NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260430_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)
=== ===== === ==== ==== === === === ====
Thu 04/30 12Z 0.15 1916  98  97   3 0.00
Thu 04/30 13Z 0.20 2015  97  95   3 0.01
Thu 04/30 14Z 0.25 2113  98  92   2 0.01
Thu 04/30 15Z 0.28 2211  99  92   2 0.01
Thu 04/30 16Z 0.33 2310  99  91   2 0.01
Thu 04/30 17Z 0.36 2509  99  92   1 0.02
Thu 04/30 18Z 0.38 2708  99  91   1 0.02
Thu 04/30 19Z 0.35 2908  97  90   1 0.01
Thu 04/30 20Z 0.27 2907  95  87   1 0.01
Thu 04/30 21Z 0.26 3007  94  84   1 0.01
Thu 04/30 22Z 0.39 3010  94  76   0 0.01
Thu 04/30 23Z 0.39 3112  95  61   0 0.01
Fri 05/01 00Z 0.42 3115  95  54   0 0.01
Fri 05/01 01Z 0.46 3117  95  55   0 0.01
Fri 05/01 02Z 0.45 3117  95  60  -1 0.01
Fri 05/01 03Z 0.42 3117  94  67  -1 0.01
Fri 05/01 04Z 0.38 3117  93  69  -1 0.01
Fri 05/01 05Z 0.37 3218  93  68  -1 0.01
Fri 05/01 06Z 0.36 3218  91  71  -2 0.01
Fri 05/01 07Z 0.34 3218  87  78  -2 0.00
Fri 05/01 08Z 0.36 3219  83  79  -2 0.00
Fri 05/01 09Z 0.36 3220  81  76  -2 0.00
Fri 05/01 10Z 0.36 3220  80  65  -2 0.00
Fri 05/01 11Z 0.38 3221  79  62  -3 0.00
Fri 05/01 12Z 0.40 3220  79  53  -3 0.00
Fri 05/01 13Z 0.48 3118  80  44  -4 0.00
Fri 05/01 14Z 1.43 3117  79  41  -4 0.00
Fri 05/01 15Z 3.28 3115  76  47  -4 0.00
Fri 05/01 16Z 4.21 3112  72  47  -3 0.00
Fri 05/01 17Z 4.88 3011  69  47  -2 0.00
Fri 05/01 18Z 5.37 3010  67  45  -1 0.00
Fri 05/01 19Z 4.90 3008  64  35  -1 0.00
Fri 05/01 20Z 4.65 2908  63  34   0 0.00
Fri 05/01 21Z 4.24 2907  65  38   0 0.00
Fri 05/01 22Z 3.06 2807  67  41   0 0.00
Fri 05/01 23Z 1.15 2706  68  54   0 0.00
Sat 05/02 00Z 0.60 2706  69  63   0 0.00
Sat 05/02 01Z 0.52 2706  70  71   0 0.00
Sat 05/02 02Z 0.41 2604  71  77   0 0.00
Sat 05/02 03Z 0.20 2702  72  80   0 0.00
Sat 05/02 04Z 0.15 0700  72  83   0 0.00
Sat 05/02 05Z 0.28 0904  75  90  -1 0.00
Sat 05/02 06Z 0.21 0904  83  92  -1 0.03
Sat 05/02 07Z 0.17 1005  91  92  -1 0.07
Sat 05/02 08Z 0.16 1205  93  91  -2 0.06
Sat 05/02 09Z 0.13 1305  95  90  -2 0.03
Sat 05/02 10Z 0.11 1505  97  88  -2 0.02
Sat 05/02 11Z 0.12 1705  99  83  -2 0.02
Sat 05/02 12Z 0.13 1904  99  82  -2 0.01
Sat 05/02 13Z 0.13 2104  99  81  -2 0.01
Sat 05/02 14Z 0.16 2304  99  84  -3 0.01
Sat 05/02 15Z 0.21 2505  99  77  -3 0.00
Sat 05/02 16Z 0.26 2706  98  71  -3 0.00
Sat 05/02 17Z 0.27 2906  97  61  -3 0.00
Sat 05/02 18Z 0.32 3105  94  48  -2 0.00
Sat 05/02 19Z 0.47 3104  90  35  -2 0.00
Sat 05/02 20Z 0.58 3103  86  30  -1 0.00
Sat 05/02 21Z 0.82 3004  84  40  -1 0.00
Sat 05/02 22Z 0.60 3005  86  63  -1 0.00
Sat 05/02 23Z 0.25 3206  87  69  -1 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