NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260120_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)
=== ===== === ==== ==== === === === ====
Tue 01/20 18Z 5.96 2514  81  66 -20 0.00
Tue 01/20 19Z 7.67 2618  83  66 -20 0.00
Tue 01/20 20Z 8.43 2621  82  66 -20 0.01
Tue 01/20 21Z 6.91 2622  77  62 -20 0.01
Tue 01/20 22Z 5.03 2623  78  51 -20 0.00
Tue 01/20 23Z 2.47 2625  78  44 -21 0.00
Wed 01/21 00Z 1.83 2627  75  39 -20 0.00
Wed 01/21 01Z 1.41 2629  70  38 -20 0.00
Wed 01/21 02Z 1.27 2630  72  39 -20 0.00
Wed 01/21 03Z 1.10 2629  76  43 -20 0.00
Wed 01/21 04Z 1.01 2629  77  41 -19 0.00
Wed 01/21 05Z 1.00 2629  81  40 -19 0.00
Wed 01/21 06Z 0.91 2528  84  39 -19 0.00
Wed 01/21 07Z 0.88 2529  82  35 -19 0.00
Wed 01/21 08Z 0.84 2529  78  22 -18 0.00
Wed 01/21 09Z 0.81 2530  78  14 -18 0.00
Wed 01/21 10Z 0.72 2528  76   9 -17 0.00
Wed 01/21 11Z 0.70 2528  74   9 -17 0.00
Wed 01/21 12Z 0.64 2527  71   9 -17 0.00
Wed 01/21 13Z 0.57 2526  65   9 -16 0.00
Wed 01/21 14Z 0.56 2525  61  12 -16 0.00
Wed 01/21 15Z 0.53 2424  57  33 -15 0.00
Wed 01/21 16Z 0.49 2422  55  65 -15 0.00
Wed 01/21 17Z 0.43 2320  51  87 -14 0.00
Wed 01/21 18Z 0.40 2220  46  91 -14 0.00
Wed 01/21 19Z 0.39 2122  41  87 -13 0.00
Wed 01/21 20Z 0.36 2122  54  85 -13 0.00
Wed 01/21 21Z 0.27 2025  69  89 -12 0.00
Wed 01/21 22Z 0.21 1928  80  88 -11 0.00
Wed 01/21 23Z 0.20 1932  88  88 -11 0.02
Thu 01/22 00Z 0.33 2037  90  89 -12 0.04
Thu 01/22 01Z 0.51 2041  91  89 -12 0.03
Thu 01/22 02Z 0.57 2141  92  87 -10 0.01
Thu 01/22 03Z 0.62 2239  93  88  -9 0.01
Thu 01/22 04Z 0.66 2236  93  91  -9 0.01
Thu 01/22 05Z 0.67 2334  94  82  -8 0.01
Thu 01/22 06Z 0.68 2333  93  67  -8 0.01
Thu 01/22 07Z 0.69 2331  92  54  -8 0.00
Thu 01/22 08Z 0.68 2330  93  45  -8 0.00
Thu 01/22 09Z 0.67 2328  93  43  -9 0.00
Thu 01/22 10Z 0.66 2327  93  49  -9 0.00
Thu 01/22 11Z 0.68 2327  94  59  -9 0.00
Thu 01/22 12Z 0.74 2328  94  72  -9 0.00
Thu 01/22 13Z 0.80 2329  94  81  -9 0.01
Thu 01/22 14Z 0.84 2330  92  74  -9 0.01
Thu 01/22 15Z 1.00 2331  94  71  -9 0.01
Thu 01/22 16Z 1.33 2431  91  60 -10 0.01
Thu 01/22 17Z 1.67 2427  89  49 -10 0.00
Thu 01/22 18Z 3.47 2426  92  60 -10 0.00
Thu 01/22 19Z 5.05 2426  92  44 -10 0.01
Thu 01/22 20Z 4.07 2428  89  44 -10 0.01
Thu 01/22 21Z 5.90 2431  85  40 -11 0.01
Thu 01/22 22Z 4.07 2432  85  45 -11 0.00
Thu 01/22 23Z 4.13 2434  88  46 -12 0.00
Fri 01/23 00Z 3.30 2534  89  43 -13 0.01
Fri 01/23 01Z 2.98 2533  87  38 -14 0.01
Fri 01/23 02Z 1.98 2533  82  33 -14 0.00
Fri 01/23 03Z 1.69 2533  82  33 -15 0.00
Fri 01/23 04Z 2.00 2631  83  32 -16 0.00
Fri 01/23 05Z 1.78 2629  84  31 -17 0.00

Run total areal avg QPF for the greens is: 0.25 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