Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260125_1200
Using the MB925 to MB850 layer.
Assumes Central_Greens are orthogonal to 260 degree winds.

925-850 mb Layer Average Wind/RH. Avg QPF over the Central_Greens.
Date   Time        Wind RH% 700 850T QPF
Day mm/dd hhZ Frd# DDFF (%) RH% (C) (in)
=== ===== === ==== ==== === === === ====
Sun 01/25 12Z 0.02 0800  50  28 -17 0.00
Sun 01/25 13Z 0.05 1204  45  53 -16 0.00
Sun 01/25 14Z 0.06 1305  44  80 -15 0.00
Sun 01/25 15Z 0.07 1408  52  85 -15 0.00
Sun 01/25 16Z 0.12 1312  75  87 -16 0.00
Sun 01/25 17Z 0.13 1415  85  87 -16 0.01
Sun 01/25 18Z 0.16 1418  85  87 -15 0.04
Sun 01/25 19Z 0.15 1420  85  87 -15 0.06
Sun 01/25 20Z 0.19 1424  85  88 -15 0.06
Sun 01/25 21Z 0.24 1427  86  89 -14 0.06
Sun 01/25 22Z 0.27 1327  86  89 -14 0.08
Sun 01/25 23Z 0.33 1330  86  90 -13 0.08
Mon 01/26 00Z 0.40 1233  86  91 -13 0.07
Mon 01/26 01Z 0.45 1235  86  92 -13 0.07
Mon 01/26 02Z 0.47 1133  85  92 -13 0.06
Mon 01/26 03Z 0.46 1130  85  90 -13 0.06
Mon 01/26 04Z 0.46 1029  85  91 -12 0.06
Mon 01/26 05Z 0.50 1031  84  92 -12 0.05
Mon 01/26 06Z 0.55 1030  86  91 -13 0.02
Mon 01/26 07Z 0.55 1029  86  95 -13 0.02
Mon 01/26 08Z 0.51 1026  87  95 -13 0.01
Mon 01/26 09Z 0.47 1025  87  93 -12 0.01
Mon 01/26 10Z 0.41 1022  87  92 -12 0.01
Mon 01/26 11Z 0.37 1121  87  91 -12 0.01
Mon 01/26 12Z 0.36 1121  87  89 -12 0.01
Mon 01/26 13Z 0.33 1119  89  89 -13 0.01
Mon 01/26 14Z 0.33 1118  89  88 -12 0.01
Mon 01/26 15Z 0.30 1216  89  90 -12 0.00
Mon 01/26 16Z 0.25 1314  89  87 -12 0.01
Mon 01/26 17Z 0.19 1312  90  86 -12 0.01
Mon 01/26 18Z 0.16 1411  91  89 -12 0.01
Mon 01/26 19Z 0.12 1509  92  95 -12 0.01
Mon 01/26 20Z 0.06 1607  92  93 -12 0.02
Mon 01/26 21Z 0.12 1906  92  94 -13 0.02
Mon 01/26 22Z 0.38 2409  90  89 -14 0.03
Mon 01/26 23Z 0.51 2513  88  81 -14 0.03
Tue 01/27 00Z 0.72 2815  89  81 -16 0.02
Tue 01/27 01Z 0.67 3020  90  84 -17 0.01
Tue 01/27 02Z 0.63 3021  90  81 -18 0.01
Tue 01/27 03Z 0.57 3022  92  78 -18 0.01
Tue 01/27 04Z 0.53 3123  91  79 -19 0.01
Tue 01/27 05Z 0.47 3126  90  74 -19 0.01
Tue 01/27 06Z 0.49 3128  91  66 -19 0.00
Tue 01/27 07Z 0.52 3129  89  57 -19 0.00
Tue 01/27 08Z 0.58 3029  88  51 -19 0.00
Tue 01/27 09Z 0.57 3029  85  44 -19 0.00
Tue 01/27 10Z 0.54 3028  74  28 -19 0.00
Tue 01/27 11Z 0.48 3026  63  25 -19 0.00
Tue 01/27 12Z 0.45 3026  65  41 -18 0.00
Tue 01/27 13Z 0.40 3022  72  46 -18 0.00
Tue 01/27 14Z 0.38 2817  79  60 -18 0.00
Tue 01/27 15Z 0.37 2613  83  77 -18 0.00
Tue 01/27 16Z 0.37 2412  80  81 -18 0.00
Tue 01/27 17Z 0.42 2313  70  82 -18 0.00
Tue 01/27 18Z 0.52 2315  57  79 -18 0.00
Tue 01/27 19Z 0.41 2315  58  81 -17 0.00
Tue 01/27 20Z 0.52 2315  52  82 -17 0.00
Tue 01/27 21Z 1.20 2319  74  83 -19 0.00
Tue 01/27 22Z 1.20 2322  86  80 -19 0.00
Tue 01/27 23Z 1.61 2423  93  77 -19 0.00

Run total areal avg QPF for the Central_Greens is: 1.08 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