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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
324 am CDT sun Sep 22 2019

Short term...
320 am CDT

Through Monday...

Main forecast concerns continue to be with heavy rainfall and the
potential for flash flooding this morning through this evening,
as well as a threat for severe storms this afternoon into
early/mid evening.

Despite somewhat of a delay of the more widespread shower and
thunderstorm development overnight, overall setup and latest
trends with upstream development and latest short term guidance
still supporting the idea of heavy rainfall with the threat of
flash flooding across the Flash Flood Watch area this morning
through this evening. Much of the development overnight has been
more scattered, with rain rates and duration not really supporting
much rainfall or the threat of flash flooding. This includes what
is currently ongoing across northeast Illinois and parts of northwest
in, with latest radar imagery depicting an area of light to
occasionally moderate rain. This area, supported by a weak mid
level vort and warm air advection, will likely continue over the next several
hours but then should see coverage lower. Focus then turns back to
the widespread showers and thunderstorms lifting across Iowa and
MO. As the upper level trough over the central Continental U.S. Shifts east
this morning, more energetic flow will steadily lift across the
region, while surface trough and front inch closer to the area.
This large scale ascent with warm air advection and moisture transport focused
into northern IL, will bring showers and storms and the threat of
flash flooding to north central and northeast Illinois around day break.
The significant increase in ascent coinciding with high pwats
around 2 inches, will support high rainfall rates of 1 to 2 inches
per hour this morning. While much of the watch area will likely
observe showers/storms this morning, the initial focus for more
persistent heavier rainfall may be more so across the Rockford
area. This might be the trend through mid to late morning, but as
the upper level trough and surface trough continue east, should
see this precip shield spread further east/southeast into
remaining areas in northeast and east central Illinois.

It's quite possible that locations across the southeast third of
the County Warning Area stay dry through midday and possibly early afternoon. This
is actually a little concerning if this trend continues to appear
more of a likely scenario, as this would support a higher threat
of severe weather later this afternoon through mid evening. With
latest trends with upstream precip and short term guidance, have
trended toward this idea with the forecast for later today, and
have increased messaging for possible severe weather. Latest rap
guidance is showing increasing instability with little to no cap
by mid afternoon, for areas along and southeast of a line from La
Salle to Oak Lawn. In this location, forecast soundings are
indicating that steepening low level lapse rates will be the trend
through early/mid afternoon. Strong height falls with a strong
upper level jet will support some deepening to the surface trough
shifting through the region this afternoon. Strong large scale
ascent once again expected this afternoon in response to these
features, with an uptick with the showers and storms likely. This
will occur across the entire area with increasing intensity of
ongoing storms likely supporting high rain rates and the
possibility for additional flash flooding. Of most concern, will
be where the instability axis with little/no cin and now higher
bulk shear will reside. Once again, at this time, this appears to
along and southeast of a line from La Salle to Oak Lawn. In this
location, will need to monitor the potential for more robust
development which will be capable of producing damaging winds.
Given the strong dynamics, wind field, and instability, can't rule
out an isolated tornado threat. The severe threat will likely
last through early to mid evening, as the risk lowers and shifts
out of the area. Showers and storms will still be possible through
just after midnight, but expect the coverage to lower before
completely drying out early Monday morning as the trough/front
push through.

Rodriguez

&&

Long term...
209 am CDT

Monday night through Saturday...

Strong subsidence will continue to overspread the region Monday
night and into Tuesday as shortwave ridging (temporarily) builds
in overhead. Still think that the potential for fog development
looks fairly low even though moisture does look to remain trapped
in the lowest 5 kft under the developing inversion. A dense plume
of cirrus combined with soundings that show a propensity for
things to remain pretty well mixed immediately off the surface
should greatly curtail the potential for dense fog.

Surface high pressure will slide off to our south and east through
the day on Tuesday, allowing southerly return flow to develop back
across the region. Moisture looks to remain insufficient to
support precipitation chances until later Tuesday night into
Wednesday as the next robust shortwave trough pivots across the
upper Midwest (but even then, moisture quality doesn't look all
that impressive). A substantial 130 kt jet streak is expected to
nose into parts of Minnesota and southern Wisconsin during this
time, which will drive a notable low-level ageostrophic response
and ramp up a 50-55 kt low-level jet. While the dynamics with
this system will be robust, it looks to come in essentially at the
diurnal instability minimum. Forecast soundings look pretty
capped to surface-based convection, and it may even be difficult
to force elevated convection given a minimal amount of time for
more appreciable moisture return. The blended guidance chance pops
look reasonable at this point an no notable changes were made.
Could get a bit gusty late Wednesday morning and into the
afternoon as we tap into a belt of faster flow immediately off the
surface.

Looks like we'll get another brief break on Thursday before the
next storm system brings a renewed potential for showers and
thunderstorms to the region to close out the week and into the
upcoming weekend.

Carlaw

&&

Aviation...
for the 06z tafs...

Plentiful aviation weather concerns this period as a potent storm
system is expected to bring several rounds of rain and storms to
the c90 tracon.

We're currently monitoring two areas of showers late this
evening. The first is very near the Chicago-area terminals and
should roll across through 07z or so with periodic vsbys
reductions down below 2-3 sm and some occasional lightning. A
second area of generally light to moderate precipitation is moving
north of kpia/kbmi, and this seems to be associated with a
remnant mesoscale convective vortex that developed on Friday night
across northeastern Oklahoma. Some subtle increase in warm
advection on this feature's eastern periphery may result in an
expansion of precipitation through 08-09z or so and this could
bring a brief period of heavier showers/tsra to the Chicago-area
sites. Rfd may stay more-or-less precipitation-free through 10z.

Once this initial batch of activity moves out of the region, we'll
turn our attention to the large mass of convection spreading into
Iowa and northwestern Missouri. The parent upper trough
responsible for this activity may be digging a bit more than
earlier anticipated, and this has slowed the evolution of
convective chances a bit. Latest thinking has this activity
getting close to rfd after 10-11z, and then late this
morning/early afternoon at the Chicago-area sites. This slightly
slower progression may allow a bit more instability to
materialize, which could spell a somewhat greater potential for
+tsra into the afternoon hours. Uncertainties regarding the
highest chances for on-station ts preclude a tempo group at this
time, but one will probably be necessary for the 09z amendments or
12z set.

With the slower progression may also come a slower frontal passage. We'll
show a west-northwest wind shift at rfd towards 23/01z and then at Ord/dpa/mdw
towards 23/03z. Shower chances will continue behind the front, but
ts potential will drop markedly as drier air begins to filter into
the region.

Carlaw

&&

Lot watches/warnings/advisories...
Illinois...Flash Flood Watch...ilz003-ilz004-ilz005-ilz006-ilz008-ilz010-
ilz011-ilz012-ilz013-ilz014-ilz019-ilz020-ilz021-ilz022
until 1 am Monday.

In...none.
Lm...Small Craft Advisory...Illinois nearshore waters until 10 PM Sunday.

Small Craft Advisory...in nearshore waters until 4 PM Monday.

&&

$$

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