NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260112_0600
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)
=== ===== === ==== ==== === === === ====
Mon 01/12 06Z 2.81 2828  88  75 -16 0.00
Mon 01/12 09Z 1.00 2939  90  37 -14 0.00
Mon 01/12 12Z 0.73 3037  88  18 -12 0.00
Mon 01/12 15Z 0.86 2933  90  82 -11 0.00
Mon 01/12 18Z 1.09 2631  94  79  -9 0.00
Mon 01/12 21Z 1.56 2632  95  79  -9 0.01
Tue 01/13 00Z 1.50 2630  94  84 -10 0.01
Tue 01/13 03Z 1.11 2628  94  74  -9 0.01
Tue 01/13 06Z 0.87 2727  93  68  -8 0.01
Tue 01/13 09Z 0.72 2723  92  78  -7 0.01
Tue 01/13 12Z 0.67 2621  94  32  -7 0.00
Tue 01/13 15Z 0.51 2617  92  14  -6 0.01
Tue 01/13 18Z 0.28 2313  77  12  -4 0.00
Tue 01/13 21Z 0.15 2018  67  26   0 0.00
Wed 01/14 00Z 0.18 1929  70  80   0 0.00
Wed 01/14 03Z 0.35 2038  80  92   0 0.00
Wed 01/14 06Z 0.65 2138  97  91  -3 0.08
Wed 01/14 09Z 0.70 2333  94  50  -2 0.00
Wed 01/14 12Z 0.69 2331  94  51  -2 0.00
Wed 01/14 15Z 0.72 2329  95  85  -3 0.00
Wed 01/14 18Z 0.80 2324  97  87  -4 0.01
Wed 01/14 21Z 0.78 2419  96  75  -4 0.00
Thu 01/15 00Z 0.63 2414  96  86  -5 0.01
Thu 01/15 03Z 0.74 2419  96  83  -4 0.01
Thu 01/15 06Z 0.13 3106  96  94  -5 0.00
Thu 01/15 09Z 0.07 3511  93  90  -6 0.03
Thu 01/15 12Z 0.07 0012  90  90  -7 0.04
Thu 01/15 15Z 0.04 3514  87  90  -9 0.04
Thu 01/15 18Z 0.11 3417  84  90 -14 0.07

Run total areal avg QPF for the greens is: 0.35 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