NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260401_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 04/01 18Z 0.26 3114  88  18   0 0.00
Wed 04/01 19Z 0.28 3116  82  20   0 0.00
Wed 04/01 20Z 0.22 3214  79  23  -1 0.00
Wed 04/01 21Z 0.14 3313  75  27  -1 0.00
Wed 04/01 22Z 0.09 3412  72  33  -1 0.00
Wed 04/01 23Z 0.04 3512  70  40  -1 0.00
Thu 04/02 00Z 0.05 3511  70  39  -1 0.00
Thu 04/02 01Z 0.06 0109  68  37  -1 0.00
Thu 04/02 02Z 0.11 0308  62  36  -1 0.00
Thu 04/02 03Z 0.11 0506  54  42   0 0.00
Thu 04/02 04Z 0.11 0705  52  41   0 0.00
Thu 04/02 05Z 0.12 0906  53  41   0 0.00
Thu 04/02 06Z 0.13 1106  56  39   0 0.00
Thu 04/02 07Z 0.12 1308  62  40  -1 0.00
Thu 04/02 08Z 0.10 1408  68  43  -1 0.00
Thu 04/02 09Z 0.11 1409  73  44  -1 0.00
Thu 04/02 10Z 0.13 1309  75  40   0 0.00
Thu 04/02 11Z 0.12 1410  78  41   0 0.00
Thu 04/02 12Z 0.12 1410  83  43  -1 0.00
Thu 04/02 13Z 0.11 1408  88  45  -1 0.00
Thu 04/02 14Z 0.13 1309  89  45  -1 0.00
Thu 04/02 15Z 0.15 1309  89  48  -1 0.00
Thu 04/02 16Z 0.18 1210  90  53  -1 0.00
Thu 04/02 17Z 0.23 1210  90  57  -1 0.00
Thu 04/02 18Z 0.22 1208  90  63  -1 0.00
Thu 04/02 19Z 0.25 1210  87  65  -1 0.00
Thu 04/02 20Z 0.24 1211  86  78  -1 0.00
Thu 04/02 21Z 0.24 1312  86  87   0 0.00
Thu 04/02 22Z 0.24 1314  86  91   0 0.00
Thu 04/02 23Z 0.26 1418  86  96   0 0.00
Fri 04/03 00Z 0.25 1421  84  91   0 0.00
Fri 04/03 01Z 0.20 1524  84  94   1 0.00
Fri 04/03 02Z 0.16 1627  86  96   2 0.01
Fri 04/03 03Z 0.09 1629  89  95   2 0.02
Fri 04/03 04Z 0.04 1730  93  86   3 0.03
Fri 04/03 05Z 0.04 1832  96  79   4 0.02
Fri 04/03 07Z 0.13 1935  97  59   6 0.02
Fri 04/03 08Z 0.17 1935  97  74   8 0.03
Fri 04/03 09Z 0.25 2036  97  93   8 0.04
Fri 04/03 10Z 0.31 2135  97  87   9 0.06
Fri 04/03 11Z 0.36 2237  97  77  10 0.08
Fri 04/03 13Z 0.44 2336  97  74  10 0.03
Fri 04/03 14Z 0.49 2336  96  76  10 0.02
Fri 04/03 15Z 0.50 2336  94  74  11 0.03
Fri 04/03 16Z 0.58 2437  96  72  11 0.05
Fri 04/03 17Z 0.63 2438  96  82  11 0.05
Fri 04/03 18Z 0.67 2437  94  80  11 0.05
Fri 04/03 19Z 0.76 2439  94  79  11 0.03
Fri 04/03 20Z 0.84 2439  92  80  11 0.01
Fri 04/03 21Z 0.95 2439  91  67  11 0.00
Fri 04/03 22Z 1.07 2440  92  44  10 0.00
Fri 04/03 23Z 1.19 2541  95  26   9 0.00
Sat 04/04 00Z 1.21 2542  97  19   9 0.00
Sat 04/04 01Z 1.28 2542  98  15   9 0.01
Sat 04/04 02Z 1.45 2640  99  14   8 0.01
Sat 04/04 03Z 1.30 2738  98  13   6 0.02
Sat 04/04 04Z 1.04 2834  95  12   4 0.04
Sat 04/04 05Z 0.68 2929  92  11   3 0.06

Run total areal avg QPF for the greens is: 0.72 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