NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260218_1800
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)
=== ===== === ==== ==== === === === ====
Wed 02/18 18Z 0.17 3211  55  25  -6 0.00
Wed 02/18 19Z 0.11 3411  48  17  -6 0.00
Wed 02/18 20Z 0.07 3410  46  16  -6 0.00
Wed 02/18 21Z 0.06 3509  46  23  -6 0.00
Wed 02/18 22Z 0.05 0010  47  25  -5 0.00
Thu 02/19 00Z 0.10 0113  41  32  -5 0.00
Thu 02/19 01Z 0.14 0214  40  33  -6 0.00
Thu 02/19 02Z 0.18 0215  41  34  -6 0.00
Thu 02/19 03Z 0.17 0215  41  33  -6 0.00
Thu 02/19 04Z 0.14 0214  40  33  -7 0.00
Thu 02/19 05Z 0.13 0214  37  32  -7 0.00
Thu 02/19 06Z 0.10 0212  33  32  -6 0.00
Thu 02/19 07Z 0.11 0211  30  31  -6 0.00
Thu 02/19 08Z 0.10 0210  27  26  -6 0.00
Thu 02/19 09Z 0.09 0209  26  19  -6 0.00
Thu 02/19 10Z 0.07 0207  24  12  -6 0.00
Thu 02/19 11Z 0.08 0305  22   9  -6 0.00
Thu 02/19 12Z 0.07 0404  20  15  -5 0.00
Thu 02/19 13Z 0.06 0404  19  14  -5 0.00
Thu 02/19 14Z 0.05 0403  19  14  -5 0.00
Thu 02/19 15Z 0.04 0601  19  16  -4 0.00
Thu 02/19 16Z 0.02 1400  22  20  -4 0.00
Thu 02/19 17Z 0.02 1902  25  29  -4 0.00
Thu 02/19 18Z 0.04 2103  29  38  -4 0.00
Thu 02/19 19Z 0.07 2203  34  45  -3 0.00
Thu 02/19 20Z 0.11 2305  37  49  -4 0.00
Thu 02/19 21Z 0.12 2305  35  42  -3 0.00
Thu 02/19 22Z 0.12 2305  32  46  -3 0.00
Thu 02/19 23Z 0.12 2306  31  48  -3 0.00
Fri 02/20 00Z 0.10 2205  34  32  -2 0.00
Fri 02/20 01Z 0.08 2204  39  19  -2 0.00
Fri 02/20 02Z 0.09 2106  44   9  -3 0.00
Fri 02/20 03Z 0.09 2106  50  12  -2 0.00
Fri 02/20 04Z 0.13 2108  58  12  -3 0.00
Fri 02/20 05Z 0.14 2109  67  12  -3 0.00
Fri 02/20 06Z 0.13 2110  73  18  -2 0.00
Fri 02/20 07Z 0.10 2010  77  20  -2 0.00
Fri 02/20 08Z 0.08 2009  77  18  -1 0.00
Fri 02/20 09Z 0.07 2009  75  19  -1 0.00
Fri 02/20 10Z 0.04 1908  72  21  -1 0.00
Fri 02/20 11Z 0.02 1808  72  17  -1 0.00
Fri 02/20 12Z 0.02 1709  73  17  -1 0.00
Fri 02/20 13Z 0.04 1711  73  17  -1 0.00
Fri 02/20 14Z 0.05 1613  69  19  -1 0.00
Fri 02/20 15Z 0.08 1616  66  33  -1 0.00
Fri 02/20 16Z 0.08 1618  64  51  -1 0.00
Fri 02/20 17Z 0.08 1618  77  66  -1 0.00
Fri 02/20 18Z 0.08 1618  84  74  -1 0.01
Fri 02/20 19Z 0.13 1518  88  87   0 0.04
Fri 02/20 20Z 0.20 1419  92  96   0 0.08
Fri 02/20 21Z 0.27 1322  94  98   0 0.13
Fri 02/20 22Z 0.31 1324  96  98  -1 0.12
Fri 02/20 23Z 0.35 1326  95  98  -1 0.13
Sat 02/21 00Z 0.39 1327  95  96  -1 0.12
Sat 02/21 01Z 0.43 1327  96  93  -2 0.10
Sat 02/21 02Z 0.41 1325  96  90  -3 0.08
Sat 02/21 03Z 0.36 1419  97  92  -5 0.07
Sat 02/21 04Z 0.31 1417  95  94  -6 0.06
Sat 02/21 05Z 0.23 1515  95  94  -6 0.04

Run total areal avg QPF for the greens is: 0.98 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