NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20250323_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)
=== ===== === ==== ==== === === === ====
Sun 03/23 12Z 0.59 3125  76   8 -16 0.00
Sun 03/23 13Z 0.49 3226  70   8 -16 0.00
Sun 03/23 14Z 0.47 3224  65   9 -15 0.00
Sun 03/23 15Z 0.47 3122  58   9 -15 0.00
Sun 03/23 16Z 0.46 3120  50   9 -14 0.00
Sun 03/23 17Z 0.51 3119  45  10 -14 0.00
Sun 03/23 18Z 0.58 3118  42  11 -13 0.00
Sun 03/23 19Z 0.62 3017  41  10 -12 0.00
Sun 03/23 20Z 0.49 3015  39   9 -11 0.00
Sun 03/23 21Z 0.45 2914  38   8 -11 0.00
Sun 03/23 22Z 0.45 2915  37   8 -10 0.00
Sun 03/23 23Z 0.38 2915  34   8  -9 0.00
Mon 03/24 00Z 0.32 2913  32   7  -8 0.00
Mon 03/24 01Z 0.27 2812  30   8  -7 0.00
Mon 03/24 02Z 0.23 2710  29   8  -6 0.00
Mon 03/24 03Z 0.17 2608  27  10  -5 0.00
Mon 03/24 04Z 0.10 2306  25  13  -4 0.00
Mon 03/24 05Z 0.08 2008  25  17  -4 0.00
Mon 03/24 06Z 0.04 1911  24  18  -4 0.00
Mon 03/24 07Z 0.04 1816  24  19  -4 0.00
Mon 03/24 08Z 0.05 1822  30  20  -4 0.00
Mon 03/24 09Z 0.09 1827  35  39  -4 0.00
Mon 03/24 10Z 0.04 1829  43  81  -3 0.00
Mon 03/24 11Z 0.06 1731  63  97  -2 0.00
Mon 03/24 12Z 0.07 1734  88  98  -1 0.02
Mon 03/24 13Z 0.03 1737  94  97  -1 0.05
Mon 03/24 14Z 0.06 1837  95  87  -2 0.04
Mon 03/24 15Z 0.08 1835  96  62  -2 0.03
Mon 03/24 16Z 0.07 1831  97  34  -2 0.03
Mon 03/24 17Z 0.07 1830  97  20  -2 0.02
Mon 03/24 18Z 0.05 1827  97   8  -1 0.02
Mon 03/24 19Z 0.04 1824  97   7  -1 0.01
Mon 03/24 20Z 0.05 1820  96   7  -1 0.01
Mon 03/24 21Z 0.04 1818  94   9  -1 0.01
Mon 03/24 22Z 0.10 1916  91   8  -1 0.01
Mon 03/24 23Z 0.19 2116  88  15  -1 0.00
Tue 03/25 00Z 0.29 2217  91  62  -1 0.00
Tue 03/25 01Z 0.60 2422  97  73  -3 0.05
Tue 03/25 02Z 0.63 2619  89  74  -3 0.02
Tue 03/25 03Z 0.74 2519  92  86  -4 0.00
Tue 03/25 04Z 0.96 2522  94  83  -5 0.01
Tue 03/25 05Z 1.14 2423  94  75  -5 0.01
Tue 03/25 06Z 1.18 2524  94  68  -6 0.00
Tue 03/25 07Z 1.12 2526  91  68  -7 0.00
Tue 03/25 08Z 1.06 2526  90  72  -7 0.00
Tue 03/25 09Z 0.99 2525  91  77  -7 0.00
Tue 03/25 10Z 1.05 2526  93  77  -7 0.00
Tue 03/25 11Z 1.02 2525  93  76  -7 0.00
Tue 03/25 12Z 1.04 2524  94  75  -8 0.00
Tue 03/25 13Z 1.19 2522  95  75  -8 0.00
Tue 03/25 14Z 1.79 2520  95  76  -8 0.00
Tue 03/25 15Z 3.20 2518  94  79  -8 0.00
Tue 03/25 16Z 2.49 2418  94  84  -7 0.01
Tue 03/25 17Z 1.79 2419  95  87  -7 0.02
Tue 03/25 18Z 2.12 2419  96  86  -7 0.03
Tue 03/25 19Z 2.90 2520  95  84  -7 0.03
Tue 03/25 20Z 1.70 2520  95  84  -7 0.02
Tue 03/25 21Z 1.37 2521  96  83  -7 0.02
Tue 03/25 22Z 1.38 2621  96  83  -7 0.02
Tue 03/25 23Z 1.26 2621  93  82  -7 0.01

Run total areal avg QPF for the greens is: 0.50 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