NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260514_0600
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)
=== ===== === ==== ==== === === === ====
Thu 05/14 06Z 0.07 1823  89  87   6 0.00
Thu 05/14 07Z 0.06 1725  91  86   6 0.00
Thu 05/14 08Z 0.06 1727  92  82   6 0.01
Thu 05/14 09Z 0.03 1726  93  87   6 0.01
Thu 05/14 10Z 0.04 1824  96  86   5 0.01
Thu 05/14 11Z 0.04 1822  98  96   5 0.02
Thu 05/14 14Z 0.03 1814  99  98   4 0.05
Thu 05/14 15Z 0.07 1712  99  98   4 0.06
Thu 05/14 16Z 0.14 1512  99  98   5 0.06
Thu 05/14 17Z 0.21 1515  99  98   5 0.05
Thu 05/14 18Z 0.31 1417  98  98   5 0.05
Thu 05/14 19Z 0.34 1416  97  98   5 0.07
Thu 05/14 20Z 0.35 1313  96  98   5 0.10
Thu 05/14 21Z 0.44 1114  99  99   5 0.10
Thu 05/14 22Z 0.48 1114  97  98   5 0.08
Thu 05/14 23Z 0.62 0816  98  97   5 0.11
Fri 05/15 00Z 0.80 0819  98  97   4 0.15
Fri 05/15 01Z 0.86 0822  97  98   4 0.17
Fri 05/15 02Z 0.95 0825  96  98   3 0.18
Fri 05/15 03Z 0.95 0927  96  95   3 0.15
Fri 05/15 04Z 1.00 0930  95  93   3 0.12
Fri 05/15 05Z 0.99 0930  94  91   3 0.11
Fri 05/15 06Z 0.93 0929  94  87   3 0.08
Fri 05/15 07Z 0.86 1027  93  82   3 0.07
Fri 05/15 08Z 0.77 1026  95  83   4 0.07
Fri 05/15 09Z 0.74 1025  94  83   3 0.06
Fri 05/15 10Z 0.71 1023  95  78   3 0.05
Fri 05/15 11Z 0.66 1021  94  76   3 0.04
Fri 05/15 12Z 0.60 1019  95  78   3 0.03
Fri 05/15 13Z 0.55 1017  95  81   3 0.03
Fri 05/15 14Z 0.57 1017  95  93   3 0.03
Fri 05/15 15Z 0.61 1017  96  91   2 0.02
Fri 05/15 16Z 0.60 1116  96  89   2 0.02
Fri 05/15 17Z 0.58 1114  95  87   2 0.02
Fri 05/15 18Z 0.51 1110  95  86   2 0.01
Fri 05/15 19Z 0.38 1107  95  86   2 0.01
Fri 05/15 20Z 0.28 1205  94  88   3 0.00
Fri 05/15 21Z 0.19 1404  94  91   3 0.00
Fri 05/15 22Z 0.15 1603  94  88   3 0.00
Fri 05/15 23Z 0.10 1803  94  84   3 0.01
Sat 05/16 00Z 0.09 1903  93  76   4 0.01
Sat 05/16 01Z 0.14 2204  93  71   4 0.01
Sat 05/16 02Z 0.20 2406  90  66   4 0.00
Sat 05/16 03Z 0.26 2508  86  60   4 0.00
Sat 05/16 04Z 0.33 2510  83  63   4 0.00
Sat 05/16 05Z 0.31 2610  84  64   5 0.00
Sat 05/16 06Z 0.37 2612  86  60   5 0.00
Sat 05/16 07Z 0.46 2615  88  61   4 0.00
Sat 05/16 08Z 0.41 2614  87  57   5 0.00
Sat 05/16 09Z 0.45 2615  92  55   5 0.00
Sat 05/16 10Z 0.54 2717  94  56   5 0.00
Sat 05/16 11Z 0.57 2717  90  44   5 0.00
Sat 05/16 12Z 0.59 2717  87  24   4 0.01
Sat 05/16 13Z 0.66 2718  83  27   5 0.01
Sat 05/16 14Z 0.78 2718  81  33   5 0.00
Sat 05/16 15Z 1.07 2716  77  42   5 0.00
Sat 05/16 16Z 1.35 2614  71  48   6 0.00
Sat 05/16 17Z 1.95 2514  69  48   7 0.00

Run total areal avg QPF for the greens is: 2.25 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