NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260421_0000
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 00Z 1.82 3111  73  41  -9 0.00
Tue 04/21 01Z 2.58 3113  67  31  -9 0.00
Tue 04/21 02Z 2.58 3115  60  20  -9 0.00
Tue 04/21 03Z 2.35 3114  53  10 -10 0.00
Tue 04/21 04Z 1.86 3114  50   6 -10 0.00
Tue 04/21 05Z 1.52 3213  48   5 -10 0.00
Tue 04/21 06Z 1.06 3212  46   6 -10 0.00
Tue 04/21 07Z 0.70 3211  45   7 -10 0.00
Tue 04/21 08Z 0.56 3109  45   8 -10 0.00
Tue 04/21 09Z 0.59 3009  45   9 -10 0.00
Tue 04/21 10Z 0.57 2908  45  11 -10 0.00
Tue 04/21 11Z 0.45 2806  45  10 -10 0.00
Tue 04/21 12Z 0.50 2605  46   9 -10 0.00
Tue 04/21 13Z 1.61 2404  47  10 -10 0.00
Tue 04/21 14Z 3.06 2405  46  10  -9 0.00
Tue 04/21 15Z 3.49 2405  46  11  -9 0.00
Tue 04/21 16Z 3.51 2306  45  12  -8 0.00
Tue 04/21 17Z 3.65 2306  43  20  -7 0.00
Tue 04/21 18Z 3.84 2307  42  42  -6 0.00
Tue 04/21 19Z 3.73 2307  42  66  -6 0.00
Tue 04/21 20Z 3.14 2207  41  70  -5 0.00
Tue 04/21 21Z 2.06 2207  41  79  -5 0.00
Tue 04/21 22Z 0.92 2107  40  72  -4 0.00
Tue 04/21 23Z 0.31 2009  39  72  -4 0.00
Wed 04/22 00Z 0.31 2110  38  67  -4 0.00
Wed 04/22 01Z 0.36 2112  39  73  -4 0.00
Wed 04/22 02Z 0.39 2114  40  79  -3 0.00
Wed 04/22 03Z 0.40 2117  42  84  -3 0.00
Wed 04/22 04Z 0.43 2119  44  90  -2 0.00
Wed 04/22 05Z 0.49 2221  44  91  -2 0.00
Wed 04/22 06Z 0.53 2222  44  89  -2 0.00
Wed 04/22 07Z 0.45 2122  52  88  -2 0.00
Wed 04/22 08Z 0.29 2118  53  94  -1 0.00
Wed 04/22 09Z 0.22 2116  61  95   0 0.00
Wed 04/22 10Z 0.21 2116  77  91   0 0.00
Wed 04/22 11Z 0.20 2017  87  93   0 0.00
Wed 04/22 12Z 0.16 2017  91  89   0 0.00
Wed 04/22 13Z 0.13 1916  94  83   0 0.00
Wed 04/22 14Z 0.14 1916  95  74  -1 0.00
Wed 04/22 15Z 0.16 2016  96  72  -1 0.01
Wed 04/22 16Z 0.17 2015  98  62  -1 0.01
Wed 04/22 17Z 0.19 2015  98  50  -1 0.01
Wed 04/22 18Z 0.21 2113  98  30  -1 0.01
Wed 04/22 19Z 0.19 2111  98  15  -1 0.01
Wed 04/22 20Z 0.22 2210  98   7  -1 0.00
Wed 04/22 21Z 0.22 2308  98   4  -1 0.01
Wed 04/22 22Z 0.18 2406  98   6   0 0.00
Wed 04/22 23Z 0.16 2605  98   6   0 0.00
Thu 04/23 00Z 0.17 2705  98   8   0 0.00
Thu 04/23 01Z 0.16 2906  98  10  -1 0.00
Thu 04/23 02Z 0.16 3006  98   9  -1 0.00
Thu 04/23 03Z 0.15 3106  98  10  -1 0.00
Thu 04/23 04Z 0.16 3106  98  12  -2 0.00
Thu 04/23 05Z 0.24 3008  97  14  -2 0.00
Thu 04/23 06Z 0.25 3009  95  30  -2 0.00
Thu 04/23 07Z 0.27 3010  95  38  -2 0.00
Thu 04/23 08Z 0.35 3012  95  56  -2 0.01
Thu 04/23 09Z 0.41 3014  94  79  -1 0.01
Thu 04/23 10Z 0.46 2916  94  93  -1 0.02
Thu 04/23 11Z 0.62 3022  96  93  -1 0.06

Run total areal avg QPF for the greens is: 0.16 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