NAM3 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260113_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)
=== ===== === ==== ==== === === === ====
Tue 01/13 18Z 0.27 2214  90   9  -4 0.00
Tue 01/13 19Z 0.20 2015  85   6  -3 0.00
Tue 01/13 20Z 0.20 2018  83  16  -2 0.00
Tue 01/13 21Z 0.16 1921  81  26  -1 0.00
Tue 01/13 22Z 0.19 1925  81  35  -1 0.00
Tue 01/13 23Z 0.21 2028  75  46   0 0.00
Wed 01/14 00Z 0.29 2031  69  71   0 0.00
Wed 01/14 01Z 0.39 2035  67  89   0 0.00
Wed 01/14 02Z 0.43 2134  86  93  -1 0.00
Wed 01/14 03Z 0.52 2136  89  92  -2 0.01
Wed 01/14 04Z 0.58 2137  89  94  -2 0.03
Wed 01/14 05Z 0.68 2235  87  56  -3 0.01
Wed 01/14 06Z 0.70 2233  87  52  -2 0.00
Wed 01/14 07Z 0.69 2331  86  61  -2 0.00
Wed 01/14 08Z 0.65 2330  85  56  -2 0.00
Wed 01/14 09Z 0.59 2328  85  56  -1 0.00
Wed 01/14 10Z 0.52 2227  83  67  -1 0.00
Wed 01/14 11Z 0.49 2227  82  74   0 0.00
Wed 01/14 12Z 0.46 2227  80  74   0 0.00
Wed 01/14 13Z 0.44 2227  80  74   0 0.00
Wed 01/14 14Z 0.47 2228  80  72   0 0.00
Wed 01/14 16Z 0.54 2226  86  58   0 0.00
Wed 01/14 17Z 0.59 2324  87  48   0 0.00
Wed 01/14 18Z 0.61 2323  88  48  -1 0.00
Wed 01/14 20Z 0.54 2319  90  64  -1 0.00
Wed 01/14 21Z 0.48 2317  91  71  -2 0.00
Wed 01/14 22Z 0.38 2215  90  81  -1 0.00
Wed 01/14 23Z 0.32 2214  91  88  -1 0.00
Thu 01/15 00Z 0.25 2113  93  93  -1 0.00
Thu 01/15 01Z 0.12 2011  96  94  -1 0.00
Thu 01/15 02Z 0.04 1811  98  97  -1 0.00
Thu 01/15 03Z 0.03 1714  98  94  -1 0.01
Thu 01/15 04Z 0.07 1716  95  93   0 0.01
Thu 01/15 05Z 0.10 1618  90  95   0 0.03
Thu 01/15 06Z 0.10 1619  92  95   0 0.03
Thu 01/15 07Z 0.08 1720  97  95   0 0.02
Thu 01/15 08Z 0.07 1719  98  95   0 0.02
Thu 01/15 09Z 0.06 1719  98  95   0 0.03
Thu 01/15 10Z 0.05 1720  98  95  -1 0.04
Thu 01/15 11Z 0.06 1819  98  95  -1 0.05
Thu 01/15 12Z 0.09 1819  98  93  -1 0.05
Thu 01/15 13Z 0.15 1917  98  90  -1 0.05
Thu 01/15 14Z 0.24 2018  98  91  -1 0.05
Thu 01/15 15Z 0.33 2117  98  87  -2 0.03
Thu 01/15 16Z 0.39 2116  99  84  -3 0.02
Thu 01/15 17Z 0.30 2216  97  74  -3 0.01
Thu 01/15 18Z 0.32 2317  95  61  -5 0.02
Thu 01/15 19Z 0.39 2418  95  49  -7 0.02
Thu 01/15 20Z 0.55 2518  93  33 -10 0.01
Thu 01/15 21Z 1.01 2519  91  20 -12 0.01
Thu 01/15 22Z 1.81 2619  90  18 -14 0.01
Thu 01/15 23Z 2.33 2520  89  20 -15 0.01
Fri 01/16 00Z 2.43 2619  91  25 -16 0.01
Fri 01/16 01Z 4.79 2519  89  30 -17 0.00
Fri 01/16 02Z 2.55 2520  84  36 -18 0.00
Fri 01/16 03Z 1.60 2520  81  33 -18 0.00
Fri 01/16 04Z 1.61 2620  82  26 -19 0.00
Fri 01/16 05Z 2.21 2517  83  44 -19 0.00

Run total areal avg QPF for the greens is: 0.59 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