NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260111_1800
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)
=== ===== === ==== ==== === === === ====
Sun 01/11 18Z 2.75 2615  91  46  -7 0.00
Sun 01/11 19Z 2.50 2618  89  60  -8 0.00
Sun 01/11 20Z 2.49 2618  89  68  -8 0.00
Sun 01/11 21Z 1.56 2519  88  68  -8 0.00
Sun 01/11 22Z 2.51 2623  88  70  -9 0.01
Sun 01/11 23Z 3.40 2726  89  67 -10 0.02
Mon 01/12 00Z 1.90 2724  90  79 -10 0.00
Mon 01/12 01Z 1.80 2726  93  80 -10 0.01
Mon 01/12 02Z 2.14 2828  94  81 -12 0.03
Mon 01/12 03Z 2.00 2930  92  81 -13 0.02
Mon 01/12 04Z 2.50 2932  93  77 -15 0.01
Mon 01/12 05Z 2.38 2933  93  65 -16 0.01
Mon 01/12 06Z 1.96 2933  94  68 -16 0.01
Mon 01/12 07Z 1.39 2934  94  72 -16 0.01
Mon 01/12 08Z 1.14 2934  94  41 -15 0.01
Mon 01/12 09Z 0.91 2935  90  27 -13 0.01
Mon 01/12 10Z 0.72 3035  81  16 -12 0.00
Mon 01/12 11Z 0.66 3034  78  11 -12 0.00
Mon 01/12 12Z 0.68 2933  78  18 -12 0.00
Mon 01/12 13Z 0.71 2933  76  36 -11 0.00
Mon 01/12 14Z 0.71 2932  84  66 -11 0.00
Mon 01/12 15Z 0.76 2932  88  74 -10 0.00
Mon 01/12 16Z 0.85 2831  87  74 -10 0.00
Mon 01/12 17Z 0.84 2730  85  69  -9 0.00
Mon 01/12 18Z 0.92 2629  88  72  -9 0.00
Mon 01/12 19Z 0.95 2629  92  83  -9 0.00
Mon 01/12 20Z 1.01 2628  95  82  -9 0.00
Mon 01/12 21Z 1.11 2529  97  82 -10 0.01
Mon 01/12 22Z 1.32 2529  97  87 -10 0.01
Mon 01/12 23Z 1.33 2529  97  86 -10 0.01
Tue 01/13 00Z 1.37 2629  95  74 -10 0.01
Tue 01/13 01Z 1.30 2629  95  58 -10 0.01
Tue 01/13 02Z 1.23 2629  96  46 -10 0.01
Tue 01/13 03Z 1.16 2629  96  50  -9 0.01
Tue 01/13 04Z 1.15 2629  96  53  -9 0.01
Tue 01/13 05Z 1.09 2728  96  51  -9 0.01
Tue 01/13 06Z 1.09 2727  96  51  -9 0.01
Tue 01/13 07Z 1.09 2726  96  48  -9 0.01
Tue 01/13 08Z 1.09 2725  97  45  -9 0.01
Tue 01/13 09Z 1.06 2725  96  43  -9 0.01
Tue 01/13 10Z 1.05 2724  96  53  -9 0.01
Tue 01/13 11Z 0.97 2724  96  58  -9 0.01
Tue 01/13 12Z 0.91 2724  95  23  -8 0.01
Tue 01/13 13Z 0.73 2622  93  21  -7 0.01
Tue 01/13 14Z 0.63 2621  91  19  -6 0.00
Tue 01/13 15Z 0.60 2521  91  19  -6 0.00
Tue 01/13 16Z 0.46 2519  85  23  -4 0.00
Tue 01/13 17Z 0.35 2417  78  25  -3 0.00
Tue 01/13 18Z 0.30 2317  74  21  -2 0.00
Tue 01/13 19Z 0.27 2318  69  19  -1 0.00
Tue 01/13 20Z 0.28 2220  67  17   0 0.00
Tue 01/13 21Z 0.25 2220  68  21   0 0.00
Tue 01/13 22Z 0.26 2123  72  34   0 0.00
Tue 01/13 23Z 0.26 2026  72  40   0 0.00
Wed 01/14 00Z 0.24 2029  65  25   0 0.00
Wed 01/14 01Z 0.27 2033  63  29   0 0.00
Wed 01/14 02Z 0.34 2035  68  71   0 0.00
Wed 01/14 03Z 0.42 2136  77  91  -1 0.00
Wed 01/14 04Z 0.50 2138  89  92  -2 0.02
Wed 01/14 05Z 0.50 2138  97  92  -2 0.07

Run total areal avg QPF for the greens is: 0.41 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