Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260117_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)
=== ===== === ==== ==== === === === ====
Sat 01/17 12Z 0.20 2116  87  66  -9 0.00
Sat 01/17 13Z 0.15 2015  86  75  -8 0.00
Sat 01/17 14Z 0.20 2017  86  87  -8 0.00
Sat 01/17 15Z 0.18 2016  87  89  -7 0.00
Sat 01/17 16Z 0.16 2016  91  87  -7 0.00
Sat 01/17 17Z 0.18 2017  93  88  -7 0.01
Sat 01/17 18Z 0.22 2118  94  86  -6 0.02
Sat 01/17 19Z 0.25 2119  94  87  -6 0.03
Sat 01/17 20Z 0.32 2119  95  86  -7 0.03
Sat 01/17 21Z 0.42 2220  95  85  -7 0.04
Sat 01/17 22Z 0.44 2418  94  87  -6 0.03
Sat 01/17 23Z 0.46 2518  92  80  -6 0.01
Sun 01/18 00Z 0.53 2520  92  68  -6 0.00
Sun 01/18 01Z 0.59 2521  91  54  -7 0.00
Sun 01/18 02Z 0.62 2522  89  48  -6 0.00
Sun 01/18 03Z 0.65 2523  89  50  -7 0.00
Sun 01/18 04Z 0.71 2424  91  48  -7 0.00
Sun 01/18 05Z 0.75 2426  94  37  -7 0.00
Sun 01/18 06Z 0.75 2425  94  29  -8 0.01
Sun 01/18 07Z 0.80 2425  93  24  -8 0.00
Sun 01/18 08Z 0.80 2326  91  28  -8 0.00
Sun 01/18 09Z 0.96 2427  94  30  -9 0.01
Sun 01/18 10Z 1.09 2428  87  50 -10 0.01
Sun 01/18 11Z 0.98 2427  76  37 -10 0.00
Sun 01/18 12Z 1.85 2428  72  34 -11 0.00
Sun 01/18 13Z 2.01 2528  78   8 -12 0.00
Sun 01/18 14Z 1.91 2527  84  15 -12 0.00
Sun 01/18 15Z 2.01 2424  86  36 -12 0.00
Sun 01/18 16Z 1.32 2420  87  16 -12 0.00
Sun 01/18 17Z 1.68 2417  89  13 -12 0.00
Sun 01/18 18Z 2.27 2415  89  14 -12 0.00
Sun 01/18 19Z 1.61 2412  86  14 -12 0.00
Sun 01/18 20Z 0.82 2509  83  14 -12 0.00
Sun 01/18 21Z 0.50 2507  81  16 -12 0.00
Sun 01/18 22Z 0.27 2405  79  13 -12 0.00
Sun 01/18 23Z 0.26 2405  77  12 -11 0.00
Mon 01/19 00Z 0.12 2702  74  20 -11 0.00
Mon 01/19 01Z 0.08 2802  74  22 -11 0.00
Mon 01/19 02Z 0.05 0001  76  25 -11 0.00
Mon 01/19 03Z 0.08 3002  79  25 -11 0.00
Mon 01/19 04Z 0.10 3204  82  30 -12 0.00
Mon 01/19 05Z 0.16 3106  84  36 -12 0.00
Mon 01/19 06Z 0.22 3108  87  51 -12 0.00
Mon 01/19 07Z 0.22 3108  91  33 -13 0.00
Mon 01/19 08Z 0.16 3106  92  29 -13 0.00
Mon 01/19 09Z 0.18 2905  92  27 -13 0.00
Mon 01/19 10Z 0.25 2706  93  30 -14 0.00
Mon 01/19 11Z 0.29 2507  93  25 -14 0.00
Mon 01/19 12Z 0.37 2409  92  31 -14 0.00
Mon 01/19 13Z 0.42 2410  91  42 -14 0.00
Mon 01/19 14Z 0.40 2212  89  59 -14 0.00
Mon 01/19 15Z 0.37 2113  87  82 -14 0.00
Mon 01/19 16Z 0.53 2115  86  85 -14 0.00
Mon 01/19 17Z 1.05 2117  85  83 -14 0.00
Mon 01/19 18Z 1.55 2117  84  82 -14 0.00
Mon 01/19 19Z 4.29 2217  88  86 -14 0.00
Mon 01/19 20Z 3.49 2219  90  82 -14 0.00
Mon 01/19 21Z 1.88 2222  92  72 -14 0.01
Mon 01/19 22Z 1.84 2324  89  68 -14 0.01
Mon 01/19 23Z 1.48 2326  85  65 -14 0.00

Run total areal avg QPF for the greens is: 0.22 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