Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 07/21/19

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 647 PM CDT Sat Jul 20 2019

.SHORT TERM...
319 PM CDT

Through Sunday night...

We will be able to drop some counties from the heat warning
within the next hour or so as the outflow boundary continues to
progress southward into northern IL. Expect gusty northerly winds
in the wake of the boundary along with much cooler temperatures.

South of the approaching outflow we have temperatures in the low
to mid 90s atop mid to upper 70s dewpoints. While 700 mb
temperatures are also very warm at around 14C, it appears that
area is not capped all that strongly, per some AMDAR soundings out
of MDW. For this reason, it appears likely that scattered
thunderstorms will continue to develop with the arrival of the
outflow boundary as it pushes southward into northern IL through
the late afternoon hours. This also appears likely as the
west-east orientation of the outflow boundary along with the
gusty southwesterly prefrontal winds will continue to favor some
decent low-level convergence. There remains concerns for some
severe storms as well, though confidence with severe storms
remains on the lower side. Isolated damaging winds would be the
primary threats with any strong storms that collapse.

Expect the storms to wane for a period early this evening, before
additional storms develop over the area later this evening and
overnight. It appears that these elevated storms will develop in
association with the elevated frontal boundary. These storms will
be capable of producing very heavy rainfall, and with easterly
storm motions we could have some training of cells over portions
of the area tonight. This does cause concern for flash flooding
as storms could easily produce 2+ inches per hour. We thought
about issuing a flash flood watch for northern IL, but due to
some lower confidence on the overall convective setup over
northern IL we will hold off. A short fused watch could need to be
issued this evening, however, if conditions continue to point
towards this possibility.

Otherwise, some lingering storms will be around on Sunday morning,
but the main focus will shift over my southern CWA for convection.
Some strong storms will be possible in this area on Sunday, as the
surface frontal boundary remain gradually sags into central IL
Sunday. Areas north will be drying out, and temperatures will be
much cooler, especially near the lake.

KJB

&&

.LONG TERM...
319 PM CDT

Tuesday through Friday...

Nothing has changed with the long term forecast thinking below.

A period of quiet, dry conditions is expected for the long term
forecast period.  A large, sprawling area of surface high pressure
is expected to spread through the Northern and Central Plains and
across the Upper Mississippi Valley to the Great Lakes region.  In
the upper levels, a high amplitude long wave pattern, with little
progression, will set up, with a broad ridging centered over the
continental Divide and a broad trough over the ern CONUS. The local
area will be under nwly flow aloft through the period. The center of
the surface high is expected to slowly slide south and east and
should be centered over IL/IN by Wednesday night and Thursday. The
center of the sfc high is expected to shift to the Mid atlantic
region by Friday. After a relatively cool Monday, temperatures
should begin to moderate back into the mid to upper 80s by mid week.
The next chance for pcpn may not be until next Saturday when some
of the long range models are suggesting a cold front may push
through the area.

&&

.AVIATION...
For the 00Z TAFs...

647 PM...Main forecast concern is thunderstorm chances through
Sunday morning.

Made no significant changes with the 00z tafs. Trends support
isolated thunder developing this evening and then increasing in
coverage by late evening and especially overnight. With the
current cell near ksfy in far northwest IL...these trends seem
reasonable and will need to monitor trends for possible changes
with the 03z tafs. This activity should move southeast of the
terminals by mid morning Sunday but low confidence for an end
time.

Winds have become light easterly behind the earlier convection
and outflow boundary and may continue to become light southeast
this evening. Confidence for when winds will turn back to the
northeast is low but likely sometime later this evening so
maintained the northeast winds...which once they return will
continue through the rest of the period. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Excessive Heat Warning...ILZ019-ILZ021-ILZ022-ILZ023-ILZ032-
     ILZ033-ILZ039 until 7 PM Saturday.

IN...Excessive Heat Warning...INZ001-INZ002-INZ010-INZ011-INZ019
     until 7 PM Saturday.

LM...None.
&&

$$

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