03Z 0.54 3217  84  68  -3 0.00
Tue 05/12 04Z 0.47 3218  81  61  -3 0.00
Tue 05/12 05Z 0.41 3319  80  59  -3 0.00
Tue 05/12 06Z 0.32 3319  73  52  -3 0.00
Tue 05/12 07Z 0.28 3418  70  41  -4 0.00
Tue 05/12 08Z 0.25 3417  68  24  -4 0.00
Tue 05/12 09Z 0.24 3416  67  21  -4 0.00
Tue 05/12 10Z 0.29 3315  67  29  -4 0.00
Tue 05/12 11Z 0.35 3315  68  40  -5 0.00
Tue 05/12 12Z 0.50 3214  70  43  -5 0.00
Tue 05/12 13Z 2.49 3113  73  47  -5 0.00
Tue 05/12 14Z 5.31 3112  74  44  -5 0.00
Tue 05/12 15Z 6.21 3012  72  37  -5 0.00
Tue 05/12 16Z 6.63 3011  72  36  -4 0.00
Tue 05/12 17Z 7.02 2911  72  42  -4 0.00

Run total areal avg QPF for the greens is: 0.25 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