Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20260115_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)
=== ===== === ==== ==== === === === ====
Thu 01/15 06Z 0.14 1512  97  91  -1 0.00
Thu 01/15 09Z 0.11 1615  95  90  -1 0.07
Thu 01/15 12Z 0.08 1813  93  75  -1 0.04
Thu 01/15 15Z 0.16 2014  97  91  -2 0.04
Thu 01/15 18Z 0.34 2215  97  53  -3 0.02
Thu 01/15 21Z 0.54 2319  93  15  -8 0.02
Fri 01/16 00Z 1.23 2619  85  33 -14 0.01
Fri 01/16 03Z 4.88 2519  84  49 -17 0.01
Fri 01/16 06Z 8.64 2719  86  76 -19 0.00
Fri 01/16 09Z 1.85 2821  83  84 -20 0.00
Fri 01/16 12Z 0.72 2822  86  65 -19 0.00
Fri 01/16 15Z 0.53 2823  79  21 -17 0.00
Fri 01/16 18Z 0.45 2822  51   5 -13 0.00
Fri 01/16 21Z 0.39 2817  38   9 -12 0.00
Sat 01/17 00Z 0.26 2611  32  74 -10 0.00
Sat 01/17 03Z 0.34 2116  57  88 -12 0.00
Sat 01/17 06Z 0.35 2120  89  88 -11 0.02
Sat 01/17 09Z 0.38 2122  88  86 -10 0.01
Sat 01/17 12Z 0.43 2223  90  66  -9 0.00
Sat 01/17 15Z 0.51 2322  89  67  -9 0.00
Sat 01/17 18Z 0.56 2318  86  74  -8 0.00
Sat 01/17 21Z 0.44 2215  83  72  -8 0.00
Sun 01/18 00Z 0.55 2319  87  84  -7 0.00
Sun 01/18 03Z 0.51 2317  85  77  -8 0.00
Sun 01/18 06Z 1.05 2522  86  80  -9 0.00
Sun 01/18 09Z 1.32 2721  87  75 -11 0.01
Sun 01/18 12Z 1.41 2618  84  25 -13 0.00
Sun 01/18 15Z 1.14 2518  83  21 -14 0.00
Sun 01/18 18Z 9.65 2517  84  39 -16 0.00

Run total areal avg QPF for the greens is: 0.25 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