Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 191936
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
236 PM CDT FRI AUG 19 2016
236 PM CDT
Several concerns exist for the overnight period. The immediate
concern is on leftover remnants of the southeastward moving MCS
which has now stalled across areas that have been hit hard with
rainfall over the last few days. There is not much of a reason for
these showers and storms to move southward anymore as this area
destabilized the most today, and therefore this axis will tend to
shift a bit northward or remain in place at this is right alon g
the instability/cap gradient. Rainfall rates have been as high as
1.75 inches per hour. Flooding may become more of a concern,
though these should enter a weakening phase in a few hours.
Second concern centers around the increasing area of thunderstorms
across Iowa. There is a shortwave, slightly convectively enhanced,
that will continue eastward this afternoon and early evening. IR
satellite suggests storms are not weakening, and the atmosphere
has already recovered across northwest and north central Illinois,
so expect that coverage will increase westward. There is still
some capping closer to Chicago. Many short term guidance sources
weaken this area as it approaches Lake Michigan, but a few showers
and storms may get in late this afternoon and early this evening
all the way to the lakefront. The southward extent will keep areas
south of I-80 and into Northwest Indiana (outside of current
storms), largely storm free from this wave.
The focus overnight should shift back westward as we await the
arrival of the initial lead shortwave across the central Plains
that will again increase shower coverage across western and
northern Illinois late tonight ahead of the arrival of the
impressive upper trough and strengthening surface low.
350 AM CDT
Sunday through Thursday...
Much cooler and less humid air is expected for Sunday following
the Saturday cold frontal passage. Temperatures should be in the
mid 70s Sunday, with only slightly warmer conditions expected by
Monday. Sunday nigh could be a rather chilly night across the
area as surface high pressure settles over the area. Clear skies
and light winds could allow temperatures to radiate down into the
low 50s outside of Chicago.
Temperatures Tuesday through Thursday look to warm back near
seasonal normals (lower 80s). However, another storm system is
forecast to take shape over the central CONUS by midweek, and this
will result in our next good chances for storms, late Wednesday
and into Thursday. This should also result in a push of cooler air
over the area later next week.
For the 18Z TAFs...
Concerns on winds, both direction and speeds, in the near term,
then a transition to periods of showers and thunderstorms with the
most widespread coverage on Saturday.
Winds have been chaotic in lee of the morning MCS but have
followed the climatologically favored shift to NW in the storms,
then around to SE where they are currently in numerous locations,
before a shift back to SW will resume as has occurred near KRFD.
Expect the SW will hold except in any other storms. Focus shifts
to the first area of storms across Iowa. Recoveries across NW/NC
IL may allow some of this to enter the picture late this
afternoon. Most guidance weakens this as it gets closer to the
Chicago terminals in the 23z-01z time frame, and have just
carried a VCSH for now, as the better forcing would be tied to the
shortwave farther upstream. This could have some thunder in it.
Additional development overnight would be more scattered in nature
until the cold front and associated low approaches Saturday.
Deteriorating conditions with lowered cigs, occasional lower vsby
can be expected with showers turning to showers/storms through the
day. Winds will increase out of the SW and gust into the mid 20s,
higher with any convective elements, with winds shifting W then
NW late in the day.
319 AM CDT
Weak area of low pressure will lift northeast across the northern
portions of Lake Michigan today, with another low pressure
developing across the Central Plains. This next low will then lift
northeast Sat and then deepen as it slides north across the Upper
Peninsula of Michigan. The gradient is expected to tighten quickly
midday Saturday and likely bring strong west winds to the Lake
between 20-25kt then increase late Sat ngt to 30kt with occasional
gale force gusts to 35kt through Sun morning. Winds will then turn
northwest and diminish as high pressure builds in from the
High pressure will then shift southeast of Lake Michigan late
Sunday night/early Monday, with a tightening gradient redeveloping
Mon over Lake Michigan from the southwest ahead of another low
pressure system that will lift northeast from the Central Plains
to the Upper Midwest late Tuesday night.
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