NAM12 EXPERIMENTAL Froude Number for the Green Mountains

Subject: NAM12 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM12_20250813_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)
=== ===== === ==== ==== === === === ====
Wed 08/13 18Z 2.10 2607  83  86  17 0.00
Wed 08/13 21Z 0.29 2707  82  80  17 0.06
Thu 08/14 00Z 0.37 2809  84  84  16 0.04
Thu 08/14 03Z 0.34 2907  93  92  15 0.05
Thu 08/14 06Z 0.33 3215  92  82  15 0.09
Thu 08/14 09Z 0.07 3514  82  66  15 0.00
Thu 08/14 12Z 0.05 3513  79  56  13 0.00
Thu 08/14 15Z 0.10 3412  76  38  13 0.00
Thu 08/14 18Z 0.24 3411  77  23  12 0.00
Thu 08/14 21Z 0.20 3510  68   9  13 0.00
Fri 08/15 00Z 0.03 3510  59   3  12 0.00
Fri 08/15 03Z 0.04 0011  45   2  12 0.00
Fri 08/15 06Z 0.06 0109  33   3  12 0.00
Fri 08/15 09Z 0.07 0305  18   4  13 0.00
Fri 08/15 12Z 0.03 0901  17   4  12 0.00
Fri 08/15 15Z 0.04 2001  31   4  12 0.00
Fri 08/15 18Z 0.09 2301  47   4  13 0.00
Fri 08/15 21Z 0.09 1801  52   5  13 0.00
Sat 08/16 00Z 0.03 1806  59   5  13 0.00
Sat 08/16 03Z 0.29 2109  64   5  13 0.00
Sat 08/16 06Z 0.58 2409  72   5  12 0.00
Sat 08/16 09Z 0.40 2509  80   8  12 0.00
Sat 08/16 12Z 0.36 2508  86  29  12 0.00
Sat 08/16 15Z 0.42 2509  79  14  12 0.00
Sat 08/16 18Z 1.33 2305  85  40  13 0.00
Sat 08/16 21Z 1.14 2406  78  11  14 0.00
Sun 08/17 00Z 0.46 2509  82  17  14 0.00
Sun 08/17 03Z 0.49 2611  79  61  14 0.00
Sun 08/17 06Z 0.56 2814  67  57  15 0.00

Run total areal avg QPF for the greens is: 0.24 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