Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251908
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
208 PM CDT Tue Sep 25 2018

.SHORT TERM...
207 PM CDT

Through tonight...

Mainly a mesoscale discussion...Concerns center on thunderstorms
later this afternoon and evening. A severe thunderstorm watch has
been issued for a good portion of northern Illinois.

Forcing and instability is still working its way east ahead of a
cold front across Iowa. While isolated development is possible
ahead of the front, the majority of the forcing is tied to the
front itself. The corridor downstream of more active thunderstorms
across Iowa is favorable for continued development or
intensification as we have been able to heat up a bit after
morning cloudiness. There is an 18z sounding out of the Quad
Cities, KDVN, has depicted some mixing out of the dewpoints which
is fairly representative of what has been occurring across
northeast Illinois.

As far as hazards, the storms being on the front have largely
taken on a linear shape already this afternoon, with stronger
portions of the line depicting a decent wind signature on a few of
the storms. Therefore, damaging winds remains the primary concern.
Upstream in Iowa, effective shear is 45-55 kt which is more than
sufficient for rotating storms, which has been observed by radar
and spotters in Iowa. There is a closed surface low across Iowa
that will spread northeast into Wisconsin. This feature is likely
contributing to higher helicity values and will be the corridor
where if any tornadoes were to form, this would be area. This
corridor will shift into north central Illinois and the I-39
corridor later this afternoon, and thus will be something to watch
for. As the low continues northeast, this threat becomes lower
farther east, but we do expect shear to increase as storms
approach our area given the approach of the positively tilted
trough. A severe thunderstorm watch was issued as damaging winds
do remain the main hazard, but the watch mentions possible
tornadoes as well. Mid level lapse rates suggest hail is not as
big of a concern, but certainly possible with any supercells.

Most of the thunderstorms will be confined to a several hour
period around the cold front. Expect gusty southwest winds to
around 20 kt ahead of the front. A few isolated/widely scattered
showers or storms are possible by mid afternoon, but the main
window for severe storms will be late afternoon around the
Rockford area, and 7-9 pm around the Chicago area. The storms will
be arriving closer to Chicago after peak heating, and thus
confidence on how strong/severe storms will be closer to Lake
Michigan is lower. Certainly the forcing and shear will still be
sufficient. But thunderstorms should survive to the lake
regardless.

Behind the front winds will head to NW and gust into the middle
20s overnight, with lows falling into the 40s outside of Chicago.

KMD

&&

.LONG TERM...
322 AM CDT

Wednesday through Monday...

Quick hitting shot of seasonably cool air Wednesday should be
followed by a rebound in temps Thursday as southwesterly winds
increase. Cold front trailing from a low tracking up toward James
Bay should move through the area with no precip and not much
fanfare Thursday night.

This boundary is then forecast to stall out east-west across the
region, likely leaving most of our CWA on the cool side of the
boundary Friday into Saturday. Medium range models suggest a low
amplitude wave rippling through the broad trough over the region
will tighten the baroclinic zone and lead to an area of chilly
f-gen driven rain to the north of this boundary. Still some
significant timing differences wrt this wave between the
operational GFS and ECMWF runs. Friday evening looks wet in the
ECMWF with remainder of the weekend dry, while the GFS provides
for a good soaking rain over the heart of the CWA Saturday into
Saturday night.

The model disagreement shouldn`t be a huge shock given the low
amplitude nature of the wave and typical struggles in modeling
mid-level f-gen driven precip 5+ days out. Have opted to stick
with the blended model initialization for now this weekend, which
carries low chances of rain each period. It is worth noting that a
solution more in line with the rainy Saturday in the GFS would
have significant temperature implications as well. As it stands,
Saturday looks to be the chilliest day in months, with highs north
of I-80 in the 50s, however, should the rainy GFS solution pan out
then some areas might struggle to get out of the upper 40s
Saturday afternoon.

Medium range guidance is in decent agreement in reloading the
western U.S. trough over the weekend and ejecting this system out
toward the Great Lakes region early next week. Current model runs
track this wave sufficiently far north to drag unseasonably warm
and moist air back north into the area for a time early next week.
Still a lot of time for this solution to change, so stay tuned...

- Izzi

&&

.AVIATION...
For the 18Z TAFs...

Concerns center on thunderstorms later this afternoon and evening.
MVFR clouds have generally gone scattered with bases generally
VFR. Cannot rule out some MVFR bases but with warming they should
generally remain VFR.

Forcing and instability is still working its way east. Most of the
thunderstorms will be confined to a several hour period around the
cold front. Expect gusty southwest winds to around 20 kt ahead of
the front. A few isolated/widely scattered showers or storms are
possible by mid afternoon, but the main window for gusty storms
will be 22z- 24z around RFD, and 00z-03z around the Chicago
terminals. There could be a period of strong winds with these
storms, highest chances near RFD. Confidence on thunderstorm
occurrence at the terminals is medium-high with medium to high
confidence on timing within an hour or so. Confidence on coverage
duration of severe storms is low.

Behind the front winds will head to NW and gust into the middle
20s overnight, accompanied with MVFR cigs. West winds will then be
the rule tomorrow with VFR conditions.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...7 PM
     Tuesday to 10 AM Wednesday.

     Small Craft Advisory...LMZ744-LMZ745...10 PM Tuesday to 1 PM
     Wednesday.

&&

$$

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