NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260927_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)
=== ===== === ==== ==== === === === ====
Sun 09/27 06Z 0.84 0730  77  68   9 0.00
Sun 09/27 07Z 0.86 0633  70  72   9 0.00
Sun 09/27 08Z 0.87 0734  67  84   9 0.00
Sun 09/27 09Z 0.84 0735  69  86  10 0.00
Sun 09/27 10Z 0.79 0736  76  88  10 0.01
Sun 09/27 11Z 0.77 0737  91  89  10 0.02
Sun 09/27 12Z 0.77 0838  93  87  10 0.03
Sun 09/27 13Z 0.75 0838  92  85  10 0.04
Sun 09/27 14Z 0.71 0837  91  81  11 0.02
Sun 09/27 15Z 0.66 0833  93  76  11 0.02
Sun 09/27 16Z 0.62 0831  94  77  11 0.02
Sun 09/27 17Z 0.62 0830  94  79  11 0.03
Sun 09/27 18Z 0.60 0829  93  81  11 0.03
Sun 09/27 19Z 0.62 0829  94  81  11 0.03
Sun 09/27 20Z 0.62 0828  94  72  11 0.01
Sun 09/27 21Z 0.62 0928  94  51  11 0.01
Sun 09/27 22Z 0.60 0929  94  33  11 0.01
Sun 09/27 23Z 0.61 0930  93  44  11 0.01
Mon 09/28 00Z 0.59 0931  93  57  11 0.01
Mon 09/28 01Z 0.58 0931  93  62  11 0.01
Mon 09/28 02Z 0.58 0931  93  63  11 0.01
Mon 09/28 03Z 0.57 1031  92  67  12 0.01
Mon 09/28 04Z 0.55 1030  92  72  12 0.01
Mon 09/28 05Z 0.53 1029  92  68  12 0.01
Mon 09/28 06Z 0.51 1028  92  71  12 0.01
Mon 09/28 07Z 0.48 1027  92  84  12 0.02
Mon 09/28 08Z 0.46 1026  92  87  12 0.02
Mon 09/28 09Z 0.44 1024  91  85  12 0.02
Mon 09/28 10Z 0.41 1022  91  85  12 0.01
Mon 09/28 11Z 0.37 1020  93  83  12 0.02
Mon 09/28 12Z 0.36 0919  93  89  12 0.02
Mon 09/28 13Z 0.35 0918  93  90  12 0.01
Mon 09/28 14Z 0.37 0818  95  95  11 0.04
Mon 09/28 15Z 0.39 0818  96  94  11 0.09
Mon 09/28 16Z 0.41 0819  97  94  10 0.14
Mon 09/28 17Z 0.37 0817  97  89  10 0.06
Mon 09/28 18Z 0.34 0816  98  91  10 0.04
Mon 09/28 19Z 0.33 0816  97  84  10 0.03
Mon 09/28 20Z 0.32 0715  97  82  10 0.02
Mon 09/28 21Z 0.29 0714  97  88  10 0.01
Mon 09/28 22Z 0.26 0613  97  89  10 0.01
Mon 09/28 23Z 0.24 0612  96  91  10 0.02
Tue 09/29 00Z 0.22 0611  96  87  10 0.02
Tue 09/29 01Z 0.21 0610  96  90   9 0.02
Tue 09/29 02Z 0.21 0610  96  93   9 0.03
Tue 09/29 03Z 0.18 0609  96  95   9 0.03
Tue 09/29 04Z 0.13 0507  96  95   9 0.02
Tue 09/29 05Z 0.09 0406  97  96   8 0.02
Tue 09/29 06Z 0.08 0306  98  94   9 0.01
Tue 09/29 07Z 0.07 0207  97  94   9 0.01
Tue 09/29 08Z 0.06 0109  98  94   9 0.00
Tue 09/29 09Z 0.08 0110  98  92   9 0.00
Tue 09/29 10Z 0.10 0210  97  90   9 0.00
Tue 09/29 11Z 0.11 0210  96  88   9 0.00
Tue 09/29 12Z 0.09 0209  97  86   9 0.00
Tue 09/29 13Z 0.05 0108  98  87   9 0.00
Tue 09/29 14Z 0.04 0009  98  87   9 0.00
Tue 09/29 15Z 0.04 0010  98  88   9 0.01
Tue 09/29 16Z 0.05 0010  97  89   9 0.01
Tue 09/29 17Z 0.03 0009  97  88  10 0.00

Run total areal avg QPF for the greens is: 1.12 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