NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20261029_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)
=== ===== === ==== ==== === === === ====
Thu 10/29 12Z 0.47 2516  75  39   7 0.00
Thu 10/29 13Z 0.49 2517  76  39   7 0.00
Thu 10/29 14Z 0.52 2418  76  37   7 0.00
Thu 10/29 15Z 0.54 2420  76  36   7 0.00
Thu 10/29 16Z 0.59 2422  75  49   7 0.00
Thu 10/29 17Z 0.68 2424  73  53   8 0.00
Thu 10/29 18Z 0.77 2426  69  52   8 0.00
Thu 10/29 19Z 0.84 2426  66  50   8 0.00
Thu 10/29 20Z 0.85 2427  66  37   9 0.00
Thu 10/29 21Z 0.84 2428  66  52   9 0.00
Thu 10/29 22Z 0.86 2430  68  79   9 0.00
Thu 10/29 23Z 0.88 2532  72  90   9 0.00
Fri 10/30 00Z 0.90 2532  74  91   9 0.00
Fri 10/30 01Z 0.95 2533  77  92   9 0.00
Fri 10/30 02Z 1.02 2535  85  95   8 0.02
Fri 10/30 03Z 1.02 2534  88  94   8 0.04
Fri 10/30 04Z 1.02 2634  90  94   8 0.04
Fri 10/30 05Z 0.95 2632  94  92   8 0.05
Fri 10/30 06Z 0.88 2630  96  85   8 0.05
Fri 10/30 07Z 0.86 2728  94  75   7 0.03
Fri 10/30 08Z 0.84 2726  93  50   7 0.03
Fri 10/30 09Z 0.83 2826  95  27   6 0.02
Fri 10/30 10Z 0.82 2826  92  11   6 0.02
Fri 10/30 11Z 0.81 2826  87   8   5 0.02
Fri 10/30 12Z 0.81 2826  85   8   4 0.02
Fri 10/30 13Z 0.81 2826  83   7   3 0.02
Fri 10/30 14Z 0.81 2825  85   5   2 0.02
Fri 10/30 15Z 0.83 2925  88   8   1 0.02
Fri 10/30 16Z 0.96 2925  92  16   0 0.02
Fri 10/30 17Z 1.17 2926  92  14   0 0.02
Fri 10/30 18Z 1.24 2926  92  13   0 0.01
Fri 10/30 19Z 1.41 2928  93  14  -1 0.01
Fri 10/30 20Z 1.54 2929  95  18  -1 0.02
Fri 10/30 21Z 1.33 3029  95  18  -2 0.01
Fri 10/30 22Z 1.10 3030  95  17  -2 0.01
Fri 10/30 23Z 1.08 3031  96  23  -2 0.01
Sat 10/31 00Z 1.12 3131  97  24  -3 0.01
Sat 10/31 01Z 1.12 3132  97  25  -3 0.02
Sat 10/31 02Z 0.99 3132  96  29  -3 0.02
Sat 10/31 03Z 0.85 3132  95  32  -4 0.01
Sat 10/31 04Z 0.75 3230  92  33  -5 0.01
Sat 10/31 05Z 0.63 3229  89  32  -5 0.00
Sat 10/31 06Z 0.55 3229  88  20  -6 0.00
Sat 10/31 07Z 0.50 3328  89  10  -6 0.00
Sat 10/31 08Z 0.55 3228  93   7  -6 0.00
Sat 10/31 09Z 0.45 3224  92   9  -6 0.00
Sat 10/31 10Z 0.36 3222  88   8  -6 0.00
Sat 10/31 11Z 0.33 3221  82  19  -5 0.00
Sat 10/31 12Z 0.32 3221  74  24  -4 0.00
Sat 10/31 13Z 0.29 3220  67  25  -4 0.00
Sat 10/31 14Z 0.27 3219  60  26  -3 0.00
Sat 10/31 15Z 0.26 3118  53  27  -3 0.00
Sat 10/31 16Z 0.25 3117  47  24  -2 0.00
Sat 10/31 17Z 0.25 3116  42  21  -2 0.00
Sat 10/31 18Z 0.26 3116  40  21  -2 0.00
Sat 10/31 19Z 0.25 3115  38  21  -1 0.00
Sat 10/31 20Z 0.27 3115  39  26  -1 0.00
Sat 10/31 21Z 0.29 3014  45  31  -1 0.00
Sat 10/31 22Z 0.34 2914  50  29  -1 0.00
Sat 10/31 23Z 0.36 2815  51  27  -1 0.00

Run total areal avg QPF for the greens is: 0.58 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