NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260326_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 03/26 12Z 0.58 2324  77  92   0 0.00
Thu 03/26 13Z 0.57 2325  74  91   1 0.00
Thu 03/26 14Z 0.59 2326  76  96   2 0.00
Thu 03/26 15Z 0.63 2327  76  87   3 0.00
Thu 03/26 16Z 0.65 2424  73  64   3 0.00
Thu 03/26 17Z 0.75 2423  71  70   4 0.00
Thu 03/26 20Z 1.34 2524  89  98   3 0.00
Thu 03/26 21Z 0.85 2621  93  98   2 0.01
Thu 03/26 22Z 0.62 2717  95  97   2 0.05
Thu 03/26 23Z 0.47 2516  94  97   3 0.07
Fri 03/27 00Z 0.45 2515  98  98   3 0.08
Fri 03/27 01Z 0.30 2411  98  97   3 0.04
Fri 03/27 02Z 0.19 2406  98  98   3 0.09
Fri 03/27 03Z 0.26 2909  99  98   2 0.10
Fri 03/27 06Z 0.20 3316  98  96   0 0.05
Fri 03/27 07Z 0.26 3320  98  96  -1 0.18
Fri 03/27 08Z 0.27 3323  97  95  -3 0.10
Fri 03/27 09Z 0.24 3324  96  88  -4 0.07
Fri 03/27 10Z 0.23 3427  95  74  -6 0.04
Fri 03/27 12Z 0.22 3426  91  37 -10 0.04
Fri 03/27 13Z 0.24 3325  85  17 -11 0.04
Fri 03/27 14Z 0.25 3324  74  10 -11 0.02
Fri 03/27 15Z 0.28 3323  67   6 -12 0.01
Fri 03/27 16Z 0.28 3322  63   5 -13 0.01
Fri 03/27 17Z 0.39 3321  65   4 -14 0.01
Fri 03/27 18Z 0.76 3321  66   4 -14 0.00
Fri 03/27 19Z 1.14 3320  63   5 -14 0.00
Fri 03/27 20Z 1.27 3220  62   5 -14 0.00
Fri 03/27 21Z 0.95 3219  60   5 -14 0.00
Fri 03/27 22Z 0.47 3217  57   5 -14 0.00
Fri 03/27 23Z 0.37 3215  57   6 -14 0.00
Sat 03/28 00Z 0.34 3215  56   7 -14 0.00
Sat 03/28 01Z 0.34 3115  58   8 -14 0.00
Sat 03/28 02Z 0.31 3115  60   9 -14 0.00
Sat 03/28 03Z 0.39 3116  65  12 -15 0.00
Sat 03/28 04Z 0.42 3117  64  17 -15 0.00
Sat 03/28 05Z 0.44 3118  64  23 -16 0.00
Sat 03/28 06Z 0.45 3118  62  32 -16 0.00
Sat 03/28 07Z 0.41 3218  64  41 -17 0.00
Sat 03/28 08Z 0.33 3217  65  51 -17 0.00
Sat 03/28 09Z 0.28 3215  67  67 -17 0.00
Sat 03/28 10Z 0.24 3313  70  76 -18 0.00
Sat 03/28 11Z 0.23 3313  72  78 -18 0.00
Sat 03/28 12Z 0.26 3212  72  76 -19 0.00
Sat 03/28 13Z 0.41 3211  72  75 -19 0.00
Sat 03/28 14Z 0.92 3110  71  73 -19 0.00
Sat 03/28 15Z 2.02 3111  69  69 -19 0.00
Sat 03/28 16Z 3.75 3011  67  63 -19 0.00
Sat 03/28 17Z 5.05 3012  67  59 -18 0.00
Sat 03/28 18Z 6.07 2912  66  51 -17 0.00
Sat 03/28 19Z 6.37 2911  63  47 -17 0.00
Sat 03/28 20Z 6.39 2911  61  47 -16 0.00
Sat 03/28 21Z 5.53 2811  60  54 -16 0.00
Sat 03/28 22Z 3.61 2811  62  62 -15 0.00
Sat 03/28 23Z 2.30 2712  65  60 -15 0.00

Run total areal avg QPF for the greens is: 1.01 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