NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260223_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)
=== ===== === ==== ==== === === === ====
Mon 02/23 12Z 0.45 0419  80  76 -10 0.00
Mon 02/23 13Z 0.43 0421  69  68  -9 0.00
Mon 02/23 14Z 0.43 0423  57  69  -9 0.00
Mon 02/23 15Z 0.43 0325  47  57  -9 0.00
Mon 02/23 16Z 0.38 0325  49  55  -9 0.00
Mon 02/23 17Z 0.35 0223  58  51 -10 0.00
Mon 02/23 18Z 0.33 0222  67  57 -10 0.00
Mon 02/23 19Z 0.18 0122  69  61 -10 0.00
Mon 02/23 20Z 0.10 0023  74  61 -10 0.00
Mon 02/23 21Z 0.07 3524  75  62 -10 0.00
Mon 02/23 22Z 0.12 3526  78  54 -10 0.00
Mon 02/23 23Z 0.14 3530  77  50 -10 0.01
Tue 02/24 00Z 0.14 3533  83  43 -11 0.00
Tue 02/24 01Z 0.21 3433  90  40 -12 0.00
Tue 02/24 02Z 0.29 3432  92  36 -12 0.00
Tue 02/24 03Z 0.38 3431  93  35 -13 0.01
Tue 02/24 04Z 0.47 3330  95  36 -13 0.01
Tue 02/24 05Z 0.55 3329  95  34 -14 0.01
Tue 02/24 06Z 0.55 3329  95  34 -14 0.01
Tue 02/24 07Z 0.59 3328  95  33 -15 0.01
Tue 02/24 08Z 0.53 3226  95  38 -15 0.01
Tue 02/24 09Z 0.45 3225  92  42 -15 0.01
Tue 02/24 10Z 0.41 3224  92  45 -16 0.01
Tue 02/24 11Z 0.38 3223  92  50 -16 0.01
Tue 02/24 12Z 0.36 3123  91  52 -16 0.01
Tue 02/24 13Z 0.35 3123  91  54 -16 0.00
Tue 02/24 14Z 0.33 3122  90  56 -16 0.00
Tue 02/24 15Z 0.31 3120  89  57 -16 0.00
Tue 02/24 16Z 0.32 3119  89  55 -16 0.00
Tue 02/24 17Z 0.34 3118  88  53 -16 0.00
Tue 02/24 18Z 0.41 3016  88  51 -16 0.00
Tue 02/24 19Z 0.50 2916  89  51 -16 0.00
Tue 02/24 20Z 0.48 2916  87  51 -15 0.00
Tue 02/24 21Z 0.42 2915  84  44 -15 0.00
Tue 02/24 22Z 0.48 2916  85  32 -15 0.00
Tue 02/24 23Z 0.45 2916  86  18 -14 0.00
Wed 02/25 00Z 0.40 2815  83   7 -14 0.00
Wed 02/25 01Z 0.31 2713  79   2 -13 0.00
Wed 02/25 02Z 0.30 2613  77   3 -13 0.00
Wed 02/25 03Z 0.23 2512  69   7 -12 0.00
Wed 02/25 04Z 0.16 2310  57  18 -11 0.00
Wed 02/25 05Z 0.17 2213  50  43 -11 0.00
Wed 02/25 06Z 0.22 2216  43  68 -11 0.00
Wed 02/25 07Z 0.24 2218  34  85 -11 0.00
Wed 02/25 08Z 0.20 2118  46  72 -11 0.00
Wed 02/25 09Z 0.16 2020  60  73 -11 0.00
Wed 02/25 10Z 0.18 1924  69  88 -11 0.00
Wed 02/25 11Z 0.14 1927  84  85 -10 0.01
Wed 02/25 12Z 0.16 1928  91  85 -10 0.03
Wed 02/25 13Z 0.24 2025  92  89 -10 0.03
Wed 02/25 14Z 0.28 2122  94  89 -10 0.01
Wed 02/25 15Z 0.33 2118  95  87 -10 0.01
Wed 02/25 16Z 0.36 2215  94  74 -10 0.01
Wed 02/25 17Z 0.44 2213  93  72  -9 0.00
Wed 02/25 18Z 0.67 2212  92  66  -9 0.00
Wed 02/25 19Z 1.46 2212  89  62  -9 0.00
Wed 02/25 20Z 2.39 2313  87  70  -8 0.00
Wed 02/25 21Z 1.50 2215  88  74  -8 0.00
Wed 02/25 22Z 0.74 2217  92  74  -8 0.00
Wed 02/25 23Z 0.66 2218  93  85  -8 0.00

Run total areal avg QPF for the greens is: 0.21 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