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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

                            
000
FXUS63 KLOT 191914
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
214 PM CDT Thu Sep 19 2019

.UPDATE...
1055 AM CDT

Scattered showers and some thunderstorms are expected to make 
inroads further east into the forecast area through early this 
afternoon, possibly into the western Chicago suburbs by 2 pm. A 
weakening of this convection is still the expectation with fairly
decent confidence during the afternoon. Another note is that it 
is more likely there will be at least isolated thunderstorms in 
the region tonight.

A broad, loosely-defined MCS is centered roughly over the 
Iowa/Wisconsin/Illinois border region and shifting very slowly 
east. There is a low amplitude short wave, likely convectively
enhanced, in the Iowa/Wisconsin broader area that is drifting 
east on the northern periphery of the upper ridge. This short 
wave is helping to gradually advect eastward a moist and stable 
elevated air mass noted on the 12Z DVN sounding. On the leading 
edge of this higher theta-e air mass has been convective towers, a
handful of which have sprouted into storms just west of I-39. 
This convective corridor almost seems to be a small warm- 
advection wing as part of the main MCS. Convection has been fairly
behaved, though one cell earlier south of Dixon likely had a core
with small hail.

Satellite trends on the large scale indicate notably warming cloud
tops and weakening of the MCS as a whole, along with outflow
spilling southeastward. However, the warm advection wing across 
north central Illinois has maintained its own. While expectations
are for a continued overall weakening, some festering showers or
storms are likely to ride that leading edge instability gradient 
and approach Kane and Kendall Counties early this afternoon. In 
addition, there may be some festering showers mid to late 
afternoon within gradual isentropic ascent, including into
Chicago, though the flow looks quite light. With the amount of 
clouds and loss of stronger warm advection for that period, the 
chance for thunderstorms 3-7 p.m. looks fairly low. Going into 
tonight, the warm advection begins to slightly renew and the 
parent short wave is still meandering eastward. That combination 
indicates there could be scattered thunderstorms to develop, and 
several convection allowing models are showing that. With weak 
steering flow and high ambient moisture the concerns would be for 
locally heavy rain.

MTF

&&

.SHORT TERM...
240 AM CDT

Through Friday...

Satellite and radar pictures show what will be a slow but 
eventual breakdown of the upper ridge across the region. 
Convection which has largely been confined farther upstream is now
slowly inching its way closer to northern Illinois. The activity 
across northeast Iowa is largely attributed to a weak upper wave 
and some increased moisture transport with more robust 
thunderstorms into western Iowa where instability is much greater.
Meanwhile the showers ahead of the stronger forcing quickly erode
with the instability gradient being fairly sharp across northwest
and into north central IL. 

High pressure at the surface will also only slowly shift east, and 
thus the lower levels across much of the local area will still be 
influenced by the drier airmass to our east and will keep the 
instability/theta-e gradient fairly large across Illinois. With 
the subtle forcing and weak steering flow, any activity this 
morning will be tied closer to the forcing with the thunderstorm 
complex that could briefly impact the I-39 corridor this morning 
before hitting the instability gradient.

Following this, isentropic lift with the southwest flow may continue 
to drive some showers and storms across northwest IL, northern IL 
and southern WI, but we will still feel the influence of the surface 
high to keep most of the area dry.  It should become fairly warm yet 
again today, but there will likely be quite a bit of higher cloud 
cover. 

Subtle energy will remain embedded in the upper ridge, but it is the 
warm front/composite outflow boundary from this upstream 
thunderstorm complex will serve as focus late tonight into early 
Friday, but more focused to our south and west through the day.

KMD

&&

.LONG TERM...
213 PM CDT

Saturday through Thursday...

Forecast concerns during the long term period continue to focus 
on the increasing threat of very heavy rainfall and possible flash
flooding across the area, particularly late Saturday through 
Sunday. 

An upper level ridge axis, currently in place across the area, 
will be shunted to the east by Saturday, as a large upper trough 
over the western CONUS shifts across the Rockies. As this occurs,
strong and deep south-southwesterly flow will be induced across 
much of the Mississippi River Valley and western Great Lakes 
region on Saturday. This will thus allow tropical moisture from 
the remnants of Imelda (featuring precipitable water values 
around 2') to be transported northward into our area this weekend 
setting up the stage for some very heavy rainfall, and possibly 
some significant flooding. 

While we will begin to see chances for some showers and storms
early on Saturday our best chances for more widespread showers 
and storms looks to be Saturday night and especially Sunday. This
is the period in which we will see the large scale dynamic 
forcing for ascent maximize as the upper trough and surface cold 
front approaches the area. With deep tropical moisture over our 
area ahead of this front, deep warm cloud depths will result in 
some very efficient rainfall producing showers and storms. As a 
result, it appears that at least localized rainfall rates 
exceeding 1.5" per hour will be possible at times into Sunday. 

Total rainfall amounts across much of northern IL and portions of
northwestern IN through Sunday evening could exceed 2", with 
locally higher amounts in excess of 3" possible. While there does 
continue to be some uncertainty with where the heaviest rainfall 
axis will occur, trends continue to suggest that much of northern
IL could be near this heavy rainfall axis. For this reason, flash
flooding and river flooding will be a concern that we will need 
to monitor as we get closer to this event. Since it is too early 
to consider any type of watch, we will issue an ESF (Hydro 
outlook) to increase awareness of this increasing potential for 
very heavy rainfall. 

Surface high pressure is expected to build across the area Monday
following Sunday evenings frontal passage. This should result in a
couple of quiet and pleasant weather days early next week before
the possibility for showers and storms returns to the area mid to
late next week.

KJB

&&

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

Concerns for the Chicago area airports through Friday are:

- Lake breeze wind shift to easterly later this afternoon

- Small chance for showers late this afternoon

- Chance for scattered showers and storms late tonight into Friday
  morning

Showers and some embedded thunder are likely to continue across 
northwest and north central Illinois this afternoon. The air mass
is less favorable over Chicago, so our certainty in no thunder 
at ORD and MDW is high while for no showers this afternoon is 
medium. A lake breeze boundary is evident on Terminal Doppler 
Weather Radar (TDWR) near the shore and this will creep inland 
through the afternoon. Confidence in this wind shift reaching ORD 
and MDW is medium-high, with confidence in speeds of 8+ kts being 
medium.

Later tonight, a weak upper level disturbance will interact with 
elevated moist air spreading over the area. This is likely to 
develop at least isolated showers and thunderstorms in the 
region, maybe most favored across northwestern/western Illinois 
into north central Illinois. Confidence in both the coverage near 
the terminals and how far east this activity spreads is low.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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