NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260127_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)
=== ===== === ==== ==== === === === ====
Tue 01/27 06Z 0.66 3023  86  70 -21 0.00
Tue 01/27 07Z 0.69 3026  89  60 -21 0.00
Tue 01/27 08Z 0.71 3027  87  54 -21 0.00
Tue 01/27 09Z 0.68 3028  85  46 -21 0.00
Tue 01/27 10Z 0.58 3026  81  32 -20 0.00
Tue 01/27 11Z 0.56 3027  80  30 -20 0.00
Tue 01/27 12Z 0.52 3027  75  40 -20 0.00
Tue 01/27 13Z 0.49 2924  72  45 -19 0.00
Tue 01/27 14Z 0.46 2820  72  52 -19 0.00
Tue 01/27 15Z 0.49 2718  79  71 -19 0.00
Tue 01/27 16Z 0.52 2616  84  79 -19 0.00
Tue 01/27 17Z 0.50 2514  84  80 -19 0.01
Tue 01/27 18Z 0.45 2412  82  80 -18 0.01
Tue 01/27 19Z 0.46 2310  85  81 -18 0.01
Tue 01/27 20Z 1.42 2311  85  81 -19 0.01
Tue 01/27 21Z 0.87 2213  86  80 -19 0.01
Tue 01/27 22Z 1.15 2315  88  79 -19 0.01
Tue 01/27 23Z 1.63 2418  90  77 -19 0.01
Wed 01/28 00Z 1.34 2620  85  74 -20 0.01
Wed 01/28 01Z 1.29 2720  79  72 -20 0.01
Wed 01/28 02Z 1.03 2720  79  73 -20 0.00
Wed 01/28 03Z 0.79 2719  84  72 -20 0.00
Wed 01/28 04Z 0.78 2719  87  71 -20 0.00
Wed 01/28 05Z 0.78 2819  89  71 -20 0.01
Wed 01/28 06Z 0.75 2819  89  68 -20 0.01
Wed 01/28 07Z 0.70 2919  90  64 -20 0.00
Wed 01/28 08Z 0.69 2920  90  61 -20 0.00
Wed 01/28 09Z 0.65 2920  90  57 -20 0.00
Wed 01/28 10Z 0.59 3020  90  54 -20 0.00
Wed 01/28 11Z 0.50 3019  90  46 -20 0.00
Wed 01/28 12Z 0.49 3019  91  39 -20 0.00
Wed 01/28 13Z 0.44 3017  89  32 -20 0.00
Wed 01/28 14Z 0.39 3015  86  30 -20 0.00
Wed 01/28 15Z 0.39 2912  85  32 -20 0.00
Wed 01/28 16Z 0.43 2810  84  33 -20 0.00
Wed 01/28 17Z 0.73 2708  82  35 -19 0.00
Wed 01/28 18Z 1.75 2707  81  37 -19 0.00
Wed 01/28 19Z 3.13 2707  81  39 -19 0.00
Wed 01/28 20Z 2.30 2707  80  41 -19 0.00
Wed 01/28 21Z 0.81 2807  80  42 -19 0.00
Wed 01/28 22Z 0.53 2808  84  45 -19 0.00
Wed 01/28 23Z 0.52 2709  88  47 -19 0.00
Thu 01/29 00Z 0.51 2709  91  53 -19 0.00
Thu 01/29 01Z 0.50 2909  91  64 -19 0.00
Thu 01/29 02Z 0.45 3010  87  76 -19 0.00
Thu 01/29 03Z 0.42 3010  86  80 -19 0.00
Thu 01/29 04Z 0.42 3111  85  79 -19 0.01
Thu 01/29 05Z 0.47 3012  86  79 -20 0.01
Thu 01/29 06Z 0.54 3013  88  78 -20 0.01
Thu 01/29 07Z 0.56 3114  89  77 -20 0.01
Thu 01/29 08Z 0.55 3115  91  76 -20 0.00
Thu 01/29 09Z 0.53 3117  92  75 -20 0.00
Thu 01/29 10Z 0.54 3117  92  72 -20 0.00
Thu 01/29 11Z 0.50 3117  91  70 -20 0.00
Thu 01/29 12Z 0.47 3117  91  69 -20 0.00
Thu 01/29 13Z 0.46 3116  91  71 -19 0.00
Thu 01/29 14Z 0.47 3015  91  71 -20 0.00
Thu 01/29 15Z 0.51 3015  91  69 -20 0.00
Thu 01/29 16Z 0.64 3014  90  67 -19 0.00
Thu 01/29 17Z 1.41 2915  90  64 -19 0.00

Run total areal avg QPF for the greens is: 0.15 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