Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260119_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)
=== ===== === ==== ==== === === === ====
Mon 01/19 18Z 3.41 2212  87  71 -14 0.00
Mon 01/19 21Z 1.34 2321  89  86 -14 0.00
Tue 01/20 00Z 1.61 2430  82  67 -14 0.00
Tue 01/20 03Z 8.88 2431  80  59 -16 0.00
Tue 01/20 06Z 1.44 2429  83  50 -17 0.00
Tue 01/20 09Z 1.72 2628  89  56 -17 0.00
Tue 01/20 12Z 3.04 2624  81  63 -20 0.00
Tue 01/20 15Z 4.97 2720  80  64 -21 0.00
Tue 01/20 18Z 8.64 2618  80  70 -21 0.00
Tue 01/20 21Z 4.41 2622  80  60 -20 0.00
Wed 01/21 00Z 2.10 2626  83  48 -21 0.00
Wed 01/21 03Z 1.20 2728  77  47 -20 0.00
Wed 01/21 06Z 1.00 2627  78  41 -20 0.00
Wed 01/21 09Z 0.83 2626  82  17 -19 0.00
Wed 01/21 12Z 0.72 2526  74  13 -17 0.00
Wed 01/21 15Z 0.53 2423  59  24 -16 0.00
Wed 01/21 18Z 0.32 2022  46  86 -13 0.00
Wed 01/21 21Z 0.21 1931  76  85 -10 0.01
Thu 01/22 00Z 0.40 2044  91  89 -10 0.06
Thu 01/22 03Z 0.63 2143  93  89  -8 0.04
Thu 01/22 06Z 0.76 2338  89  71  -8 0.01
Thu 01/22 09Z 0.99 2436  85  55  -9 0.00
Thu 01/22 12Z 1.40 2534  87  64 -11 0.00
Thu 01/22 15Z 0.97 2436  87  63  -9 0.00
Thu 01/22 18Z 1.88 2424  87  89 -11 0.00
Thu 01/22 21Z 2.06 2428  88  83 -11 0.01
Fri 01/23 00Z 2.21 2532  84  50 -12 0.01
Fri 01/23 03Z 2.48 2532  86  51 -14 0.01
Fri 01/23 06Z 3.20 2532  82  55 -17 0.01

Run total areal avg QPF for the greens is: 0.16 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