NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260112_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)
=== ===== === ==== ==== === === === ====
Mon 01/12 18Z 0.90 2623  91  77 -10 0.00
Mon 01/12 19Z 1.04 2627  92  73  -9 0.00
Mon 01/12 20Z 1.17 2529  96  75  -9 0.00
Mon 01/12 21Z 1.29 2531  96  79  -9 0.01
Mon 01/12 22Z 1.37 2532  96  80  -9 0.01
Mon 01/12 23Z 1.31 2531  96  83  -9 0.01
Tue 01/13 00Z 1.34 2531  97  84  -9 0.01
Tue 01/13 01Z 1.43 2531  96  79  -9 0.01
Tue 01/13 02Z 1.47 2631  96  75  -9 0.01
Tue 01/13 03Z 1.25 2630  95  73  -9 0.01
Tue 01/13 04Z 1.09 2629  95  74  -8 0.00
Tue 01/13 05Z 0.97 2628  95  79  -8 0.01
Tue 01/13 06Z 0.92 2627  95  80  -8 0.01
Tue 01/13 07Z 0.82 2627  94  80  -7 0.00
Tue 01/13 08Z 0.67 2625  92  85  -6 0.00
Tue 01/13 09Z 0.65 2624  93  88  -6 0.01
Tue 01/13 10Z 0.67 2624  93  79  -6 0.00
Tue 01/13 11Z 0.66 2623  94  50  -6 0.01
Tue 01/13 12Z 0.65 2623  94  37  -7 0.01
Tue 01/13 13Z 0.56 2620  93  31  -6 0.01
Tue 01/13 14Z 0.53 2519  93  28  -6 0.00
Tue 01/13 15Z 0.47 2518  90  25  -5 0.00
Tue 01/13 16Z 0.39 2417  87  20  -4 0.00
Tue 01/13 17Z 0.30 2316  82  15  -3 0.00
Tue 01/13 18Z 0.26 2216  80  14  -2 0.00
Tue 01/13 19Z 0.23 2216  80  22  -2 0.00
Tue 01/13 20Z 0.20 2118  81  34  -1 0.00
Tue 01/13 21Z 0.22 2023  82  40  -1 0.00
Tue 01/13 22Z 0.23 2026  76  53  -1 0.00
Tue 01/13 23Z 0.26 2028  74  87  -1 0.00
Wed 01/14 00Z 0.22 2028  68  83   0 0.00
Wed 01/14 01Z 0.25 2031  69  74   0 0.00
Wed 01/14 02Z 0.37 2036  69  83   0 0.00
Wed 01/14 03Z 0.47 2136  95  95  -2 0.03
Wed 01/14 04Z 0.52 2135  95  82  -2 0.05
Wed 01/14 05Z 0.52 2134  87  61  -2 0.00
Wed 01/14 06Z 0.58 2234  84  54  -2 0.00
Wed 01/14 07Z 0.62 2232  84  51  -2 0.00
Wed 01/14 08Z 0.61 2330  85  46  -1 0.00
Wed 01/14 09Z 0.56 2328  86  41  -1 0.00
Wed 01/14 10Z 0.51 2326  85  51  -1 0.00
Wed 01/14 11Z 0.51 2227  86  62  -1 0.00
Wed 01/14 12Z 0.50 2226  86  71  -1 0.00
Wed 01/14 13Z 0.51 2227  90  83  -1 0.00
Wed 01/14 14Z 0.48 2226  93  90  -1 0.01
Wed 01/14 15Z 0.49 2225  96  91  -2 0.01
Wed 01/14 16Z 0.54 2224  96  90  -2 0.02
Wed 01/14 17Z 0.57 2323  95  89  -2 0.02
Wed 01/14 18Z 0.58 2322  94  88  -2 0.01
Wed 01/14 19Z 0.55 2320  94  87  -3 0.01
Wed 01/14 20Z 0.54 2319  95  89  -3 0.01
Wed 01/14 21Z 0.48 2217  96  88  -3 0.01
Wed 01/14 22Z 0.44 2216  97  90  -3 0.01
Wed 01/14 23Z 0.44 2216  97  90  -3 0.02
Thu 01/15 00Z 0.43 2215  97  89  -3 0.01
Thu 01/15 01Z 0.41 2314  98  86  -3 0.01
Thu 01/15 02Z 0.40 2314  98  81  -3 0.01
Thu 01/15 03Z 0.41 2313  97  70  -4 0.01
Thu 01/15 04Z 0.41 2314  96  61  -4 0.01
Thu 01/15 05Z 0.37 2213  97  53  -4 0.01

Run total areal avg QPF for the greens is: 0.40 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