NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260113_0000
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 00Z 1.40 2425  92  82 -10 0.00
Tue 01/13 03Z 1.24 2630  88  78  -9 0.00
Tue 01/13 06Z 0.97 2627  94  83  -9 0.01
Tue 01/13 09Z 0.72 2623  92  87  -7 0.03
Tue 01/13 12Z 0.59 2622  92  38  -6 0.02
Tue 01/13 15Z 0.51 2618  90  19  -6 0.00
Tue 01/13 18Z 0.31 2316  79  16  -3 0.00
Tue 01/13 21Z 0.21 2021  79  47   0 0.00
Wed 01/14 00Z 0.37 2132  72  78   0 0.00
Wed 01/14 03Z 0.37 2134  85  87   0 0.01
Wed 01/14 06Z 0.61 2136  97  91  -3 0.08
Wed 01/14 09Z 0.61 2334  85  78  -1 0.02
Wed 01/14 12Z 0.58 2331  81  56   0 0.00
Wed 01/14 15Z 0.59 2327  87  47  -1 0.00
Wed 01/14 18Z 0.47 2319  89  82  -2 0.00
Wed 01/14 21Z 0.25 2114  95  93  -2 0.00
Thu 01/15 00Z 0.08 1912  97  92  -2 0.05
Thu 01/15 03Z 0.16 2012  97  90  -3 0.09
Thu 01/15 06Z 0.22 2111  97  89  -3 0.06
Thu 01/15 09Z 0.50 2714  95  88  -6 0.07
Thu 01/15 12Z 0.87 2822  89  81 -13 0.04
Thu 01/15 15Z 7.52 2719  89  57 -17 0.02
Thu 01/15 18Z 11.73 2621  84  83 -19 0.01
Thu 01/15 21Z 2.14 2720  87  74 -16 0.01
Fri 01/16 00Z 9.02 2725  79  80 -24 0.01
Fri 01/16 03Z 1.46 2724  81  53 -25 0.00
Fri 01/16 06Z 0.69 2724  65  49 -23 0.00
Fri 01/16 09Z 0.55 2724  65  37 -19 0.00
Fri 01/16 12Z 0.66 2527  82  17 -17 0.00

Run total areal avg QPF for the greens is: 0.53 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