NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20250410_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)
=== ===== === ==== ==== === === === ====
Thu 04/10 18Z 0.90 2004  69  40  -4 0.00
Thu 04/10 19Z 0.81 1905  65  56  -4 0.00
Thu 04/10 20Z 0.48 1806  65  75  -3 0.00
Thu 04/10 21Z 0.54 1608  67  90  -3 0.00
Thu 04/10 22Z 0.22 1710  71  92  -3 0.00
Thu 04/10 23Z 0.07 1712  75  90  -3 0.00
Fri 04/11 00Z 0.10 1814  79  90  -3 0.00
Fri 04/11 01Z 0.15 1815  83  93  -3 0.00
Fri 04/11 02Z 0.15 1914  87  94  -3 0.00
Fri 04/11 03Z 0.16 1914  91  95  -3 0.01
Fri 04/11 04Z 0.16 1914  94  95  -3 0.01
Fri 04/11 05Z 0.16 1916  96  97  -3 0.01
Fri 04/11 06Z 0.18 1917  98  98  -3 0.01
Fri 04/11 07Z 0.14 1917  96  94  -4 0.01
Fri 04/11 08Z 0.18 1918  95  95  -4 0.01
Fri 04/11 09Z 0.18 1919  95  93  -4 0.01
Fri 04/11 10Z 0.16 1918  90  91  -4 0.01
Fri 04/11 11Z 0.15 1917  84  91  -4 0.00
Fri 04/11 12Z 0.08 1916  77  91  -3 0.00
Fri 04/11 13Z 0.04 1815  74  83  -3 0.00
Fri 04/11 14Z 0.02 1814  72  84  -3 0.00
Fri 04/11 15Z 0.03 1714  72  76  -3 0.00
Fri 04/11 16Z 0.07 1712  74  71  -3 0.00
Fri 04/11 17Z 0.14 1710  80  71  -3 0.00
Fri 04/11 18Z 0.21 1709  84  72  -3 0.00
Fri 04/11 19Z 0.14 1709  87  77  -3 0.00
Fri 04/11 20Z 0.08 1608  90  77  -2 0.00
Fri 04/11 21Z 0.08 1608  90  74  -2 0.00
Fri 04/11 22Z 0.09 1609  89  79  -2 0.00
Fri 04/11 23Z 0.10 1509  87  78  -1 0.00
Sat 04/12 00Z 0.08 1608  87  77  -1 0.00
Sat 04/12 01Z 0.07 1607  86  78  -1 0.00
Sat 04/12 02Z 0.07 1506  84  75   0 0.00
Sat 04/12 03Z 0.08 1406  83  74   0 0.00
Sat 04/12 04Z 0.09 1406  81  77   0 0.00
Sat 04/12 05Z 0.10 1306  78  81  -1 0.00
Sat 04/12 06Z 0.12 1306  75  82  -1 0.00
Sat 04/12 07Z 0.15 1207  73  84  -1 0.00
Sat 04/12 08Z 0.17 1208  71  87  -1 0.00
Sat 04/12 09Z 0.19 1109  72  89  -1 0.00
Sat 04/12 10Z 0.21 1109  77  90  -1 0.00
Sat 04/12 11Z 0.24 1010  77  91  -1 0.00
Sat 04/12 12Z 0.27 1011  77  90  -1 0.00
Sat 04/12 13Z 0.29 1012  79  92  -1 0.00
Sat 04/12 14Z 0.32 1013  84  94  -1 0.01
Sat 04/12 15Z 0.30 0913  86  91  -1 0.01
Sat 04/12 16Z 0.30 0913  89  90  -1 0.01
Sat 04/12 17Z 0.31 0812  92  92  -1 0.02
Sat 04/12 18Z 0.34 0813  93  93  -1 0.03
Sat 04/12 19Z 0.35 0814  95  95  -1 0.05
Sat 04/12 20Z 0.37 0715  97  96  -2 0.06
Sat 04/12 21Z 0.39 0716  97  96  -2 0.06
Sat 04/12 22Z 0.42 0717  97  96  -2 0.04
Sat 04/12 23Z 0.46 0819  97  97  -2 0.03
Sun 04/13 00Z 0.44 0918  96  97  -1 0.02
Sun 04/13 01Z 0.39 0917  95  95  -1 0.01
Sun 04/13 02Z 0.34 1016  94  93   0 0.01
Sun 04/13 03Z 0.31 1015  95  93   0 0.01
Sun 04/13 04Z 0.29 0914  95  94   0 0.01
Sun 04/13 05Z 0.25 0912  96  91   0 0.01

Run total areal avg QPF for the greens is: 0.47 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