NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260926_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)
=== ===== === ==== ==== === === === ====
Sat 09/26 18Z 4.79 0415  78  28   8 0.00
Sat 09/26 19Z 4.00 0416  77  30   8 0.00
Sat 09/26 20Z 2.01 0418  76  31   8 0.01
Sat 09/26 21Z 1.16 0421  74  34   8 0.00
Sat 09/26 22Z 0.86 0424  70  43   9 0.00
Sat 09/26 23Z 0.90 0428  69  49   9 0.00
Sun 09/27 00Z 1.25 0531  67  26   9 0.00
Sun 09/27 01Z 1.23 0532  57  22   9 0.00
Sun 09/27 02Z 1.10 0632  50  18   9 0.00
Sun 09/27 03Z 1.07 0632  49  25   9 0.00
Sun 09/27 04Z 1.06 0633  57  50   9 0.00
Sun 09/27 05Z 0.95 0633  56  68   9 0.00
Sun 09/27 06Z 0.89 0733  58  74   9 0.00
Sun 09/27 07Z 0.87 0734  64  68   9 0.01
Sun 09/27 08Z 0.82 0734  73  69   9 0.02
Sun 09/27 09Z 0.84 0736  80  81   8 0.02
Sun 09/27 10Z 0.81 0736  82  80   9 0.01
Sun 09/27 11Z 0.73 0734  90  84   9 0.01
Sun 09/27 12Z 0.67 0735  90  89  10 0.02
Sun 09/27 13Z 0.67 0736  90  88  11 0.02
Sun 09/27 14Z 0.67 0836  91  91  11 0.04
Sun 09/27 15Z 0.66 0736  92  92  11 0.03
Sun 09/27 16Z 0.64 0734  93  92  11 0.04
Sun 09/27 17Z 0.64 0833  94  84  11 0.06
Sun 09/27 18Z 0.62 0833  95  89  11 0.08
Sun 09/27 19Z 0.61 0832  95  82  12 0.07
Sun 09/27 20Z 0.61 0832  95  81  12 0.06
Sun 09/27 21Z 0.59 0931  95  86  12 0.04
Sun 09/27 22Z 0.56 0930  94  77  12 0.04
Sun 09/27 23Z 0.54 0930  94  68  12 0.03
Mon 09/28 00Z 0.54 0930  94  42  12 0.02
Mon 09/28 01Z 0.54 1030  93  59  12 0.01
Mon 09/28 02Z 0.55 0931  93  65  12 0.01
Mon 09/28 03Z 0.52 1029  91  64  13 0.01
Mon 09/28 04Z 0.49 0928  91  70  13 0.01
Mon 09/28 05Z 0.50 0928  91  67  12 0.01
Mon 09/28 06Z 0.49 0928  90  75  13 0.01
Mon 09/28 07Z 0.48 1027  91  73  12 0.02
Mon 09/28 08Z 0.47 1026  94  78  12 0.02
Mon 09/28 09Z 0.45 0925  93  79  12 0.01
Mon 09/28 10Z 0.42 0924  90  81  13 0.01
Mon 09/28 11Z 0.39 0922  89  78  13 0.01
Mon 09/28 12Z 0.35 1020  90  76  12 0.01
Mon 09/28 13Z 0.31 1018  90  79  12 0.01
Mon 09/28 14Z 0.29 1016  91  86  12 0.01
Mon 09/28 15Z 0.28 0915  91  87  11 0.01
Mon 09/28 16Z 0.28 0915  91  85  11 0.01
Mon 09/28 17Z 0.28 0914  92  85  11 0.01
Mon 09/28 18Z 0.28 0813  93  84  11 0.02
Mon 09/28 19Z 0.27 0812  94  85  11 0.02
Mon 09/28 20Z 0.29 0713  94  90  10 0.03
Mon 09/28 21Z 0.31 0714  95  92  10 0.05
Mon 09/28 22Z 0.30 0813  95  96  10 0.04
Mon 09/28 23Z 0.28 0813  96  96  10 0.05
Tue 09/29 00Z 0.27 0912  96  93  10 0.04
Tue 09/29 01Z 0.26 0912  97  93  10 0.03
Tue 09/29 02Z 0.26 0912  97  84  10 0.03
Tue 09/29 03Z 0.24 0911  97  78  10 0.02
Tue 09/29 04Z 0.22 0910  97  75  10 0.01
Tue 09/29 05Z 0.19 0908  97  70   9 0.01

Run total areal avg QPF for the greens is: 1.17 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