NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260420_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 04/20 18Z 5.34 3010  83  58  -9 0.00
Mon 04/20 19Z 4.77 3112  77  54  -8 0.00
Mon 04/20 20Z 4.70 3113  74  56  -8 0.00
Mon 04/20 21Z 4.14 3113  74  51  -8 0.00
Mon 04/20 22Z 3.84 3213  72  41  -9 0.00
Mon 04/20 23Z 3.39 3114  67  31  -9 0.00
Tue 04/21 00Z 2.25 3214  63  23  -9 0.00
Tue 04/21 01Z 2.18 3214  60  18  -9 0.00
Tue 04/21 02Z 1.82 3114  58  16  -9 0.00
Tue 04/21 03Z 1.46 3114  56  16 -10 0.00
Tue 04/21 04Z 1.21 3213  55  14 -10 0.00
Tue 04/21 05Z 0.88 3212  54  10 -10 0.00
Tue 04/21 06Z 0.67 3111  53   8 -10 0.00
Tue 04/21 07Z 0.60 3110  53   8 -10 0.00
Tue 04/21 08Z 0.54 3008  53   7 -10 0.00
Tue 04/21 09Z 0.49 3007  54   7 -10 0.00
Tue 04/21 10Z 0.55 2907  55   7 -10 0.00
Tue 04/21 11Z 0.48 2806  57   8 -10 0.00
Tue 04/21 12Z 0.84 2606  57   9 -10 0.00
Tue 04/21 13Z 2.27 2405  57   9 -10 0.00
Tue 04/21 14Z 3.32 2306  56  10  -9 0.00
Tue 04/21 15Z 3.35 2306  54  10  -9 0.00
Tue 04/21 16Z 3.26 2306  51  14  -8 0.00
Tue 04/21 17Z 3.26 2206  48  25  -7 0.00
Tue 04/21 18Z 3.30 2207  46  64  -6 0.00
Tue 04/21 19Z 2.79 2206  43  83  -6 0.00
Tue 04/21 20Z 2.48 2306  42  84  -5 0.00
Tue 04/21 21Z 1.52 2206  40  84  -5 0.00
Tue 04/21 22Z 0.71 2206  39  84  -4 0.00
Tue 04/21 23Z 0.27 2107  38  79  -4 0.00
Wed 04/22 00Z 0.24 2108  38  79  -4 0.00
Wed 04/22 01Z 0.31 2110  41  84  -4 0.00
Wed 04/22 02Z 0.34 2112  44  85  -4 0.00
Wed 04/22 03Z 0.38 2113  46  84  -3 0.00
Wed 04/22 04Z 0.39 2115  48  79  -3 0.00
Wed 04/22 05Z 0.42 2117  50  81  -3 0.00
Wed 04/22 06Z 0.42 2117  52  84  -3 0.00
Wed 04/22 07Z 0.39 2117  56  87  -3 0.00
Wed 04/22 08Z 0.29 2116  61  82  -3 0.00
Wed 04/22 09Z 0.19 2015  65  76  -2 0.00
Wed 04/22 10Z 0.12 1914  71  67  -2 0.00
Wed 04/22 11Z 0.08 1914  79  57  -3 0.00
Wed 04/22 12Z 0.08 1914  87  49  -3 0.00
Wed 04/22 13Z 0.08 1914  90  33  -3 0.00
Wed 04/22 14Z 0.12 1913  91  21  -3 0.00
Wed 04/22 15Z 0.13 1913  92  14  -3 0.01
Wed 04/22 16Z 0.13 1911  94  13  -3 0.01
Wed 04/22 17Z 0.14 1910  95  10  -3 0.01
Wed 04/22 18Z 0.12 1909  97   7  -3 0.01
Wed 04/22 19Z 0.11 2007  98   5  -3 0.01
Wed 04/22 20Z 0.14 2106  98   5  -3 0.01
Wed 04/22 21Z 0.15 2205  98   7  -2 0.00
Wed 04/22 22Z 0.15 2404  98   7  -2 0.00
Wed 04/22 23Z 0.17 2704  97   9  -2 0.00
Thu 04/23 00Z 0.15 2904  97  12  -3 0.00
Thu 04/23 01Z 0.18 3005  97  12  -3 0.00
Thu 04/23 02Z 0.14 3105  96  13  -3 0.00
Thu 04/23 03Z 0.15 3105  95  17  -2 0.00
Thu 04/23 04Z 0.16 3105  95  23  -2 0.00
Thu 04/23 05Z 0.20 3006  95  28  -3 0.00

Run total areal avg QPF for the greens is: 0.06 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