Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20251228_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 12/28 12Z 0.11 3212  47  26   1 0.00
Sun 12/28 15Z 0.10 2906  38  16   2 0.00
Sun 12/28 18Z 0.17 2411  30  27   4 0.00
Sun 12/28 21Z 0.22 2218  29  23   5 0.00
Mon 12/29 00Z 0.25 2124  28  20   6 0.00
Mon 12/29 03Z 0.16 2024  72  87   3 0.01
Mon 12/29 06Z 0.11 1731  97  97   3 0.22
Mon 12/29 09Z 0.20 2030  98  95   6 0.34
Mon 12/29 12Z 0.28 2130  98  95   8 0.15
Mon 12/29 15Z 0.35 2228  98  96   6 0.19
Mon 12/29 18Z 0.39 2226  98  92   3 0.06
Mon 12/29 21Z 1.17 2624  97  66  -4 0.10
Tue 12/30 00Z 8.42 2527  86  76  -9 0.02
Tue 12/30 03Z 2.44 2426  86  75 -11 0.00
Tue 12/30 06Z 7.04 2427  90  84 -13 0.00
Tue 12/30 09Z 18.29 2627  81  91 -19 0.00
Tue 12/30 12Z 17.33 2726  79  83 -22 0.01
Tue 12/30 15Z 17.80 2726  78  83 -24 0.00
Tue 12/30 18Z 17.77 2726  76  82 -23 0.00
Tue 12/30 21Z 13.60 2829  77  81 -22 0.00
Wed 12/31 00Z 1.96 2833  83  81 -21 0.01
Wed 12/31 03Z 1.82 2834  82  80 -19 0.02
Wed 12/31 06Z 0.95 2833  52  36 -17 0.02
Wed 12/31 09Z 1.32 2833  72  65 -19 0.00
Wed 12/31 12Z 0.78 2626  87  82 -13 0.01
Wed 12/31 15Z 0.75 2423  90  82 -12 0.01
Wed 12/31 18Z 0.86 2226  90  91 -11 0.01
Wed 12/31 21Z 1.14 2331  92  44 -10 0.01
Thu 01/01 00Z 3.52 2724  88  83 -13 0.04

Run total areal avg QPF for the greens is: 1.23 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