Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260312_0600
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)
=== ===== === ==== ==== === === === ====
Thu 03/12 06Z 0.56 2128  89  80   9 0.00
Thu 03/12 07Z 0.70 2233  95  90   9 0.02
Thu 03/12 08Z 0.83 2332  98  92   9 0.07
Thu 03/12 09Z 1.03 2527  98  95   6 0.10
Thu 03/12 10Z 1.15 2631  97  88   2 0.08
Thu 03/12 11Z 1.12 2733  94  82  -1 0.07
Thu 03/12 14Z 1.68 2828  90  53  -9 0.02
Thu 03/12 15Z 1.74 2826  87  33  -9 0.00
Thu 03/12 16Z 1.96 2825  87  21 -10 0.00
Thu 03/12 17Z 3.04 2825  87  24 -11 0.00
Thu 03/12 18Z 3.30 2924  86  34 -11 0.00
Thu 03/12 19Z 3.40 2923  86  28 -12 0.00
Thu 03/12 20Z 2.45 2923  86  38 -12 0.00
Thu 03/12 21Z 2.39 3024  84  39 -13 0.00
Thu 03/12 22Z 1.87 3025  82  20 -13 0.00
Thu 03/12 23Z 1.27 3025  81  25 -13 0.00
Fri 03/13 00Z 1.41 3025  84  32 -13 0.00
Fri 03/13 01Z 1.83 3025  85  40 -14 0.00
Fri 03/13 02Z 2.04 3026  87  46 -14 0.00
Fri 03/13 03Z 1.78 3025  88  47 -14 0.00
Fri 03/13 04Z 1.62 3126  89  50 -14 0.00
Fri 03/13 05Z 1.22 3025  88  49 -14 0.00
Fri 03/13 06Z 0.94 3023  88  50 -14 0.00
Fri 03/13 07Z 0.84 3022  88  52 -13 0.00
Fri 03/13 08Z 0.80 2921  88  53 -13 0.00
Fri 03/13 09Z 0.61 2920  84  46 -12 0.00
Fri 03/13 10Z 0.56 2919  82  38 -12 0.00
Fri 03/13 11Z 0.59 2820  84  33 -12 0.00
Fri 03/13 12Z 0.40 2814  81  14 -12 0.00
Fri 03/13 13Z 0.36 2711  80   8 -12 0.00
Fri 03/13 14Z 0.27 2508  77   5 -11 0.00
Fri 03/13 15Z 0.20 2207  70   4 -11 0.00
Fri 03/13 16Z 0.08 1807  63  10 -10 0.00
Fri 03/13 18Z 0.20 1813  59  67  -9 0.00
Fri 03/13 19Z 0.47 1716  57  87  -9 0.00
Fri 03/13 20Z 0.38 1720  79  88 -10 0.00
Fri 03/13 21Z 0.23 1625  93  89  -9 0.00
Fri 03/13 22Z 0.15 1728  93  90  -9 0.00
Fri 03/13 23Z 0.10 1728  93  89  -9 0.00
Sat 03/14 00Z 0.13 1829  94  91  -8 0.04
Sat 03/14 01Z 0.19 1930  95  90  -8 0.04
Sat 03/14 02Z 0.23 1930  95  86  -7 0.02
Sat 03/14 03Z 0.18 1929  95  92  -5 0.01
Sat 03/14 04Z 0.26 1929  96  89  -6 0.01
Sat 03/14 05Z 0.40 2027  97  84  -6 0.01
Sat 03/14 06Z 0.61 2225  97  85  -6 0.02
Sat 03/14 07Z 0.87 2325  96  86  -7 0.03
Sat 03/14 08Z 1.05 2426  96  85  -7 0.02
Sat 03/14 09Z 1.25 2427  96  85  -8 0.02
Sat 03/14 10Z 1.44 2428  96  83  -9 0.01
Sat 03/14 11Z 1.43 2527  95  82  -9 0.02
Sat 03/14 12Z 1.46 2427  94  85  -9 0.02
Sat 03/14 13Z 1.46 2426  94  85 -10 0.02
Sat 03/14 14Z 1.23 2425  94  81 -10 0.01
Sat 03/14 15Z 1.16 2526  94  84  -9 0.01
Sat 03/14 16Z 1.25 2527  94  84  -9 0.01
Sat 03/14 17Z 1.58 2528  93  81  -8 0.02

Run total areal avg QPF for the greens is: 0.70 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