Subject: NAM3 EXPERIMENTAL Froude Number for the Northern_Greens

Model run: BTV_GRID_D2D_NAM3_20250402_1800
Using the MB925 to MB850 layer.
Assumes Northern_Greens are orthogonal to 290 degree winds.

925-850 mb Layer Average Wind/RH. Avg QPF over the Northern_Greens.
Date   Time        Wind RH% 700 850T QPF
Day mm/dd hhZ Frd# DDFF (%) RH% (C) (in)
=== ===== === ==== ==== === === === ====
Wed 04/02 18Z 0.05 1502  37  23  -7 0.00
Wed 04/02 19Z 0.11 1504  34  29  -6 0.00
Wed 04/02 20Z 0.14 1608  34  49  -5 0.00
Wed 04/02 21Z 0.17 1612  34  73  -4 0.00
Wed 04/02 22Z 0.24 1617  32  90  -3 0.00
Wed 04/02 23Z 0.18 1820  43  95  -4 0.00
Thu 04/03 00Z 0.18 1820  76  94  -5 0.00
Thu 04/03 01Z 0.22 1722  93  96  -3 0.04
Thu 04/03 02Z 0.24 1727  94  99  -3 0.04
Thu 04/03 03Z 0.28 1732  93  99  -2 0.03
Thu 04/03 04Z 0.29 1735  94  99  -3 0.03
Thu 04/03 05Z 0.28 1838  94  99  -2 0.06
Thu 04/03 06Z 0.24 1841  95  98  -1 0.07
Thu 04/03 07Z 0.16 1843  95  98   0 0.12
Thu 04/03 08Z 0.11 1946  95  97   2 0.18
Thu 04/03 09Z 0.14 1947  94  96   3 0.34
Thu 04/03 10Z 0.14 1950  95  95   3 0.30
Thu 04/03 11Z 0.05 1950  96  93   6 0.21
Thu 04/03 12Z 0.04 1950  97  97   8 0.14
Thu 04/03 13Z 0.06 2050  97  98   7 0.20
Thu 04/03 14Z 0.17 2148  97  90   9 0.11
Thu 04/03 15Z 0.29 2245  97  84   9 0.03
Thu 04/03 16Z 0.45 2344  97  89   8 0.03
Thu 04/03 17Z 0.59 2439  97  72   8 0.02
Thu 04/03 18Z 0.66 2434  96  58   7 0.02
Thu 04/03 19Z 0.64 2433  97  42   8 0.04
Thu 04/03 20Z 0.85 2534  98  27   7 0.05
Thu 04/03 21Z 0.91 2537  92   3   7 0.02
Thu 04/03 22Z 1.03 2536  79   5   7 0.00
Thu 04/03 23Z 1.60 2538  75   3   6 0.00
Fri 04/04 00Z 2.38 2541  78   2   5 0.00
Fri 04/04 01Z 1.90 2644  86   2   4 0.00
Fri 04/04 02Z 1.60 2641  91   3   3 0.00
Fri 04/04 03Z 1.56 2737  94  14   2 0.01
Fri 04/04 04Z 1.50 2838  93   7   1 0.03
Fri 04/04 05Z 1.68 2837  87   5   0 0.01
Fri 04/04 06Z 1.66 2835  83   7  -1 0.01
Fri 04/04 07Z 1.89 2834  79  11  -2 0.00
Fri 04/04 08Z 1.73 2832  82  13  -3 0.00
Fri 04/04 09Z 1.05 2830  83  13  -3 0.00
Fri 04/04 10Z 0.94 2831  86  14  -3 0.00
Fri 04/04 11Z 0.80 2832  88  17  -3 0.00
Fri 04/04 12Z 0.71 2931  88  17  -2 0.01
Fri 04/04 13Z 0.65 2930  87   6  -2 0.01
Fri 04/04 14Z 0.63 2930  83   3  -2 0.01
Fri 04/04 15Z 0.62 2931  79   1  -1 0.01
Fri 04/04 16Z 0.60 3031  74   1  -1 0.01
Fri 04/04 17Z 0.58 3031  73   1   0 0.00
Fri 04/04 18Z 0.56 3130  72   0   0 0.01
Fri 04/04 19Z 0.53 3130  71   0   0 0.00
Fri 04/04 20Z 0.50 3230  68   1   0 0.00
Fri 04/04 21Z 0.44 3230  65   1   0 0.00
Fri 04/04 22Z 0.40 3328  66   1   0 0.00
Fri 04/04 23Z 0.35 3328  61   1   1 0.00
Sat 04/05 00Z 0.28 3426  58   1   2 0.00
Sat 04/05 01Z 0.24 3425  57   2   2 0.01
Sat 04/05 02Z 0.17 3522  54   6   3 0.01
Sat 04/05 03Z 0.11 3520  49   7   3 0.01
Sat 04/05 04Z 0.06 0017  40   7   4 0.00
Sat 04/05 05Z 0.02 0213  37   8   4 0.00

Run total areal avg QPF for the Northern_Greens is: 2.23 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