NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260429_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)
=== ===== === ==== ==== === === === ====
Wed 04/29 18Z 1.67 1608  71  72   7 0.00
Wed 04/29 19Z 2.91 1511  72  62   7 0.00
Wed 04/29 20Z 4.03 1515  74  58   7 0.00
Wed 04/29 21Z 3.51 1518  76  54   6 0.00
Wed 04/29 22Z 2.14 1520  79  65   6 0.00
Wed 04/29 23Z 0.49 1522  83  80   6 0.00
Thu 04/30 00Z 0.16 1622  86  90   6 0.00
Thu 04/30 01Z 0.10 1721  88  93   6 0.00
Thu 04/30 02Z 0.10 1623  87  94   6 0.00
Thu 04/30 03Z 0.12 1625  85  92   7 0.00
Thu 04/30 04Z 0.12 1627  84  93   7 0.00
Thu 04/30 05Z 0.07 1727  88  98   5 0.01
Thu 04/30 06Z 0.03 1727  92  98   4 0.06
Thu 04/30 07Z 0.06 1728  94  99   4 0.12
Thu 04/30 08Z 0.06 1728  95  98   4 0.11
Thu 04/30 09Z 0.06 1828  96  97   3 0.08
Thu 04/30 10Z 0.12 1926  96  97   3 0.05
Thu 04/30 11Z 0.16 1921  97  98   3 0.03
Thu 04/30 12Z 0.16 2017  97  97   3 0.02
Thu 04/30 13Z 0.19 2114  97  93   3 0.02
Thu 04/30 14Z 0.22 2112  98  88   3 0.01
Thu 04/30 15Z 0.18 2208  98  87   3 0.02
Thu 04/30 16Z 0.10 2304  98  91   3 0.03
Thu 04/30 17Z 0.05 2102  98  92   3 0.01
Thu 04/30 18Z 0.04 2002  99  92   2 0.02
Thu 04/30 19Z 0.06 2202  99  94   2 0.02
Thu 04/30 20Z 0.08 2602  99  93   2 0.03
Thu 04/30 21Z 0.12 2903  99  89   2 0.02
Thu 04/30 22Z 0.19 3006  99  87   2 0.02
Thu 04/30 23Z 0.26 3109  99  91   2 0.02
Fri 05/01 00Z 0.34 3113  98  96   1 0.02
Fri 05/01 01Z 0.41 3215  96  96   1 0.03
Fri 05/01 02Z 0.44 3117  95  97   0 0.02
Fri 05/01 03Z 0.42 3117  94  96   0 0.02
Fri 05/01 04Z 0.43 3117  94  94   0 0.02
Fri 05/01 05Z 0.43 3117  94  89   0 0.01
Fri 05/01 06Z 0.40 3118  91  87   0 0.01
Fri 05/01 07Z 0.38 3018  88  84   0 0.01
Fri 05/01 08Z 0.40 3019  85  74   0 0.00
Fri 05/01 09Z 0.42 3020  84  73   0 0.00
Fri 05/01 10Z 0.43 3020  82  71   0 0.00
Fri 05/01 11Z 0.42 3020  80  72  -1 0.00
Fri 05/01 12Z 0.44 3020  80  72  -1 0.00
Fri 05/01 13Z 0.47 3119  80  71  -1 0.00
Fri 05/01 14Z 0.55 3118  77  73  -2 0.00
Fri 05/01 15Z 2.12 3116  76  71  -2 0.00
Fri 05/01 16Z 4.66 3114  75  71  -2 0.00
Fri 05/01 17Z 7.12 3012  76  70  -2 0.00
Fri 05/01 18Z 6.89 2911  77  70  -1 0.00
Fri 05/01 19Z 5.63 2912  81  66  -1 0.00
Fri 05/01 20Z 5.06 2912  80  58  -1 0.01
Fri 05/01 21Z 5.08 2911  79  57  -1 0.00
Fri 05/01 22Z 4.37 3011  79  64  -1 0.00
Fri 05/01 23Z 2.75 3012  78  68  -1 0.01
Sat 05/02 00Z 1.65 3013  78  65  -1 0.00
Sat 05/02 01Z 2.08 3013  78  66  -2 0.00
Sat 05/02 02Z 1.56 3013  79  65  -2 0.00
Sat 05/02 03Z 1.21 3013  78  65  -2 0.00
Sat 05/02 04Z 1.31 3013  78  65  -2 0.00
Sat 05/02 05Z 0.87 3014  80  62  -2 0.00

Run total areal avg QPF for the greens is: 0.86 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