Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260404_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)
=== ===== === ==== ==== === === === ====
Sat 04/04 12Z 0.08 0112  78  10   4 0.00
Sat 04/04 13Z 0.11 0312  71   8   4 0.00
Sat 04/04 14Z 0.14 0511  65   7   5 0.00
Sat 04/04 15Z 0.16 0710  60   9   5 0.00
Sat 04/04 16Z 0.17 1010  61  16   5 0.00
Sat 04/04 17Z 0.18 1211  68  28   4 0.00
Sat 04/04 18Z 0.20 1415  78  41   3 0.00
Sat 04/04 19Z 0.27 1417  84  57   2 0.00
Sat 04/04 20Z 0.33 1520  87  76   2 0.00
Sat 04/04 21Z 0.35 1524  87  85   3 0.01
Sat 04/04 22Z 0.33 1526  79  94   4 0.00
Sat 04/04 23Z 0.24 1529  74  97   4 0.00
Sun 04/05 00Z 0.17 1632  83  93   4 0.01
Sun 04/05 01Z 0.11 1634  86  93   5 0.02
Sun 04/05 02Z 0.07 1735  90  82   7 0.02
Sun 04/05 03Z 0.06 1736  92  83   8 0.01
Sun 04/05 04Z 0.06 1739  94  89   8 0.01
Sun 04/05 05Z 0.06 1743  95  87   8 0.02
Sun 04/05 06Z 0.05 1746  96  82   9 0.02
Sun 04/05 07Z 0.05 1846  96  81   9 0.02
Sun 04/05 08Z 0.06 1845  96  80  10 0.02
Sun 04/05 09Z 0.11 1944  97  74  10 0.02
Sun 04/05 10Z 0.17 1943  97  71  10 0.03
Sun 04/05 11Z 0.29 2041  98  80   7 0.08
Sun 04/05 12Z 0.41 2234  98  94   6 0.13
Sun 04/05 13Z 0.40 2329  97  98   7 0.09
Sun 04/05 14Z 0.51 2329  97  86   6 0.08
Sun 04/05 15Z 0.59 2427  94  70   4 0.02
Sun 04/05 16Z 0.66 2526  96  56   2 0.01
Sun 04/05 17Z 0.78 2525  95  30   1 0.01
Sun 04/05 18Z 0.87 2524  93  23   0 0.01
Sun 04/05 19Z 1.23 2524  95  22  -1 0.01
Sun 04/05 20Z 1.66 2623  92  25  -1 0.01
Sun 04/05 21Z 2.35 2623  91  23  -2 0.01
Sun 04/05 22Z 2.64 2624  89  24  -3 0.00
Sun 04/05 23Z 2.42 2626  91  23  -4 0.00
Mon 04/06 00Z 2.08 2628  92  25  -5 0.00
Mon 04/06 01Z 1.82 2629  92  30  -5 0.00
Mon 04/06 02Z 1.74 2729  94  37  -6 0.00
Mon 04/06 03Z 1.56 2729  95  43  -6 0.00
Mon 04/06 04Z 1.62 2731  96  47  -7 0.01
Mon 04/06 05Z 1.70 2732  97  56  -8 0.01
Mon 04/06 06Z 1.50 2832  96  63  -8 0.01
Mon 04/06 07Z 1.39 2832  97  67  -8 0.01
Mon 04/06 08Z 1.43 2832  97  58  -8 0.01
Mon 04/06 09Z 1.67 2931  97  49  -9 0.01
Mon 04/06 10Z 1.66 2930  97  46  -9 0.01
Mon 04/06 11Z 1.52 2929  97  50 -10 0.00
Mon 04/06 12Z 1.61 2927  97  55 -10 0.01
Mon 04/06 13Z 1.67 2825  96  61 -10 0.00
Mon 04/06 14Z 1.78 2823  94  70 -10 0.00
Mon 04/06 15Z 3.69 2822  92  75 -10 0.00
Mon 04/06 16Z 6.24 2821  90  76 -10 0.00
Mon 04/06 17Z 10.03 2720  84  76  -9 0.00
Mon 04/06 18Z 12.64 2820  78  75  -9 0.00
Mon 04/06 19Z 10.42 2820  80  79  -8 0.00
Mon 04/06 20Z 7.15 2819  84  81  -8 0.00
Mon 04/06 21Z 6.22 2819  84  81  -8 0.00
Mon 04/06 22Z 3.62 2819  85  79  -8 0.00
Mon 04/06 23Z 2.09 2819  88  79  -9 0.00

Run total areal avg QPF for the greens is: 0.75 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