Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260418_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 04/18 18Z 0.34 1518  81   5   7 0.00
Sat 04/18 19Z 0.21 1522  80  18   7 0.00
Sat 04/18 20Z 0.22 1525  78  11   8 0.00
Sat 04/18 21Z 0.23 1526  77   4   7 0.00
Sat 04/18 22Z 0.19 1527  74   6   8 0.00
Sat 04/18 23Z 0.10 1628  71   7   9 0.00
Sun 04/19 00Z 0.03 1629  68   6  10 0.00
Sun 04/19 01Z 0.12 1830  67  19  10 0.00
Sun 04/19 02Z 0.18 1831  67  31  10 0.00
Sun 04/19 03Z 0.25 1931  68  51  11 0.00
Sun 04/19 04Z 0.33 2032  68  68  11 0.00
Sun 04/19 05Z 0.37 2033  69  90  11 0.00
Sun 04/19 06Z 0.51 2134  88  84   8 0.01
Sun 04/19 07Z 0.56 2233  97  91   8 0.04
Sun 04/19 08Z 0.55 2325  98  89   6 0.03
Sun 04/19 09Z 0.42 2417  98  90   6 0.03
Sun 04/19 10Z 0.31 2412  99  94   5 0.04
Sun 04/19 11Z 0.45 2515  99  99   4 0.03
Sun 04/19 12Z 0.61 2717  99  99   2 0.07
Sun 04/19 13Z 0.47 3019  98  99   0 0.05
Sun 04/19 14Z 0.42 3122  97  99   0 0.07
Sun 04/19 15Z 0.53 3124  97  99  -2 0.03
Sun 04/19 16Z 0.61 3125  97  99  -4 0.10
Sun 04/19 17Z 0.60 3124  96  98  -5 0.12
Sun 04/19 18Z 0.61 3022  96  95  -6 0.11
Sun 04/19 19Z 0.62 3122  95  91  -6 0.12
Sun 04/19 20Z 0.50 3121  93  88  -6 0.06
Sun 04/19 21Z 0.55 3120  82  58  -7 0.01
Sun 04/19 22Z 0.68 3020  81  16  -6 0.00
Sun 04/19 23Z 0.63 3022  74  18  -6 0.00
Mon 04/20 00Z 0.65 3024  69  25  -6 0.00
Mon 04/20 01Z 0.73 3024  67  24  -6 0.00
Mon 04/20 02Z 0.86 2924  71  26  -6 0.00
Mon 04/20 03Z 1.06 2922  74  34  -6 0.00
Mon 04/20 04Z 1.22 2921  75  36  -7 0.00
Mon 04/20 05Z 1.37 2821  74  41  -7 0.00
Mon 04/20 06Z 1.04 2821  75  43  -7 0.00
Mon 04/20 07Z 0.90 2720  78  48  -7 0.00
Mon 04/20 08Z 0.78 2619  84  63  -7 0.00
Mon 04/20 09Z 0.86 2622  87  68  -7 0.00
Mon 04/20 10Z 0.91 2722  90  64  -8 0.00
Mon 04/20 11Z 0.88 2821  92  70  -8 0.01
Mon 04/20 12Z 0.74 2919  93  63  -9 0.01
Mon 04/20 13Z 0.64 3116  95  54 -10 0.01
Mon 04/20 14Z 0.46 3215  88  45 -10 0.01
Mon 04/20 15Z 1.78 3213  81  38 -10 0.01
Mon 04/20 16Z 3.06 3212  75  39 -10 0.00
Mon 04/20 17Z 3.65 3111  68  39  -9 0.00
Mon 04/20 18Z 4.12 3110  70  45  -9 0.00
Mon 04/20 19Z 3.75 3110  70  52  -9 0.00
Mon 04/20 20Z 3.27 3110  69  55  -9 0.00
Mon 04/20 21Z 3.28 3111  70  58  -9 0.00
Mon 04/20 22Z 2.52 3212  72  56  -9 0.00
Mon 04/20 23Z 1.34 3213  71  47 -10 0.00
Tue 04/21 00Z 0.59 3213  70  30 -10 0.00
Tue 04/21 01Z 0.61 3213  66  18 -11 0.00
Tue 04/21 02Z 0.70 3213  62  13 -11 0.00
Tue 04/21 03Z 0.84 3214  60  11 -11 0.00
Tue 04/21 04Z 0.89 3214  60  11 -11 0.00
Tue 04/21 05Z 0.80 3114  57  11 -11 0.00

Run total areal avg QPF for the Central_Greens is: 0.97 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