NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260117_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)
=== ===== === ==== ==== === === === ====
Sat 01/17 18Z 0.24 2019  94  86  -7 0.00
Sat 01/17 21Z 0.46 2319  95  86  -7 0.02
Sun 01/18 00Z 0.60 2621  89  71  -7 0.01
Sun 01/18 03Z 0.64 2522  83  57  -6 0.00
Sun 01/18 06Z 0.82 2425  90  61  -7 0.00
Sun 01/18 09Z 0.84 2424  90  64  -8 0.00
Sun 01/18 12Z 0.81 2422  82  84  -9 0.00
Sun 01/18 15Z 1.41 2420  89  78 -11 0.00
Sun 01/18 18Z 1.30 2311  89  73 -11 0.00
Sun 01/18 21Z 0.24 2504  84  96 -11 0.00
Mon 01/19 00Z 0.09 0500  80  78 -10 0.00
Mon 01/19 03Z 0.06 0204  84  83 -10 0.00
Mon 01/19 06Z 0.03 0008  87  94 -11 0.00
Mon 01/19 09Z 0.03 3404  89  79 -11 0.01
Mon 01/19 12Z 0.10 2304  87  47 -11 0.00
Mon 01/19 15Z 0.27 2212  87  82 -12 0.00
Mon 01/19 18Z 3.71 2217  90  78 -14 0.01
Mon 01/19 21Z 2.81 2324  88  82 -14 0.00
Tue 01/20 00Z 1.75 2329  86  83 -14 0.00
Tue 01/20 03Z 4.59 2433  86  70 -15 0.00
Tue 01/20 06Z 1.65 2535  83  60 -16 0.00
Tue 01/20 09Z 3.01 2632  73  49 -19 0.00
Tue 01/20 12Z 1.82 2729  73  37 -21 0.00
Tue 01/20 15Z 1.85 2631  76  49 -18 0.00
Tue 01/20 18Z 11.55 2720  71  71 -22 0.00
Tue 01/20 21Z 6.44 2620  75  71 -21 0.00
Wed 01/21 00Z 2.22 2624  80  55 -21 0.00
Wed 01/21 03Z 1.25 2726  80  51 -21 0.00
Wed 01/21 06Z 1.02 2726  76  46 -21 0.00

Run total areal avg QPF for the greens is: 0.05 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