NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20250205_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 02/05 06Z 0.59 3127  81  11 -18 0.00
Wed 02/05 09Z 0.48 3026  78  12 -18 0.00
Wed 02/05 12Z 0.40 3025  76  17 -18 0.00
Wed 02/05 15Z 0.35 3021  75  19 -18 0.00
Wed 02/05 18Z 0.34 3018  70  13 -17 0.00
Wed 02/05 21Z 0.38 2918  58   8 -15 0.00
Thu 02/06 00Z 0.38 2919  48  13 -13 0.00
Thu 02/06 03Z 0.29 2714  38  15 -11 0.00
Thu 02/06 06Z 0.24 2512  23  15 -10 0.00
Thu 02/06 09Z 0.15 2113  19  14  -8 0.00
Thu 02/06 12Z 0.14 1923  35  33  -8 0.00
Thu 02/06 15Z 0.10 1832  70  85  -7 0.00
Thu 02/06 18Z 0.06 1837  92  96  -6 0.07
Thu 02/06 21Z 0.10 1838  94  69  -3 0.09
Fri 02/07 00Z 0.31 2128  97  15  -2 0.02
Fri 02/07 03Z 0.79 2433  97  26  -4 0.01
Fri 02/07 06Z 1.71 2535  96   3  -8 0.02
Fri 02/07 09Z 2.51 2633  90  18 -10 0.01
Fri 02/07 12Z 2.70 2737  89  85 -12 0.01
Fri 02/07 15Z 25.05 2836  79  46 -15 0.01
Fri 02/07 18Z 20.07 2832  79  40 -15 0.00
Fri 02/07 21Z 7.71 2831  83  51 -14 0.00
Sat 02/08 00Z 2.17 2835  89  30 -14 0.00
Sat 02/08 03Z 3.51 2833  84  40 -15 0.00
Sat 02/08 06Z 1.95 2930  94  26 -16 0.00
Sat 02/08 09Z 1.70 2929  92  32 -16 0.01
Sat 02/08 12Z 1.40 3028  90  30 -17 0.00
Sat 02/08 15Z 1.24 3021  79  12 -17 0.00
Sat 02/08 18Z 4.48 2914  82  17 -16 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