NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260421_1200
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)
=== ===== === ==== ==== === === === ====
Tue 04/21 12Z 0.28 2506  75   9  -9 0.00
Tue 04/21 13Z 0.62 2506  75   8  -9 0.00
Tue 04/21 14Z 2.09 2406  70   8  -9 0.00
Tue 04/21 15Z 4.68 2408  63   8  -8 0.00
Tue 04/21 16Z 4.91 2407  58  14  -8 0.00
Tue 04/21 17Z 4.65 2307  53  30  -7 0.00
Tue 04/21 18Z 3.99 2307  50  66  -6 0.00
Tue 04/21 19Z 3.66 2208  48  84  -6 0.00
Tue 04/21 20Z 3.60 2208  46  67  -5 0.00
Tue 04/21 21Z 2.35 2207  44  60  -5 0.00
Tue 04/21 22Z 0.90 2108  42  64  -4 0.00
Tue 04/21 23Z 0.34 2109  41  66  -4 0.00
Wed 04/22 00Z 0.33 2111  41  77  -3 0.00
Wed 04/22 01Z 0.35 2112  42  88  -3 0.00
Wed 04/22 02Z 0.33 2113  46  89  -3 0.00
Wed 04/22 03Z 0.44 2116  49  89  -3 0.00
Wed 04/22 04Z 0.55 2120  52  87  -3 0.00
Wed 04/22 05Z 0.57 2222  56  85  -2 0.00
Wed 04/22 06Z 0.51 2222  61  89  -2 0.00
Wed 04/22 07Z 0.44 2220  69  91  -1 0.00
Wed 04/22 08Z 0.34 2218  74  96  -1 0.00
Wed 04/22 09Z 0.30 2217  86  98  -1 0.00
Wed 04/22 10Z 0.28 2117  93  99  -1 0.00
Wed 04/22 11Z 0.21 2017  95  99  -1 0.01
Wed 04/22 12Z 0.12 1917  96  97   0 0.01
Wed 04/22 13Z 0.11 1917  97  91   0 0.01
Wed 04/22 14Z 0.11 1917  97  83   0 0.01
Wed 04/22 15Z 0.15 2017  97  66  -1 0.01
Wed 04/22 16Z 0.17 2016  97  56  -1 0.01
Wed 04/22 17Z 0.20 2014  98  47  -1 0.01
Wed 04/22 18Z 0.19 2013  98  31  -1 0.01
Wed 04/22 19Z 0.21 2112  98  24  -1 0.01
Wed 04/22 20Z 0.22 2210  98  17   0 0.01
Wed 04/22 21Z 0.19 2308  98  15   0 0.01
Wed 04/22 22Z 0.18 2507  98   7   0 0.01
Wed 04/22 23Z 0.20 2707  98   8   0 0.01
Thu 04/23 00Z 0.21 3008  98  10   0 0.01
Thu 04/23 01Z 0.21 3109  98  12  -1 0.00
Thu 04/23 02Z 0.20 3210  97  16  -1 0.00
Thu 04/23 03Z 0.17 3311  96  20  -1 0.00
Thu 04/23 04Z 0.20 3211  96  28  -1 0.00
Thu 04/23 05Z 0.25 3211  95  37  -1 0.00
Thu 04/23 06Z 0.30 3112  94  48  -1 0.01
Thu 04/23 07Z 0.41 3014  95  68  -1 0.01
Thu 04/23 08Z 0.52 3017  95  74  -1 0.01
Thu 04/23 09Z 0.58 3019  94  77  -1 0.02
Thu 04/23 10Z 0.63 3022  94  80  -1 0.03
Thu 04/23 11Z 0.61 3123  95  83  -1 0.03
Thu 04/23 12Z 0.55 3223  97  80  -2 0.02
Thu 04/23 13Z 0.51 3224  96  48  -3 0.02
Thu 04/23 14Z 0.51 3322  93  34  -4 0.00
Thu 04/23 15Z 0.72 3321  92  32  -5 0.00
Thu 04/23 16Z 3.38 3321  88  35  -5 0.00
Thu 04/23 17Z 4.47 3321  85  38  -5 0.00
Thu 04/23 18Z 4.40 3321  84  40  -5 0.00
Thu 04/23 19Z 4.48 3220  85  41  -5 0.00
Thu 04/23 20Z 5.20 3220  83  40  -5 0.00
Thu 04/23 21Z 5.03 3321  78  33  -4 0.00
Thu 04/23 22Z 4.51 3321  75  25  -4 0.00
Thu 04/23 23Z 2.43 3323  74  19  -4 0.00

Run total areal avg QPF for the greens is: 0.29 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