Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260125_1200
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)
=== ===== === ==== ==== === === === ====
Sun 01/25 12Z 0.04 3101  51  21 -17 0.00
Sun 01/25 13Z 0.05 1302  47  38 -16 0.00
Sun 01/25 14Z 0.05 1304  45  55 -16 0.00
Sun 01/25 15Z 0.07 1407  49  70 -16 0.00
Sun 01/25 16Z 0.13 1411  63  84 -16 0.00
Sun 01/25 17Z 0.14 1413  72  86 -16 0.01
Sun 01/25 18Z 0.17 1416  79  86 -16 0.02
Sun 01/25 19Z 0.17 1418  85  86 -15 0.04
Sun 01/25 20Z 0.21 1422  85  87 -15 0.05
Sun 01/25 21Z 0.26 1424  85  88 -15 0.05
Sun 01/25 22Z 0.28 1425  86  89 -14 0.06
Sun 01/25 23Z 0.33 1327  86  89 -14 0.07
Mon 01/26 00Z 0.40 1329  86  90 -14 0.06
Mon 01/26 01Z 0.44 1230  86  91 -14 0.06
Mon 01/26 02Z 0.45 1228  86  90 -14 0.06
Mon 01/26 03Z 0.44 1126  85  89 -13 0.06
Mon 01/26 04Z 0.44 1126  85  89 -12 0.05
Mon 01/26 05Z 0.47 1127  85  90 -12 0.04
Mon 01/26 06Z 0.49 1127  86  89 -13 0.03
Mon 01/26 07Z 0.48 1025  86  92 -13 0.02
Mon 01/26 08Z 0.44 1023  87  94 -13 0.01
Mon 01/26 09Z 0.41 1121  87  93 -12 0.01
Mon 01/26 10Z 0.36 1119  87  93 -12 0.01
Mon 01/26 11Z 0.34 1118  88  92 -12 0.01
Mon 01/26 12Z 0.32 1218  87  88 -12 0.01
Mon 01/26 13Z 0.29 1216  88  88 -13 0.01
Mon 01/26 14Z 0.28 1215  88  87 -12 0.01
Mon 01/26 15Z 0.25 1313  89  89 -13 0.01
Mon 01/26 16Z 0.21 1412  89  87 -12 0.01
Mon 01/26 17Z 0.17 1411  90  87 -12 0.01
Mon 01/26 18Z 0.16 1510  91  89 -12 0.01
Mon 01/26 19Z 0.15 1509  91  92 -12 0.01
Mon 01/26 20Z 0.10 1608  91  91 -13 0.02
Mon 01/26 21Z 0.09 1806  92  90 -13 0.02
Mon 01/26 22Z 0.26 2307  90  86 -14 0.03
Mon 01/26 23Z 0.38 2610  89  81 -14 0.03
Tue 01/27 00Z 0.59 2814  89  81 -15 0.02
Tue 01/27 01Z 0.67 3019  89  83 -17 0.02
Tue 01/27 02Z 0.64 3021  90  82 -18 0.01
Tue 01/27 03Z 0.62 3022  91  79 -18 0.01
Tue 01/27 04Z 0.65 3124  91  78 -19 0.01
Tue 01/27 05Z 0.63 3125  90  74 -19 0.01
Tue 01/27 06Z 0.65 3127  91  68 -19 0.01
Tue 01/27 07Z 0.70 3028  91  60 -19 0.01
Tue 01/27 08Z 0.73 3029  90  54 -19 0.00
Tue 01/27 09Z 0.70 3029  87  47 -19 0.00
Tue 01/27 10Z 0.65 3028  80  30 -19 0.00
Tue 01/27 11Z 0.58 3026  70  24 -19 0.00
Tue 01/27 12Z 0.54 3025  68  42 -19 0.00
Tue 01/27 13Z 0.46 2921  70  45 -18 0.00
Tue 01/27 14Z 0.43 2817  76  58 -19 0.00
Tue 01/27 15Z 0.39 2714  80  75 -18 0.00
Tue 01/27 16Z 0.36 2511  81  82 -18 0.00
Tue 01/27 17Z 0.37 2312  76  82 -18 0.00
Tue 01/27 18Z 0.48 2213  67  80 -18 0.00
Tue 01/27 19Z 0.37 2213  68  81 -17 0.00
Tue 01/27 20Z 0.39 2213  65  82 -18 0.00
Tue 01/27 21Z 0.87 2316  78  82 -18 0.01
Tue 01/27 22Z 0.94 2319  86  79 -19 0.01
Tue 01/27 23Z 1.27 2421  91  77 -19 0.01

Run total areal avg QPF for the greens is: 1.03 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