NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20250618_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)
=== ===== === ==== ==== === === === ====
Wed 06/18 06Z 0.50 2223  95  79  13 0.00
Wed 06/18 09Z 0.53 2420  95  80  14 0.01
Wed 06/18 12Z 0.37 2415  94  72  14 0.04
Wed 06/18 15Z 0.34 2314  95  75  14 0.05
Wed 06/18 18Z 0.29 2210  88  63  15 0.02
Wed 06/18 21Z 0.46 2309  84  69  16 0.01
Thu 06/19 00Z 0.39 2410  86  68  16 0.04
Thu 06/19 03Z 0.24 2608  91  59  16 0.03
Thu 06/19 06Z 0.17 2505  91  60  16 0.00
Thu 06/19 09Z 0.08 1906  90  69  16 0.00
Thu 06/19 12Z 0.23 2115  86  84  16 0.00
Thu 06/19 15Z 0.40 2119  87  56  16 0.00
Thu 06/19 18Z 3.23 2020  85  76  17 0.03
Thu 06/19 21Z 1.89 2025  81  84  18 0.06
Fri 06/20 00Z 1.13 2233  91  82  16 0.08
Fri 06/20 03Z 1.17 2431  96  50  14 0.06
Fri 06/20 06Z 1.00 2727  97  52  11 0.00
Fri 06/20 09Z 0.66 3024  98  60   8 0.02
Fri 06/20 12Z 0.51 3226  98  77   8 0.04
Fri 06/20 15Z 0.48 3225  97  56   9 0.03
Fri 06/20 18Z 0.87 3017  85  41  10 0.00
Fri 06/20 21Z 3.00 2915  83  19  10 0.00
Sat 06/21 00Z 1.02 2816  83  42  11 0.00
Sat 06/21 03Z 0.95 2916  83  34  10 0.01
Sat 06/21 06Z 0.69 3018  86  67  10 0.00
Sat 06/21 09Z 0.55 2915  91  47  10 0.00
Sat 06/21 12Z 0.47 2612  94  27  11 0.00
Sat 06/21 15Z 0.57 2513  90  40  11 0.00
Sat 06/21 18Z 0.79 2612  91  77  12 0.05

Run total areal avg QPF for the greens is: 0.58 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