Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260321_1800
Using the MB925 to MB850 layer.
Assumes Central_Greens are orthogonal to 260 degree winds.

925-850 mb Layer Average Wind/RH. Avg QPF over the Central_Greens.
Date   Time        Wind RH% 700 850T QPF
Day mm/dd hhZ Frd# DDFF (%) RH% (C) (in)
=== ===== === ==== ==== === === === ====
Sat 03/21 18Z 1.13 3312  90  23  -8 0.00
Sat 03/21 19Z 1.00 3215  84  12  -8 0.00
Sat 03/21 20Z 0.66 3316  79  10  -7 0.00
Sat 03/21 21Z 0.41 3314  74  16  -7 0.00
Sat 03/21 22Z 0.21 3211  73  17  -7 0.00
Sat 03/21 23Z 0.17 3210  71  22  -6 0.00
Sun 03/22 00Z 0.15 3208  68  31  -6 0.00
Sun 03/22 01Z 0.12 3209  64  55  -5 0.00
Sun 03/22 02Z 0.05 3505  54  91  -5 0.00
Sun 03/22 03Z 0.07 0403  53  93  -4 0.00
Sun 03/22 04Z 0.04 1302  58  92  -6 0.00
Sun 03/22 05Z 0.05 1803  69  93  -6 0.00
Sun 03/22 06Z 0.05 1607  73  93  -6 0.01
Sun 03/22 07Z 0.06 1712  69  96  -5 0.01
Sun 03/22 08Z 0.10 1718  77  96  -7 0.00
Sun 03/22 09Z 0.14 1823  91  96  -6 0.02
Sun 03/22 10Z 0.08 1726  95  96  -5 0.03
Sun 03/22 11Z 0.08 1730  96  98  -4 0.07
Sun 03/22 12Z 0.10 1729  97  99  -3 0.05
Sun 03/22 13Z 0.08 1728  97  98  -2 0.08
Sun 03/22 14Z 0.13 1828  98  91  -2 0.07
Sun 03/22 15Z 0.16 1827  98  91  -1 0.09
Sun 03/22 16Z 0.23 1925  98  73   0 0.05
Sun 03/22 17Z 0.29 2022  98  92   0 0.04
Sun 03/22 18Z 0.24 2018  99  95   1 0.05
Sun 03/22 19Z 0.24 2116  99  98   1 0.07
Sun 03/22 20Z 0.24 2114  99  95   1 0.08
Sun 03/22 21Z 0.20 2210  99  93   1 0.07
Sun 03/22 22Z 0.16 2307  99  79   1 0.08
Sun 03/22 23Z 0.14 2406  99  89   0 0.03
Mon 03/23 00Z 0.06 2702  99  84   0 0.04
Mon 03/23 01Z 0.03 3002 100  74   0 0.03
Mon 03/23 02Z 0.04 0202  99  65  -1 0.02
Mon 03/23 03Z 0.08 0404  99  47  -2 0.01
Mon 03/23 04Z 0.13 0406  99  28  -2 0.00
Mon 03/23 05Z 0.14 0407  99  29  -3 0.01
Mon 03/23 06Z 0.15 0407  99  32  -4 0.01
Mon 03/23 07Z 0.18 0408  98  31  -5 0.01
Mon 03/23 08Z 0.18 0308  98  29  -5 0.01
Mon 03/23 09Z 0.20 0308  98  31  -6 0.01
Mon 03/23 10Z 0.20 0309  98  30  -7 0.01
Mon 03/23 11Z 0.19 0308  98  29  -7 0.01
Mon 03/23 12Z 0.19 0307  98  28  -8 0.01
Mon 03/23 13Z 0.17 0306  98  28  -8 0.01
Mon 03/23 14Z 0.13 0206  98  30  -8 0.01
Mon 03/23 15Z 0.08 0105  98  29  -8 0.00
Mon 03/23 16Z 0.03 3405  98  32  -9 0.00
Mon 03/23 17Z 0.08 3308  97  31  -9 0.00
Mon 03/23 18Z 0.12 3310  97  39  -9 0.00
Mon 03/23 19Z 0.14 3312  95  45  -9 0.00
Mon 03/23 20Z 0.20 3315  94  36  -9 0.00
Mon 03/23 21Z 0.25 3318  94  32 -10 0.00
Mon 03/23 22Z 0.27 3321  93  26 -10 0.00
Mon 03/23 23Z 0.23 3323  94  20 -10 0.00
Tue 03/24 00Z 0.18 3324  94  18 -11 0.00
Tue 03/24 01Z 0.16 3425  91  18 -11 0.00
Tue 03/24 02Z 0.17 3425  87  18 -11 0.00
Tue 03/24 03Z 0.28 3323  93  22 -11 0.01
Tue 03/24 04Z 0.37 3322  94  20 -11 0.02
Tue 03/24 05Z 0.24 3320  95  14 -11 0.01

Run total areal avg QPF for the Central_Greens is: 1.14 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