Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1102 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 16z...
* northwest winds around 12kt gusts near 20kt through sunset...then
diminish after sunset to less than 10kt overnight.
Frontal boundary pushed south of the taf sites early this
morning...with winds remaining light from the northwest. Skies
have also begun to clear out as well...and expect within the next
couple of hours skies will become mostly sunny. Mixing will
increase later this morning...and allow winds to increase with
gusts nearing 20kt. Gusts will linger through sunset...then
quickly diminish with west/northwest winds expected overnight of
//Ord and mdw confidence...updated 16z...
* high confidence in all forecast elements.
//outlook for Ord/mdw for 00z Wednesday-12z Monday...updated 12z...
Tuesday night through Wednesday...VFR. Northwest winds turning
Thursday...mainly VFR. Chance of rain showers.
Saturday through Sunday...mainly VFR. Chance of showers and
256 am CDT
A cold front has shifted south of the lake early this morning...with
winds generally out of the northwest in its wake. A surface ridge of
high pressure will shift over the lake for Tuesday...resulting in
mainly lake breeze driven winds over the lake. However...another
area of low pressure is expected to shift eastward south of the lake
by Thursday. This looks to produce a period of east-northeasterly
winds...especially over the southern half of the lake. It currently
appears that the wind speeds may not be as strong as previously
thought...so waves may remain below Small Craft Advisory criteria.
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