NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260226_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 02/26 12Z 1.22 2722  79  45 -12 0.00
Thu 02/26 13Z 1.49 2724  79  23 -12 0.00
Thu 02/26 14Z 5.50 2621  80  15 -13 0.00
Thu 02/26 15Z 10.59 2619  81   7 -13 0.00
Thu 02/26 16Z 12.86 2619  84   4 -13 0.00
Thu 02/26 17Z 10.59 2618  85   5 -13 0.00
Thu 02/26 18Z 8.86 2616  83   9 -13 0.00
Thu 02/26 19Z 8.03 2616  82  13 -13 0.00
Thu 02/26 20Z 6.68 2616  81  20 -13 0.00
Thu 02/26 21Z 3.84 2716  81  28 -13 0.00
Thu 02/26 22Z 1.34 2716  80  36 -13 0.00
Thu 02/26 23Z 1.00 2817  78  39 -13 0.00
Fri 02/27 00Z 0.76 2818  76  40 -13 0.00
Fri 02/27 01Z 0.65 2918  74  46 -13 0.00
Fri 02/27 02Z 0.53 2917  73  44 -13 0.00
Fri 02/27 03Z 0.44 2915  71  42 -13 0.00
Fri 02/27 04Z 0.38 2915  69  47 -12 0.00
Fri 02/27 05Z 0.34 2914  68  57 -12 0.00
Fri 02/27 06Z 0.33 2814  67  59 -11 0.00
Fri 02/27 07Z 0.31 2813  66  61 -11 0.00
Fri 02/27 08Z 0.30 2813  63  60 -10 0.00
Fri 02/27 09Z 0.31 2714  61  63 -10 0.00
Fri 02/27 10Z 0.33 2715  60  63 -10 0.00
Fri 02/27 11Z 0.35 2716  62  66 -10 0.00
Fri 02/27 12Z 0.36 2617  62  73  -9 0.00
Fri 02/27 13Z 0.37 2617  64  79  -8 0.00
Fri 02/27 14Z 0.34 2517  59  84  -8 0.00
Fri 02/27 15Z 0.36 2417  55  85  -7 0.00
Fri 02/27 16Z 0.42 2419  51  86  -7 0.00
Fri 02/27 17Z 0.48 2420  49  82  -7 0.00
Fri 02/27 18Z 0.53 2421  46  78  -6 0.00
Fri 02/27 19Z 0.59 2421  47  70  -6 0.00
Fri 02/27 20Z 0.61 2420  55  53  -6 0.00
Fri 02/27 21Z 0.66 2420  63  35  -6 0.00
Fri 02/27 22Z 0.72 2421  70  32  -6 0.00
Fri 02/27 23Z 0.72 2421  73  35  -5 0.00
Sat 02/28 00Z 0.68 2422  76  32  -5 0.00
Sat 02/28 01Z 0.63 2323  76  30  -4 0.00
Sat 02/28 02Z 0.56 2324  73  33  -3 0.00
Sat 02/28 03Z 0.51 2226  66  34  -2 0.00
Sat 02/28 04Z 0.52 2230  63  26  -2 0.00
Sat 02/28 05Z 0.54 2134  62  22  -1 0.00
Sat 02/28 06Z 0.55 2136  60  20  -1 0.00
Sat 02/28 07Z 0.59 2138  61  19  -1 0.00
Sat 02/28 08Z 0.62 2140  60  25   0 0.00
Sat 02/28 09Z 0.65 2241  59  36   0 0.00
Sat 02/28 10Z 0.68 2241  56  60   0 0.00
Sat 02/28 11Z 0.73 2341  63  76   0 0.00
Sat 02/28 12Z 0.74 2340  69  82   0 0.00
Sat 02/28 13Z 0.71 2339  74  84   0 0.00
Sat 02/28 15Z 0.71 2337  81  92   0 0.00
Sat 02/28 16Z 0.75 2335  84  90   0 0.00
Sat 02/28 17Z 0.79 2433  87  92   0 0.00
Sat 02/28 18Z 0.87 2431  88  85  -1 0.00
Sat 02/28 19Z 0.82 2529  82  76  -1 0.01
Sat 02/28 20Z 0.79 2527  75  61  -1 0.01
Sat 02/28 21Z 0.80 2525  70  50  -2 0.00
Sat 02/28 22Z 0.78 2623  66  43  -2 0.00

Run total areal avg QPF for the greens is: 0.02 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