Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
254 am CDT Monday Aug 3 2015
248 am CDT
Complex of thunderstorms has pressed southeast of the forecast area
early this morning...mainly across central Indiana. The frontal
boundary will slowly progress southeast throughout the morning.
The gradient behind the boundary remains very minimal...however by
late morning the mixing heights will increase and produce gusts to
Middle-level trough axis will remain over the Great Lakes
region...keeping the surface ridge axis displaced west and stretching
from the Canadian rockies southeast through the middle-Missouri Valley.
Baroclinic zone will be much further south...as cooler air aloft
begins to filter south across the forecast area. Drier air will
continue to advect across the region with abundant sunshine
expected. Most locations should have afternoon temperatures warm into the upper
70s to the low 80s.
Weak lobe of vorticity pivots around the trough late tonight...but
it appears the associated moisture/precipitation will remain well
north/northeast of the forecast area. Plenty of dry air remains with
a continued influence from the anti-cyclonic feature located to the
west. Temperatures should easily radiate into the middle/upper 50s away from Lake
Michigan...and likely remain in the low 60s closer to the lake.
500mb trough axis begins to relax to the north...which
will allow the surface ridge across the middle-Missouri Valley to shift
east during the day. This will allow a light gradient and minimal
mixing for Tuesday. As a result expect surface winds to turn north and
likely see a lake breeze develop in the late afternoon hours. Highs
will approach 80 to the low 80s...however closer to the Lake
Michigan shore highs will hinge upon the timing of the lake breeze.
Current indications are that the timing for onshore winds will be
beyond peak heating...however if this occurs earlier in the day then
temperatures may struggle to warm beyond the middle 70s along the Lakeshore.
253 am CDT
Tuesday night through Sunday...
The main upper low...centered near the James Bay...is expected to
shift east-northward by midweek...essentially allowing the middle/upper
level flow over the region to steer the disturbances...currently
over the western Continental U.S....across the central Mississippi Valley.
However...the current thinking is that much of this precipitation
will remain to the south of much of my area. I have decided to go
mainly dry on Wednesday...especially north of Interstate 80 as a dry
east-southeasterly low level flow should keep the threat of rain out
of much of the area. Model guidance continues to diverge on the
evolution of one of the main middle-level impulses as it shifts over
the middle-Mississippi Valley late Wednesday and night. However...it
appears that the main focus for showers may continue to remain south
Wednesday night into Thursday morning. Therefore...I have focused
the highest chance probability of precipitation across my central Illinois counties...with
only slight chances probability of precipitation far north.
Later in the period it appears additional disturbances will continue
to shift eastward atop a ridge axis over the plains. This will
produce some chances of a period or two of some showers and
thunderstorms over the area again next weekend.
Temperatures through the period look to remain near...or a few
degrees below normal...with highs general near 80.
//Ord and mdw concerns...updated 06z...
* mainly northwest winds but at times variable...speeds will
remain between 3-6kt overnight. Speeds increase begin to
increase after daybreak...gusts nearing 20kt by 16z.
* A couple patches of broken ceilings around 4kft above ground level through 09z...improving
to thin cirrus and VFR conds after 9z.
Line of storms have pushed south of the taf sites with the frontal
boundary slowly sliding southeast. Winds have mainly switched to
northwest behind the boundary and diminished to less than 9kt.
Winds may become variable for a period early this morning...then
solidify from the northwest by daybreak. Mixing will increase by
16z and gusts nearing 20kt through this afternoon. Mixing will
decrease shortly after sunset...but remain northwest. Clouds look to
remain minimal as well...with only a couple areas of broken ceilings at
4kft above ground level through 9z. Then dry air will continue to pour south and
limit clouds during the day to only a few/scattered deck at 2500-3000ft
above ground level.
//Ord and mdw confidence...updated 06z...
* high confidence in wind dir/speed...and high on timing of gusts
developing and duration of gusts.
* High confidence in ceilings forecast.
//outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...
Tuesday...VFR. Northwest winds.
Wednesday...mainly VFR. Chance of showers late.
Thursday...VFR. Chance of shra/tsra.
Friday and Saturday...VFR.
440 PM CDT
Strong southwest winds will continue late this afternoon into the
evening across much of the lake including the nearshore areas...as
strong low pressure moves through the Great Lakes region. Highest
speeds are occurring across the southern half of the lake and
especially along the nearshore where a few gale force gusts are
also possible. This will persist before winds diminish late this
evening and into the overnight hours as this low exits to the
east...with winds becoming more west northwest.
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