NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260326_0000
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 03/26 00Z 0.06 1818  57  73  -2 0.00
Thu 03/26 01Z 0.18 1824  55  75  -2 0.00
Thu 03/26 02Z 0.35 1928  56  90  -2 0.00
Thu 03/26 03Z 0.90 2031  70  91  -3 0.00
Thu 03/26 04Z 0.56 2034  88  92  -2 0.01
Thu 03/26 05Z 0.64 2134  89  94  -1 0.01
Thu 03/26 06Z 0.65 2234  81  95  -1 0.00
Thu 03/26 07Z 0.67 2235  75  94   0 0.01
Thu 03/26 08Z 0.63 2232  73  95   0 0.01
Thu 03/26 09Z 0.72 2333  76  94   0 0.00
Thu 03/26 10Z 0.69 2431  90  93   0 0.02
Thu 03/26 11Z 0.66 2429  89  94   0 0.02
Thu 03/26 12Z 0.61 2429  87  96   1 0.02
Thu 03/26 13Z 0.68 2432  90  95   1 0.04
Thu 03/26 14Z 0.74 2431  88  88   1 0.03
Thu 03/26 15Z 0.72 2528  84  54   1 0.01
Thu 03/26 16Z 0.81 2527  80  49   2 0.00
Thu 03/26 17Z 0.92 2524  82  65   2 0.00
Thu 03/26 18Z 1.03 2523  82  74   1 0.00
Thu 03/26 19Z 1.24 2523  82  85   2 0.00
Thu 03/26 20Z 1.30 2522  85  93   1 0.02
Thu 03/26 21Z 0.86 2518  87  95   1 0.03
Thu 03/26 22Z 0.51 2613  89  95   1 0.05
Thu 03/26 23Z 0.37 2810  97  95   0 0.12
Fri 03/27 00Z 0.26 3008  96  97   1 0.08
Fri 03/27 01Z 0.30 3114  97  97   0 0.16
Fri 03/27 02Z 0.17 3211  96  98   1 0.08
Fri 03/27 03Z 0.19 3315  96  95   0 0.07
Fri 03/27 04Z 0.27 3217  97  98  -1 0.02
Fri 03/27 05Z 0.29 3220  97  83  -2 0.02
Fri 03/27 06Z 0.35 3223  97  78  -4 0.02
Fri 03/27 07Z 0.39 3226  96  42  -5 0.02
Fri 03/27 08Z 0.35 3326  96   7  -6 0.01
Fri 03/27 09Z 0.38 3328  96   8  -8 0.01
Fri 03/27 10Z 0.37 3329  94   6 -10 0.00
Fri 03/27 11Z 0.36 3329  90   5 -11 0.00
Fri 03/27 12Z 0.31 3329  81   5 -11 0.00
Fri 03/27 13Z 0.28 3327  69   6 -12 0.00
Fri 03/27 14Z 0.25 3326  63   4 -12 0.00
Fri 03/27 15Z 0.23 3323  60   3 -13 0.00
Fri 03/27 16Z 0.23 3321  60   2 -13 0.00
Fri 03/27 17Z 0.34 3318  60   3 -13 0.00
Fri 03/27 18Z 0.60 3315  59   4 -13 0.00
Fri 03/27 19Z 0.84 3314  56   4 -13 0.00
Fri 03/27 20Z 0.87 3212  52   5 -13 0.00
Fri 03/27 21Z 0.35 3212  47   6 -13 0.00
Fri 03/27 22Z 0.21 3311  43   7 -12 0.00
Fri 03/27 23Z 0.20 3210  43   8 -12 0.00
Sat 03/28 00Z 0.20 3210  46   9 -12 0.00
Sat 03/28 01Z 0.19 3210  51  10 -12 0.00
Sat 03/28 02Z 0.21 3110  58   9 -12 0.00
Sat 03/28 03Z 0.22 3111  61   8 -13 0.00
Sat 03/28 04Z 0.18 3211  62   8 -13 0.00
Sat 03/28 05Z 0.14 3310  63  10 -13 0.00
Sat 03/28 06Z 0.12 3309  64  13 -14 0.00
Sat 03/28 07Z 0.13 3309  64  13 -15 0.00
Sat 03/28 08Z 0.14 3309  65  21 -15 0.00
Sat 03/28 09Z 0.11 3308  66  27 -16 0.00
Sat 03/28 10Z 0.11 3308  67  34 -16 0.00
Sat 03/28 11Z 0.13 3308  69  46 -16 0.00

Run total areal avg QPF for the greens is: 0.89 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