NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260410_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)
=== ===== === ==== ==== === === === ====
Fri 04/10 06Z 0.56 2328  58  49   6 0.00
Fri 04/10 07Z 0.66 2431  65  46   6 0.00
Fri 04/10 08Z 0.71 2530  73  50   5 0.00
Fri 04/10 09Z 0.66 2526  77  42   5 0.00
Fri 04/10 10Z 0.63 2525  79  35   5 0.00
Fri 04/10 11Z 0.59 2523  79  39   5 0.00
Fri 04/10 12Z 0.49 2420  78  36   6 0.00
Fri 04/10 13Z 0.44 2321  77  48   6 0.00
Fri 04/10 14Z 0.43 2223  74  64   7 0.00
Fri 04/10 15Z 0.46 2226  69  77   7 0.00
Fri 04/10 16Z 0.53 2227  64  91   8 0.00
Fri 04/10 17Z 0.58 2227  62  91   8 0.00
Fri 04/10 18Z 0.62 2325  63  92   8 0.00
Fri 04/10 19Z 0.64 2323  65  64   8 0.00
Fri 04/10 20Z 0.71 2322  66  45   8 0.00
Fri 04/10 21Z 0.69 2222  66  31   8 0.00
Fri 04/10 22Z 0.84 2225  67  50   8 0.00
Fri 04/10 23Z 0.56 2124  69  56   8 0.00
Sat 04/11 00Z 0.50 2225  78  76   8 0.01
Sat 04/11 01Z 0.69 2426  94  96   6 0.09
Sat 04/11 02Z 0.55 2422  97  95   5 0.03
Sat 04/11 03Z 0.47 2519  98  95   5 0.06
Sat 04/11 04Z 0.48 2819  97  94   3 0.14
Sat 04/11 05Z 0.39 3020  98  87   2 0.07
Sat 04/11 06Z 0.42 3124  97  86   0 0.03
Sat 04/11 07Z 0.49 3127  97  48  -1 0.02
Sat 04/11 08Z 0.63 3128  97  36  -3 0.02
Sat 04/11 09Z 0.72 3130  96  15  -4 0.03
Sat 04/11 10Z 0.72 3129  95   7  -5 0.01
Sat 04/11 11Z 0.64 3129  90   6  -5 0.01
Sat 04/11 12Z 0.62 3128  90   4  -6 0.01
Sat 04/11 13Z 0.69 3127  90   5  -7 0.01
Sat 04/11 14Z 0.73 3127  89   6  -7 0.01
Sat 04/11 15Z 0.89 3125  91   8  -8 0.00
Sat 04/11 16Z 1.02 3124  92  11  -8 0.00
Sat 04/11 17Z 1.04 3123  91  12  -7 0.00
Sat 04/11 18Z 0.90 3123  91  13  -7 0.00
Sat 04/11 19Z 1.03 3122  91  12  -7 0.00
Sat 04/11 20Z 1.12 3021  90  11  -7 0.00
Sat 04/11 21Z 1.92 3021  87  10  -7 0.00
Sat 04/11 22Z 1.50 3022  84  11  -6 0.00
Sat 04/11 23Z 1.44 3023  83  11  -6 0.00
Sun 04/12 00Z 1.28 3125  82  10  -6 0.00
Sun 04/12 01Z 1.05 3126  81  10  -6 0.00
Sun 04/12 02Z 0.76 3126  77  12  -6 0.00
Sun 04/12 03Z 0.54 3226  69  14  -6 0.00
Sun 04/12 04Z 0.45 3225  64  12  -6 0.00
Sun 04/12 05Z 0.41 3224  62  12  -6 0.00
Sun 04/12 06Z 0.33 3323  59  12  -5 0.00
Sun 04/12 07Z 0.28 3320  59  15  -5 0.00
Sun 04/12 08Z 0.22 3317  59  19  -5 0.00
Sun 04/12 09Z 0.20 3214  58  23  -4 0.00
Sun 04/12 10Z 0.21 3113  55  24  -4 0.00
Sun 04/12 11Z 0.17 3110  48  28  -3 0.00
Sun 04/12 12Z 0.14 2907  44  34  -2 0.00
Sun 04/12 13Z 0.13 2606  41  35  -1 0.00
Sun 04/12 14Z 0.14 2406  40  35  -1 0.00
Sun 04/12 15Z 0.15 2308  38  30   0 0.00
Sun 04/12 16Z 0.26 2311  38  29  -1 0.00
Sun 04/12 17Z 0.27 2312  34  39  -1 0.00

Run total areal avg QPF for the greens is: 0.55 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