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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 211806

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
106 PM CDT Thu Jun 21 2018

1023 AM CDT

Moderate to heavy rain continues in a corridor of northeast
Illinois, as part of a larger area rotating around the broad
closed low centered across the IA/MO border region. The rain in
northeast Illinois has been around 0.75" per hour with isolated
over an inch rates. Those are rates consistent with the tropical-
like atmosphere analyzed this morning, and with recently wet 
conditions these are very close to flash flood criteria rates. Am 
concerned with a heavy rain area moving north-northeast out of 
far southern Cook County as of 1020 am, as that could push a few 
basins over. It is moving fairly quickly though which is why have
not issued at this point. We have received a handful reports of 
minor / Flood Advisory flooding, and will be watching for a 
potential issuance of a Flash Flood Warning for Cook County, 
especially if that aforementioned heavy rain area slows.

While rain looks to continue in this axis beyond this into the 
afternoon, there continues to be signs that it will ease in 
intensity. While leaning that way, any rain still may be enough 
to continue at least minor flooding issues through the afternoon.
Attention later today may temporarily shift to the southwest CWA 
as the surface low dumbbells its way into central Illinois. 
Already an axis of modest CAPE, including over 100 J/kg of 0-3km 
CAPE, that has developed across western and central Illinois in a 
clearing swath. Scattered convection has developed in the 
Iowa/Missouri/Illinois border region, and could see some low- 
topped supercells approach the southwest forecast area. Convective
winds and a tornado or two would be the threats mid-afternoon 
into early evening.

This convective area may set the stage for the heavy rain 
potential tonight as it moves northward/expands into the CWA. 
Still some variance on guidance on where exactly that may set up.
No changes to the Flash Flood Watch at this time.



330 AM CDT

Through Friday...

Slow moving upper level low pressure in a relatively high
precipitable water environment will make for messy couple of days
for some areas across our region. Expect cool and wet conditions,
with areas under and closer to the low getting breaks/showery
weather, and areas farther north potentially getting stuck in
extended periods of rain with embedded thunder. Heavy rain and
localized flooding appear the main concern, and with low flash
flood guidance due to wet grounds, flash flooding may become an
issue for some areas as well.

The 0z ILX sounding depicts the moist and relatively warm airmass,
with a 1.89" PWAT and 13,500 ft freezing level. This is a good
environment for efficient warm rain processes. Instability does 
look to remain a bit more tempered the next few days with all of 
the cloud cover. But with even lower instability, embedded thunder
remains possible which will enhance the warm rain processes. 
Certainly with more instability this becomes even more concerning.

The first wave will come through the area this morning into early
afternoon. These will certainly have heavy rain and maybe some
localized flooding potential, but this axis may be a bit more
progressive than what may be coming later tonight. Areas south of
I-80 may get in on some breaks today, and as such will 
destabilize a bit more. As the upper low continues to shift east, 
additional shortwaves will pinwheel ahead of the low. PNT forecast
soundings depict up to 1000 J/kg of CAPE and 25 kt of shear with 
decent low level lapse rates supporting some localized stronger 
wind gusts in our southern counties. Mid level lapse rates are 
weaker, thus the severe coverage does not appear to be that large,
with instability dropping off farther north, but it appears that 
some stronger storms will bear watching across west central IL and
close to our area later this afternoon. 

Tonight the upper low will build into south central IL. This will
setup and easterly feed into the area, and therefore storms will
likely train over similar areas wherever the higher Theta-e axis
on the back side of the system sets up. Model run accum QPF
certainly suggests many areas when all is said and done will see
in excess of an inch, with 3-4" totals not out of the question in
this setup. The challenge will be where this higher axis will be.
At this point areas along and north of Chicago are favored. GEFS
plumes for ORD have 1.2 to 4.5 inches of QPF indicating the
potential this system has for some areas, with mean currently 
holding at 2.75". 

With all that in mind, some type of flood message is warranted
given that rivers across northern IL are running high, storm total
QPF ranging from 1-5" both from raw guidance and GEFS ensembles 
(showing potential is there) Will go out with a flash flood 
watch through tonight, that may need to be honed in location. 
Rivers will likely rise after getting a good bump already from 
recent rains. While rates may not be at the levels for flash 
flooding or all areas (and the potential for some initial 
breaks), the potential is there, especially if these get over more
urban areas.



329 AM CDT

Friday through Wednesday...

Not too many changes to the blended model guidance given the
active short term weather. The upper low will pull away Friday
night, with the heavier rains easing Friday afternoon and even
more so in the evening. Zonal flow over the weekend and northeast
low level flow with no significant waves suggests somewhat cooler
but generally dry weather. Cannot rule out some rain showers at 
times as there is some weak energy in the zonal/NW flow aloft. 

Ridging aloft and SE flow suggests Monday will be dry with some
warming. Another upper low will cross the plains and arrive across
our region mid week, suggesting more inclement weather is
possible, depending on if the low moves through our area or if it
stays north. Beyond that time frame, there are hints of a heat 
dome building in our near the region, but it is not a clean one
that may be muddled with clouds and occasional precip. Did nudge
temps up a tad closer to 925 mb climatology. 



For the 18Z TAFs...

The forecast concerns for the Chicago area airports into Friday:

- Moderate rain becoming more showery early this afternoon

- Rain becoming more solid tonight with potential for thunder

- IFR ceilings favored through much of the TAF, though some
  variance to MVFR at time is likely

- East to northeast winds with more regular gusts developing

The morning moderate to heavy rain is easing some and that trend
should continue early this afternoon, with rain more of the light
to moderate nature. In the moderate rain, visibility briefly 
under 2SM is possible. IFR ceilings are fairly widespread, though 
as the rain eases the ceiling heights are likely to inch up and 
may briefly reach MVFR category.

Scattered thunderstorms are likely to develop across western and
central Illinois and move slowly northeast through the afternoon.
Isolated thunderstorms are possible up to the Chicago and Rockford
areas, but more favored to remain south. Rain will continue to
redevelop this evening and there is expected to be an east-to-west
corridor of heavy rain producing showers late this evening into
overnight. This is slightly more favored to be centered over the
south metro (MDW and GYY area), but confidence on precisely where
is low. The rain nature will be similar to this morning and thus
have potential for visibility down to around 1SM, primarily
wherever that axis sets up. Ceilings should drop back deeper into
the IFR category later tonight. The northeast winds will also 
continue tonight and show more gustiness.

Rain is expected to continue much of Friday morning before likely
ending in the afternoon. IFR ceilings continue to be favored 
especially in Chicago as the winds stay northeast even into the 




416 am...Low pressure over IA this morning will move to central 
IL Friday morning and then to near Lake Erie on Saturday. This 
will allow an easterly gradient to steadily increase today into 
tonight with a period of 20-25kt easterly winds expected on the 
southern portion of Lake Michigan. Some higher gusts to 30kts are 
also possible...likely on the far southern part of the lake. These
winds will turn more northeasterly on Friday and then diminish 
Friday night as they shift northerly. Northerly winds will then 
persist into Sunday when a cold front will move south across the 
region. This front will maintain northerly winds...which will 
likely increase in speed behind the front. High pressure is then 
expected to move across the Great Lakes Monday and then steadily 
shift east Tuesday. cms


IL...Flash Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     until 7 AM Friday.

     Beach Hazards Statement...ILZ006-ILZ014...7 PM Thursday to 4 AM 

IN...Beach Hazards Statement...INZ001...7 PM Thursday to 4 AM 

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744...7 PM 
     Thursday to 4 AM Saturday.