Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260219_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)
=== ===== === ==== ==== === === === ====
Thu 02/19 12Z 0.06 0702  48  23  -6 0.00
Thu 02/19 13Z 0.06 0702  43  15  -5 0.00
Thu 02/19 14Z 0.05 0702  42   9  -5 0.00
Thu 02/19 15Z 0.04 0901  41   6  -5 0.00
Thu 02/19 16Z 0.04 1302  42   6  -5 0.00
Thu 02/19 17Z 0.02 1802  44   7  -5 0.00
Thu 02/19 18Z 0.04 1603  43  11  -4 0.00
Thu 02/19 19Z 0.02 1704  45  17  -4 0.00
Thu 02/19 20Z 0.04 1904  49  25  -4 0.00
Thu 02/19 21Z 0.07 2004  52  36  -4 0.00
Thu 02/19 22Z 0.05 2003  52  42  -4 0.00
Thu 02/19 23Z 0.05 2103  53  38  -4 0.00
Fri 02/20 00Z 0.03 2002  54  35  -4 0.00
Fri 02/20 01Z 0.04 1904  56  30  -4 0.00
Fri 02/20 03Z 0.10 2007  68  26  -4 0.00
Fri 02/20 04Z 0.15 2108  73  32  -4 0.00
Fri 02/20 05Z 0.15 2108  78  28  -4 0.00
Fri 02/20 06Z 0.15 2108  81  22  -4 0.00
Fri 02/20 07Z 0.11 2107  81  17  -3 0.00
Fri 02/20 08Z 0.09 2106  81  19  -3 0.00
Fri 02/20 09Z 0.07 2105  80  16  -3 0.00
Fri 02/20 10Z 0.02 1903  77  19  -2 0.00
Fri 02/20 11Z 0.03 1603  75  24  -1 0.00
Fri 02/20 12Z 0.05 1505  74  25  -2 0.00
Fri 02/20 13Z 0.06 1507  74  23  -2 0.00
Fri 02/20 14Z 0.07 1509  75  24  -2 0.00
Fri 02/20 15Z 0.07 1609  75  28  -2 0.00
Fri 02/20 16Z 0.05 1611  74  33  -2 0.00
Fri 02/20 17Z 0.04 1711  73  34  -2 0.00
Fri 02/20 18Z 0.06 1611  70  32  -1 0.00
Fri 02/20 19Z 0.10 1512  65  45   0 0.00
Fri 02/20 20Z 0.13 1513  62  62   0 0.00
Fri 02/20 21Z 0.20 1415  61  76   0 0.00
Fri 02/20 22Z 0.26 1318  69  87   0 0.00
Fri 02/20 23Z 0.29 1320  73  94  -1 0.00
Sat 02/21 00Z 0.31 1321  83  95  -2 0.01
Sat 02/21 01Z 0.33 1322  92  97  -3 0.04
Sat 02/21 02Z 0.29 1320  95  97  -3 0.07
Sat 02/21 03Z 0.27 1319  96  97  -3 0.08
Sat 02/21 04Z 0.27 1317  95  98  -4 0.06
Sat 02/21 05Z 0.26 1415  95  98  -5 0.09
Sat 02/21 06Z 0.22 1413  95  97  -6 0.05
Sat 02/21 07Z 0.17 1511  95  95  -6 0.03
Sat 02/21 08Z 0.10 1610  95  93  -7 0.02
Sat 02/21 09Z 0.05 1709  95  92  -7 0.02
Sat 02/21 10Z 0.05 1807  95  92  -7 0.02
Sat 02/21 11Z 0.06 1906  95  93  -8 0.01
Sat 02/21 12Z 0.07 1906  95  91  -8 0.01
Sat 02/21 13Z 0.05 1905  96  89  -8 0.01
Sat 02/21 14Z 0.05 1904  97  89  -8 0.02
Sat 02/21 15Z 0.06 1903  97  90  -8 0.01
Sat 02/21 16Z 0.07 2003  97  92  -8 0.01
Sat 02/21 17Z 0.07 2102  97  91  -8 0.01
Sat 02/21 18Z 0.06 2201  97  88  -8 0.01
Sat 02/21 19Z 0.06 1700  97  86  -8 0.00
Sat 02/21 20Z 0.06 1000  97  86  -8 0.00
Sat 02/21 21Z 0.08 1001  97  88  -8 0.00
Sat 02/21 22Z 0.11 1003  96  89  -8 0.00
Sat 02/21 23Z 0.17 1004  94  90  -8 0.00

Run total areal avg QPF for the greens is: 0.58 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