Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260117_0600
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 01/17 06Z 0.33 2120  84  89  -9 0.00
Sat 01/17 07Z 0.43 2122  82  89  -9 0.00
Sat 01/17 08Z 0.48 2123  74  75  -9 0.00
Sat 01/17 09Z 0.54 2223  74  79  -9 0.00
Sat 01/17 10Z 0.49 2222  73  81  -9 0.00
Sat 01/17 11Z 0.42 2220  78  84  -9 0.00
Sat 01/17 12Z 0.34 2218  83  89  -8 0.00
Sat 01/17 13Z 0.28 2118  87  90  -7 0.00
Sat 01/17 14Z 0.29 2020  92  89  -6 0.00
Sat 01/17 15Z 0.32 2120  94  86  -6 0.01
Sat 01/17 16Z 0.34 2121  95  88  -6 0.01
Sat 01/17 17Z 0.37 2122  95  87  -6 0.02
Sat 01/17 18Z 0.38 2121  96  88  -6 0.03
Sat 01/17 19Z 0.36 2119  96  87  -5 0.04
Sat 01/17 20Z 0.38 2119  96  88  -6 0.05
Sat 01/17 22Z 0.58 2321  95  88  -6 0.05
Sat 01/17 23Z 0.60 2520  94  69  -6 0.05
Sun 01/18 00Z 0.61 2620  91  50  -7 0.03
Sun 01/18 01Z 0.54 2619  85  49  -7 0.01
Sun 01/18 02Z 0.55 2620  82  49  -6 0.00
Sun 01/18 03Z 0.59 2521  83  44  -7 0.00
Sun 01/18 04Z 0.59 2422  86  42  -7 0.00
Sun 01/18 05Z 0.69 2425  88  36  -7 0.00
Sun 01/18 06Z 0.78 2426  92  39  -7 0.00
Sun 01/18 07Z 0.79 2426  88  32  -8 0.00
Sun 01/18 08Z 0.81 2425  87  27  -8 0.00
Sun 01/18 09Z 0.85 2525  89  20  -9 0.00
Sun 01/18 10Z 0.96 2427  90  25  -9 0.00
Sun 01/18 11Z 1.02 2429  75  53  -9 0.00
Sun 01/18 12Z 0.86 2529  70  73  -9 0.01
Sun 01/18 13Z 0.98 2531  68  70 -10 0.00
Sun 01/18 14Z 0.96 2530  59  59 -10 0.00
Sun 01/18 15Z 1.02 2529  60  46 -11 0.00
Sun 01/18 16Z 1.10 2527  66  57 -11 0.00
Sun 01/18 17Z 1.45 2524  72  42 -11 0.00
Sun 01/18 18Z 1.60 2520  74  47 -11 0.00
Sun 01/18 19Z 2.18 2616  74  41 -11 0.00
Sun 01/18 20Z 1.20 2614  76  51 -11 0.00
Sun 01/18 21Z 0.67 2611  76  63 -11 0.00
Sun 01/18 22Z 0.46 2609  71  57 -10 0.00
Sun 01/18 23Z 0.30 2706  69  58 -10 0.00
Mon 01/19 00Z 0.20 2804  69  57 -10 0.00
Mon 01/19 01Z 0.13 3104  68  59 -10 0.00
Mon 01/19 02Z 0.20 2905  71  39 -10 0.00
Mon 01/19 03Z 0.17 3005  75  56 -10 0.00
Mon 01/19 04Z 0.19 3006  78  69 -11 0.00
Mon 01/19 05Z 0.16 3107  79  76 -10 0.00
Mon 01/19 06Z 0.13 3207  78  75 -10 0.00
Mon 01/19 07Z 0.11 3105  79  72 -11 0.00
Mon 01/19 08Z 0.06 2802  81  69 -11 0.00
Mon 01/19 09Z 0.04 1902  84  68 -11 0.00
Mon 01/19 10Z 0.06 1905  86  50 -11 0.00
Mon 01/19 11Z 0.11 1909  88  44 -11 0.00
Mon 01/19 12Z 0.11 1811  91  37 -10 0.00
Mon 01/19 13Z 0.14 1815  94  48 -10 0.00
Mon 01/19 14Z 0.19 1818  95  71 -10 0.00
Mon 01/19 15Z 0.26 1920  95  84  -9 0.01
Mon 01/19 16Z 0.37 1920  96  87  -9 0.01
Mon 01/19 17Z 0.63 2118  97  90  -9 0.02

Run total areal avg QPF for the Central_Greens is: 0.35 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