NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260901_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)
=== ===== === ==== ==== === === === ====
Tue 09/01 18Z 0.22 1504  93  68  11 0.00
Tue 09/01 19Z 0.34 1305  92  66  11 0.00
Wed 09/02 00Z 0.29 1415  92  37  11 0.01
Wed 09/02 01Z 0.28 1416  90  48  11 0.01
Wed 09/02 02Z 0.30 1417  91  58  11 0.00
Wed 09/02 03Z 0.25 1518  89  67  11 0.00
Wed 09/02 04Z 0.23 1519  89  70  11 0.01
Wed 09/02 05Z 0.19 1518  89  70  11 0.00
Wed 09/02 06Z 0.15 1617  88  69  11 0.00
Wed 09/02 07Z 0.16 1517  87  63  11 0.00
Wed 09/02 08Z 0.15 1516  86  57  11 0.00
Wed 09/02 09Z 0.14 1617  84  57  11 0.00
Wed 09/02 10Z 0.12 1618  83  57  11 0.00
Wed 09/02 11Z 0.08 1718  84  58  11 0.00
Wed 09/02 12Z 0.07 1718  83  56  11 0.00
Wed 09/02 13Z 0.08 1617  81  46  11 0.00
Wed 09/02 14Z 0.06 1717  80  46  11 0.00
Wed 09/02 15Z 0.07 1718  81  48  11 0.00
Wed 09/02 16Z 0.11 1618  82  47  11 0.00
Wed 09/02 17Z 0.14 1618  80  41  11 0.00
Wed 09/02 18Z 0.17 1618  76  32  11 0.00
Wed 09/02 19Z 0.18 1619  75  26  12 0.00
Wed 09/02 20Z 0.18 1619  77  33  12 0.00
Wed 09/02 21Z 0.21 1620  83  45  12 0.00
Wed 09/02 22Z 0.18 1722  84  52  12 0.00
Wed 09/02 23Z 0.11 1723  83  62  12 0.00
Thu 09/03 00Z 0.07 1724  81  68  12 0.00
Thu 09/03 01Z 0.06 1725  83  77  13 0.00
Thu 09/03 02Z 0.06 1826  87  70  13 0.00
Thu 09/03 03Z 0.11 1828  90  68  13 0.00
Thu 09/03 04Z 0.18 1928  93  61  13 0.00
Thu 09/03 05Z 0.17 1927  94  69  14 0.00
Thu 09/03 06Z 0.20 1928  96  79  14 0.00
Thu 09/03 07Z 0.14 1926  95  83  14 0.00
Thu 09/03 08Z 0.22 1931  97  92  13 0.00
Thu 09/03 09Z 0.21 1932  97  96  13 0.01
Thu 09/03 10Z 0.19 1931  95  98  13 0.01
Thu 09/03 11Z 0.23 1932  98  95  14 0.08
Thu 09/03 12Z 0.29 2034  99  98  13 0.08
Thu 09/03 13Z 0.50 2133  99  92  13 0.20
Thu 09/03 14Z 0.59 2330  99  86  13 0.20
Thu 09/03 15Z 0.57 2423  99  84  13 0.21
Thu 09/03 16Z 0.54 2519  99  73  14 0.37
Thu 09/03 17Z 0.51 2616  98  76  14 0.24
Thu 09/03 18Z 0.52 2715  98  80  13 0.12
Thu 09/03 19Z 0.48 2813  98  70  13 0.08
Thu 09/03 20Z 0.47 2813  97  69  13 0.05
Thu 09/03 21Z 0.50 3016  93  57  13 0.02
Thu 09/03 22Z 0.45 3116  93  46  13 0.02
Thu 09/03 23Z 0.38 3216  90  32  12 0.02
Fri 09/04 00Z 0.35 3215  88  17  12 0.06
Fri 09/04 01Z 0.38 3115  89  25  12 0.08
Fri 09/04 02Z 0.42 3015  89  44  12 0.01
Fri 09/04 03Z 0.46 3017  91  45  12 0.01
Fri 09/04 04Z 0.42 3016  92  39  12 0.00
Fri 09/04 05Z 0.39 3116  92  26  12 0.00

Run total areal avg QPF for the greens is: 1.90 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