NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260513_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)
=== ===== === ==== ==== === === === ====
Wed 05/13 12Z 0.04 1815  68  95   1 0.00
Wed 05/13 13Z 0.04 1817  72  91   2 0.00
Wed 05/13 14Z 0.04 1818  79  95   2 0.01
Wed 05/13 15Z 0.06 1719  85  97   2 0.01
Wed 05/13 16Z 0.07 1720  91  98   3 0.04
Wed 05/13 17Z 0.05 1719  90  84   4 0.06
Wed 05/13 18Z 0.06 1721  91  89   4 0.03
Wed 05/13 19Z 0.04 1723  94  96   4 0.04
Wed 05/13 20Z 0.05 1824  93  97   4 0.05
Wed 05/13 21Z 0.05 1825  95  98   5 0.04
Wed 05/13 22Z 0.09 1826  97  93   5 0.07
Wed 05/13 23Z 0.12 1827  97  80   6 0.04
Thu 05/14 00Z 0.19 1927  96  78   6 0.02
Thu 05/14 01Z 0.20 1926  93  72   6 0.01
Thu 05/14 02Z 0.18 1928  90  69   6 0.01
Thu 05/14 03Z 0.21 1930  83  64   7 0.01
Thu 05/14 04Z 0.25 1931  81  66   6 0.00
Thu 05/14 05Z 0.22 1930  83  85   6 0.01
Thu 05/14 06Z 0.18 1928  88  91   6 0.01
Thu 05/14 07Z 0.12 1828  94  92   5 0.01
Thu 05/14 08Z 0.08 1827  97  92   5 0.01
Thu 05/14 09Z 0.05 1826  97  96   5 0.05
Thu 05/14 10Z 0.05 1825  97  97   5 0.06
Thu 05/14 11Z 0.06 1723  98  94   5 0.05
Thu 05/14 12Z 0.08 1722  98  93   5 0.08
Thu 05/14 13Z 0.13 1621  98  97   5 0.06
Thu 05/14 14Z 0.21 1621  99  98   5 0.10
Thu 05/14 15Z 0.29 1522  97  99   5 0.12
Thu 05/14 16Z 0.32 1520  93  98   5 0.10
Thu 05/14 17Z 0.39 1417  97  98   5 0.11
Thu 05/14 18Z 0.51 1317  98  98   5 0.14
Thu 05/14 19Z 0.67 1219  98  98   5 0.14
Thu 05/14 20Z 0.76 1119  97  98   4 0.14
Thu 05/14 21Z 0.67 1018  96  96   4 0.10
Thu 05/14 22Z 0.61 0917  96  94   4 0.09
Thu 05/14 23Z 0.59 0817  98  96   4 0.08
Fri 05/15 00Z 0.62 0917  99  95   4 0.09
Fri 05/15 01Z 0.61 1018  98  96   4 0.09
Fri 05/15 02Z 0.63 1021  97  92   5 0.11
Fri 05/15 03Z 0.63 1022  97  89   5 0.08
Fri 05/15 04Z 0.76 0925  97  85   4 0.07
Fri 05/15 05Z 0.77 0924  97  92   4 0.07
Fri 05/15 06Z 0.84 0925  97  90   3 0.07
Fri 05/15 07Z 0.90 0926  96  75   3 0.04
Fri 05/15 08Z 0.85 1026  97  69   3 0.02
Fri 05/15 09Z 0.84 1025  97  61   2 0.02
Fri 05/15 10Z 0.75 1022  98  66   2 0.02
Fri 05/15 11Z 0.67 0919  98  71   2 0.04
Fri 05/15 12Z 0.62 0917  99  69   2 0.04
Fri 05/15 13Z 0.61 0816  99  73   2 0.03
Fri 05/15 14Z 0.66 0816  98  71   2 0.02
Fri 05/15 15Z 0.75 0816  98  70   1 0.02
Fri 05/15 16Z 0.81 0916  98  73   2 0.02
Fri 05/15 17Z 0.74 0915  98  80   2 0.01
Fri 05/15 18Z 0.59 1012  97  85   2 0.01
Fri 05/15 19Z 0.46 1009  95  83   3 0.00
Fri 05/15 20Z 0.26 1104  94  83   3 0.00
Fri 05/15 21Z 0.18 1103  93  78   3 0.01
Fri 05/15 22Z 0.09 1901  94  78   3 0.01
Fri 05/15 23Z 0.09 2302  91  75   4 0.02

Run total areal avg QPF for the greens is: 2.81 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