Subject: NAM3 EXPERIMENTAL Froude Number for the Central_Greens

Model run: BTV_GRID_D2D_NAM3_20260220_1800
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)
=== ===== === ==== ==== === === === ====
Fri 02/20 18Z 0.09 1515  82  87  -1 0.00
Fri 02/20 19Z 0.17 1419  90  92   0 0.02
Fri 02/20 20Z 0.19 1421  95  95  -1 0.06
Fri 02/20 21Z 0.21 1321  96  97  -1 0.10
Sat 02/21 00Z 0.36 1223  96  99  -3 0.10
Sat 02/21 01Z 0.34 1321  96  93  -4 0.13
Sat 02/21 02Z 0.29 1317  96  95  -5 0.17
Sat 02/21 03Z 0.22 1415  95  92  -6 0.12
Sat 02/21 04Z 0.14 1513  95  89  -6 0.12
Sat 02/21 05Z 0.06 1610  96  90  -6 0.06
Sat 02/21 06Z 0.05 1708  96  91  -7 0.03
Sat 02/21 07Z 0.06 1806  96  91  -7 0.03
Sat 02/21 08Z 0.12 2006  96  94  -7 0.03
Sat 02/21 09Z 0.21 2106  97  94  -7 0.02
Sat 02/21 10Z 0.26 2407  99  93  -7 0.02
Sat 02/21 11Z 0.26 2707  99  91  -7 0.02
Sat 02/21 12Z 0.24 2907  99  89  -7 0.02
Sat 02/21 13Z 0.21 3007  99  89  -7 0.02
Sat 02/21 14Z 0.19 3107  99  88  -7 0.02
Sat 02/21 15Z 0.12 3207  98  85  -7 0.01
Sat 02/21 16Z 0.08 3305  98  81  -8 0.01
Sat 02/21 17Z 0.07 3405  98  76  -8 0.00
Sat 02/21 18Z 0.07 3504  98  72  -8 0.00
Sat 02/21 19Z 0.07 3504  97  70  -8 0.00
Sat 02/21 20Z 0.05 3505  95  63  -8 0.00
Sat 02/21 21Z 0.05 0005  93  58  -8 0.00
Sat 02/21 22Z 0.08 0105  91  54  -8 0.00
Sat 02/21 23Z 0.14 0306  87  53  -8 0.00
Sun 02/22 00Z 0.17 0407  79  53  -8 0.00
Sun 02/22 01Z 0.18 0507  68  43  -8 0.00
Sun 02/22 02Z 0.19 0707  65  34  -8 0.00
Sun 02/22 03Z 0.17 0906  67  28  -8 0.00
Sun 02/22 04Z 0.16 1007  64  25  -8 0.00
Sun 02/22 05Z 0.16 1108  56  24  -8 0.00
Sun 02/22 06Z 0.15 1209  54  26  -7 0.00
Sun 02/22 07Z 0.15 1311  55  28  -7 0.00
Sun 02/22 08Z 0.14 1412  54  29  -7 0.00
Sun 02/22 09Z 0.14 1312  52  29  -7 0.00
Sun 02/22 10Z 0.16 1312  50  29  -7 0.00
Sun 02/22 11Z 0.20 1213  53  27  -7 0.00
Sun 02/22 12Z 0.22 1214  57  25  -7 0.00
Sun 02/22 13Z 0.26 1215  60  23  -7 0.00
Sun 02/22 14Z 0.23 1316  63  29  -7 0.00
Sun 02/22 15Z 0.23 1316  65  35  -8 0.00
Sun 02/22 16Z 0.29 1215  64  34  -7 0.00
Sun 02/22 17Z 0.32 1216  65  33  -7 0.00
Sun 02/22 18Z 0.36 1216  68  37  -8 0.00
Sun 02/22 19Z 0.41 1216  74  37  -8 0.00
Sun 02/22 20Z 0.46 1215  78  35  -8 0.00
Sun 02/22 21Z 0.48 1115  80  33  -8 0.00
Sun 02/22 22Z 0.47 1115  78  32  -8 0.00
Sun 02/22 23Z 0.47 1016  71  32  -7 0.00
Mon 02/23 00Z 0.49 0917  67  39  -8 0.00
Mon 02/23 01Z 0.50 0918  67  45  -8 0.00
Mon 02/23 02Z 0.49 0920  60  35  -7 0.00
Mon 02/23 03Z 0.49 0922  52  38  -7 0.00
Mon 02/23 04Z 0.52 0924  48  35  -7 0.00
Mon 02/23 05Z 0.55 0825  47  38  -7 0.00

Run total areal avg QPF for the Central_Greens is: 1.11 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