National Weather Service United States Department of Commerce

Extreme Heat in the Southwest; Severe Weather in the Northern Plains; Heavy Rain in the East

Extreme heat continues across the Southwest as temperatures may top 120 degrees. The heat may expand across the West and south-central U.S. into the middle of the month. Severe thunderstorms capable of severe wind gusts and large hail will be the primary hazards today across the northern Plains into the upper Midwest. Rainfall could be locally heavy across the Eastern U.S. today. Read More >

Subject: NAM3 EXPERIMENTAL Froude Number for the greens

Model run: BTV_GRID_D2D_NAM3_20260803_1200
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 08/03 12Z 0.10 1820  97  91  15 0.00
Mon 08/03 13Z 0.16 1919  97  91  15 0.01
Mon 08/03 14Z 0.21 1920  98  82  15 0.01
Mon 08/03 15Z 0.29 2019  98  73  15 0.01
Mon 08/03 16Z 0.28 1918  97  66  16 0.01
Mon 08/03 17Z 0.33 1919  96  74  16 0.01
Mon 08/03 18Z 0.36 2019  96  90  16 0.15
Mon 08/03 19Z 0.68 2321  94  80  15 0.23
Mon 08/03 20Z 0.81 2420  93  46  14 0.04
Mon 08/03 21Z 1.04 2621  94  58  13 0.04
Mon 08/03 22Z 0.83 2723  97  77  12 0.06
Mon 08/03 23Z 0.87 3029  99  86  11 0.10
Tue 08/04 00Z 0.86 3133  98  94  10 0.11
Tue 08/04 01Z 0.84 3134  95  73  10 0.06
Tue 08/04 02Z 0.93 3135  87  25   9 0.01
Tue 08/04 03Z 0.84 3232  78  16  10 0.00
Tue 08/04 04Z 0.81 3129  85  18  10 0.00
Tue 08/04 05Z 0.91 3127  86   8  10 0.00
Tue 08/04 06Z 1.03 3026  86   5  10 0.00
Tue 08/04 07Z 0.95 3024  81   5  10 0.00
Tue 08/04 08Z 0.95 3023  77   5  10 0.00
Tue 08/04 09Z 0.92 2923  72   4  11 0.00
Tue 08/04 10Z 0.96 3024  66   3  11 0.00
Tue 08/04 11Z 0.78 3024  60   2  12 0.00
Tue 08/04 12Z 0.67 3023  57   1  12 0.00
Tue 08/04 13Z 0.61 3022  55   2  13 0.00
Tue 08/04 14Z 0.54 3020  53   2  13 0.00
Tue 08/04 15Z 0.48 2917  49   2  14 0.00
Tue 08/04 16Z 0.42 3015  45   1  14 0.00
Tue 08/04 17Z 0.38 2912  45   1  14 0.00
Tue 08/04 18Z 0.38 2810  50   2  14 0.00
Tue 08/04 19Z 0.37 2809  51   4  15 0.00
Tue 08/04 20Z 0.35 2708  50   7  15 0.00
Tue 08/04 21Z 0.32 2707  51  10  15 0.00
Tue 08/04 22Z 0.25 2606  51  14  15 0.00
Tue 08/04 23Z 0.20 2506  50  17  16 0.00
Wed 08/05 00Z 0.17 2406  49  18  16 0.00
Wed 08/05 01Z 0.18 2307  47  19  16 0.00
Wed 08/05 02Z 0.22 2209  45  21  17 0.00
Wed 08/05 03Z 0.21 2209  43  22  17 0.00
Wed 08/05 04Z 0.23 2310  43  22  17 0.00
Wed 08/05 05Z 0.22 2310  43  21  17 0.00
Wed 08/05 06Z 0.22 2409  42  19  17 0.00
Wed 08/05 07Z 0.22 2509  42  17  17 0.00
Wed 08/05 08Z 0.21 2508  45  14  17 0.00
Wed 08/05 09Z 0.20 2508  46  12  16 0.00
Wed 08/05 10Z 0.19 2407  47  10  16 0.00
Wed 08/05 11Z 0.18 2407  47   6  16 0.00
Wed 08/05 12Z 0.18 2407  47   2  16 0.00
Wed 08/05 13Z 0.22 2408  48   1  15 0.00
Wed 08/05 14Z 0.23 2408  49   1  15 0.00
Wed 08/05 15Z 0.22 2308  53   1  15 0.00
Wed 08/05 16Z 0.21 2207  55   2  15 0.00
Wed 08/05 17Z 0.19 2107  52   3  16 0.00
Wed 08/05 18Z 0.23 2109  50   7  16 0.00
Wed 08/05 19Z 0.25 2110  50  10  16 0.00
Wed 08/05 20Z 0.28 2111  52  20  16 0.00
Wed 08/05 21Z 0.29 2111  51  26  16 0.00
Wed 08/05 22Z 0.30 2212  48  32  17 0.00
Wed 08/05 23Z 0.31 2212  48  34  17 0.00

Run total areal avg QPF for the greens is: 0.85 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