NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260221_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)
=== ===== === ==== ==== === === === ====
Sat 02/21 12Z 0.17 2905  98  84  -7 0.00
Sat 02/21 13Z 0.16 3005  98  85  -8 0.00
Sat 02/21 14Z 0.10 3305  98  79  -8 0.00
Sat 02/21 15Z 0.05 0004  98  77  -8 0.00
Sat 02/21 16Z 0.08 0203  98  75  -8 0.00
Sat 02/21 17Z 0.08 0102  98  75  -8 0.00
Sat 02/21 18Z 0.07 0101  97  73  -8 0.00
Sat 02/21 19Z 0.07 0102  95  69  -8 0.00
Sat 02/21 20Z 0.06 0103  92  64  -8 0.00
Sat 02/21 21Z 0.10 0304  87  58  -8 0.00
Sat 02/21 22Z 0.16 0505  83  53  -8 0.00
Sat 02/21 23Z 0.21 0507  80  53  -8 0.00
Sun 02/22 00Z 0.23 0608  77  48  -8 0.00
Sun 02/22 01Z 0.26 0708  77  44  -8 0.00
Sun 02/22 04Z 0.21 1208  76  39  -8 0.00
Sun 02/22 05Z 0.21 1309  75  38  -8 0.00
Sun 02/22 06Z 0.22 1310  74  36  -8 0.00
Sun 02/22 07Z 0.20 1412  69  41  -8 0.00
Sun 02/22 09Z 0.16 1513  65  49  -8 0.00
Sun 02/22 10Z 0.16 1412  62  53  -8 0.00
Sun 02/22 11Z 0.17 1412  56  59  -7 0.00
Sun 02/22 12Z 0.20 1413  51  65  -8 0.00
Sun 02/22 13Z 0.21 1413  50  70  -8 0.00
Sun 02/22 14Z 0.23 1313  52  71  -8 0.00
Sun 02/22 15Z 0.26 1314  53  70  -8 0.00
Sun 02/22 16Z 0.31 1314  54  67  -8 0.00
Sun 02/22 17Z 0.37 1215  57  69  -8 0.00
Sun 02/22 18Z 0.41 1214  64  70  -8 0.00
Sun 02/22 19Z 0.48 1215  70  71  -8 0.00
Sun 02/22 20Z 0.57 1215  74  72  -8 0.00
Sun 02/22 21Z 0.69 1116  76  71  -8 0.00
Sun 02/22 22Z 0.66 1116  79  69  -9 0.00
Sun 02/22 23Z 0.62 1117  81  68  -8 0.00
Mon 02/23 00Z 0.61 1018  80  66  -8 0.00
Mon 02/23 01Z 0.59 0920  77  62  -8 0.00
Mon 02/23 03Z 0.61 0922  72  40  -8 0.00
Mon 02/23 04Z 0.64 0922  68  40  -8 0.00
Mon 02/23 05Z 0.64 0924  58  53  -8 0.00
Mon 02/23 06Z 0.65 0826  54  56  -7 0.00
Mon 02/23 07Z 0.66 0827  52  57  -7 0.00
Mon 02/23 08Z 0.66 0728  49  64  -7 0.00
Mon 02/23 09Z 0.67 0731  44  64  -7 0.00
Mon 02/23 10Z 0.69 0733  40  71  -7 0.00
Mon 02/23 11Z 0.69 0735  35  71  -6 0.00
Mon 02/23 12Z 0.68 0635  35  71  -7 0.00
Mon 02/23 13Z 0.64 0633  36  70  -7 0.00
Mon 02/23 14Z 0.62 0531  38  73  -7 0.00
Mon 02/23 15Z 0.65 0529  48  77  -9 0.00
Mon 02/23 16Z 0.69 0426  67  82 -11 0.00
Mon 02/23 17Z 0.52 0426  78  84 -11 0.00
Mon 02/23 18Z 0.44 0327  82  86 -11 0.00
Mon 02/23 19Z 0.38 0228  88  86 -11 0.00
Mon 02/23 20Z 0.25 0129  89  86 -10 0.02
Mon 02/23 21Z 0.15 0131  90  86 -10 0.04
Mon 02/23 22Z 0.10 0032  90  86  -9 0.05
Mon 02/23 23Z 0.08 0033  88  88  -9 0.04

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