Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260310_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)
=== ===== === ==== ==== === === === ====
Tue 03/10 18Z 2.71 2316  53  40   8 0.00
Tue 03/10 19Z 1.92 2318  52  38   8 0.00
Tue 03/10 20Z 1.86 2420  55  37   8 0.00
Tue 03/10 21Z 1.68 2521  63  42   8 0.00
Tue 03/10 22Z 1.51 2622  75  39   8 0.00
Tue 03/10 23Z 0.98 2720  85  50   7 0.01
Wed 03/11 00Z 0.63 2718  91  37   7 0.01
Wed 03/11 01Z 0.55 2817  88  30   7 0.00
Wed 03/11 02Z 0.46 2817  86  32   7 0.00
Wed 03/11 03Z 0.34 2814  88  34   7 0.00
Wed 03/11 04Z 0.24 2811  90  33   7 0.00
Wed 03/11 05Z 0.18 2708  92  33   7 0.00
Wed 03/11 06Z 0.14 2506  93  71   8 0.00
Wed 03/11 07Z 0.10 2206  95  57   8 0.00
Wed 03/11 08Z 0.04 1808  96  68   8 0.00
Wed 03/11 09Z 0.02 1615  98  69   8 0.01
Wed 03/11 10Z 0.06 1622  98  57   8 0.02
Wed 03/11 11Z 0.06 1628  97  38   8 0.03
Wed 03/11 12Z 0.04 1731  97  44   9 0.03
Wed 03/11 13Z 0.06 1731  97  57   9 0.04
Wed 03/11 14Z 0.09 1732  97  68   9 0.02
Wed 03/11 15Z 0.11 1834  97  92  10 0.02
Wed 03/11 16Z 0.13 1835  95  91  10 0.03
Wed 03/11 17Z 0.24 1939  96  89   9 0.05
Wed 03/11 18Z 0.31 1940  94  44  10 0.03
Wed 03/11 19Z 0.33 2040  94  42  11 0.00
Wed 03/11 20Z 0.34 2037  90  51  12 0.00
Wed 03/11 21Z 0.33 2036  95  94  10 0.02
Wed 03/11 22Z 0.36 2039  97  94  10 0.05
Wed 03/11 23Z 0.46 2039  96  92  10 0.02
Thu 03/12 01Z 0.69 2144  98  95  11 0.04
Thu 03/12 02Z 0.87 2143  97  90  11 0.08
Thu 03/12 03Z 0.76 2138  97  86  12 0.05
Thu 03/12 04Z 0.84 2236  98  73  12 0.04
Thu 03/12 05Z 0.79 2331  98  78  11 0.18
Thu 03/12 06Z 0.85 2330  97  72  10 0.08
Thu 03/12 07Z 1.09 2430  98  73   8 0.07
Thu 03/12 08Z 1.38 2531  98  88   6 0.06
Thu 03/12 09Z 1.07 2631  96  88   2 0.06
Thu 03/12 10Z 1.06 2630  96  76  -2 0.09
Thu 03/12 11Z 1.24 2729  95  39  -5 0.05
Thu 03/12 12Z 1.18 2730  94  42  -7 0.01
Thu 03/12 13Z 1.37 2828  95  26  -9 0.00
Thu 03/12 14Z 1.70 2829  95  15 -10 0.00
Thu 03/12 15Z 2.03 2827  92   5 -11 0.00
Thu 03/12 16Z 3.82 2925  91   3 -13 0.01
Thu 03/12 17Z 4.72 2924  89   6 -13 0.00
Thu 03/12 18Z 6.45 2923  88  14 -13 0.00
Thu 03/12 19Z 7.66 2922  88  20 -13 0.00
Thu 03/12 20Z 6.75 3023  87  24 -12 0.00
Thu 03/12 21Z 4.58 3022  86  32 -12 0.00
Thu 03/12 22Z 2.28 3020  87  38 -12 0.00
Thu 03/12 23Z 1.28 3021  88  41 -13 0.00
Fri 03/13 01Z 0.93 3023  89  45 -13 0.00
Fri 03/13 02Z 0.88 3124  91  42 -13 0.00
Fri 03/13 03Z 0.83 3123  93  42 -13 0.00
Fri 03/13 04Z 0.79 3123  94  43 -13 0.00
Fri 03/13 05Z 0.73 3122  94  43 -13 0.00

Run total areal avg QPF for the Central_Greens is: 1.21 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