NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20251006_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 10/06 18Z 0.87 2209  53   9  13 0.00
Mon 10/06 19Z 0.62 2210  52   8  14 0.00
Mon 10/06 20Z 0.53 2211  50   8  14 0.00
Mon 10/06 21Z 0.49 2213  50   7  14 0.00
Mon 10/06 22Z 0.59 2315  55   7  14 0.00
Mon 10/06 23Z 0.75 2317  61   7  13 0.00
Tue 10/07 00Z 0.92 2419  66   6  13 0.00
Tue 10/07 01Z 0.98 2421  66   6  13 0.00
Tue 10/07 02Z 1.11 2423  69   7  12 0.00
Tue 10/07 03Z 1.25 2423  71   8  12 0.00
Tue 10/07 04Z 1.28 2525  72  10  12 0.00
Tue 10/07 05Z 1.32 2525  74  11  12 0.00
Tue 10/07 06Z 1.37 2525  76  13  12 0.00
Tue 10/07 07Z 1.27 2624  75  15  12 0.00
Tue 10/07 08Z 1.07 2622  74  17  12 0.00
Tue 10/07 09Z 0.99 2620  74  16  12 0.00
Tue 10/07 10Z 1.03 2619  74  15  12 0.00
Tue 10/07 11Z 0.93 2517  75  13  12 0.00
Tue 10/07 12Z 0.85 2517  77  15  12 0.00
Tue 10/07 13Z 0.81 2317  77  14  12 0.00
Tue 10/07 14Z 0.64 2217  76  16  12 0.00
Tue 10/07 15Z 0.61 2219  79  29  12 0.00
Tue 10/07 16Z 1.27 2120  82  53  12 0.00
Tue 10/07 17Z 2.41 2121  82  66  12 0.00
Tue 10/07 18Z 2.68 2123  79  84  13 0.00
Tue 10/07 19Z 1.81 2124  79  84  13 0.00
Tue 10/07 20Z 1.60 2125  84  90  13 0.01
Tue 10/07 21Z 0.79 2227  90  89  13 0.11
Tue 10/07 22Z 0.84 2229  92  92  12 0.06
Tue 10/07 23Z 1.00 2330  94  91  11 0.17
Wed 10/08 00Z 0.78 2328  92  86  12 0.06
Wed 10/08 01Z 0.79 2330  94  90  12 0.03
Wed 10/08 02Z 0.74 2229  95  90  12 0.02
Wed 10/08 03Z 0.70 2230  96  92  11 0.03
Wed 10/08 04Z 0.64 2131  96  91  12 0.08
Wed 10/08 05Z 0.64 2232  97  95  12 0.12
Wed 10/08 06Z 0.68 2232  98  94  12 0.14
Wed 10/08 07Z 0.60 2423  98  95  11 0.19
Wed 10/08 08Z 0.48 2616  98  95  10 0.17
Wed 10/08 09Z 0.38 2814  98  96   9 0.12
Wed 10/08 10Z 0.27 3113  97  95   8 0.11
Wed 10/08 11Z 0.20 3215  93  97   7 0.07
Wed 10/08 12Z 0.18 3316  93  97   6 0.05
Wed 10/08 13Z 0.25 3318  94  97   5 0.04
Wed 10/08 14Z 0.24 3319  87  87   5 0.04
Wed 10/08 15Z 0.25 3319  82  78   4 0.03
Wed 10/08 16Z 0.31 3320  79  58   4 0.02
Wed 10/08 17Z 0.36 3322  78  51   3 0.01
Wed 10/08 18Z 0.43 3223  79  24   2 0.01
Wed 10/08 19Z 0.46 3223  82   5   2 0.01
Wed 10/08 20Z 0.52 3224  89   1   1 0.01
Wed 10/08 21Z 0.55 3225  91   2   0 0.02
Wed 10/08 22Z 0.50 3226  92   2   0 0.01
Wed 10/08 23Z 0.46 3327  92   3   0 0.01
Thu 10/09 00Z 0.42 3327  90   5  -1 0.01
Thu 10/09 01Z 0.36 3327  89   5  -1 0.02
Thu 10/09 02Z 0.32 3327  88   5  -2 0.01
Thu 10/09 03Z 0.29 3326  87   4  -2 0.01
Thu 10/09 04Z 0.28 3326  87   4  -3 0.01
Thu 10/09 05Z 0.25 3326  88   7  -3 0.01

Run total areal avg QPF for the greens is: 1.82 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