Flash Flood Warning 5, NWS Chicago, IL

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
1031 PM CDT Thu May 14 2020

.MESOSCALE DISCUSSION...
1030 PM CDT

Convective trends continue to fall in line with expectations,
although there still is uncertainty on exact targeting of any
other flash flood areas outside of the current Flash Flood
Warning. The severe threat is likely gradually waning and should
continue to through 1 a.m., while the flooding threat will remain
elevated into the wee hours of the morning.

The training convection across northern Livingston County has now
moved more so into Kankakee County, with some cold pool push and
low-level outflow. This will probably ease the flash flood threat
some but given passing over the Kankakee metro area and still
cloud tops to -74C on infrared imagery, a Flash Flood Warning for
that area was issued. Also watching just north of the outflow
surge where it re-intersects the updraft, as it has shown some
curl and mesovortexgenesis signature. We do mention tornado
possible in that warning.

On the broader scale, the low-level jet nose is pointing directly
into northern Illinois. Warm-advection driven thunderstorms on
this have sprung across north central Illinois into parts of the
Chicago metro and these should continue to fill in with heavy rain
and flash flooding being the main threat. Within the updraft
areas of the convective complex, there is concern for isolated
strong winds especially in any bowing segments or in/near
mesovortices. One of these mesovortices is noted tracking into Lee
County, and another entering western Bureau County. These will be
areas we will have to also watch for some temporary QLCS tornado
threat.

The main concern remains flash flooding especially any areas with
sustained west-to-east development and any slowing/regeneration of
the main convective area as it moves out of northwest Illinois.
It's still unsure how quickly cold pool amalgamation or any push
from cold pools will occur with this, especially in the next few
hours. So the flash flood threat remains most pronounced through 2
a.m. or so and we will continue to look to target areas with
Flash Flood Warnings. Localized flooding (that to Areal Flood
Advisory level) is likely across many parts of the area with the
at least temporary heavy rainfall rates.

MTF

&&

.SHORT TERM...
249 PM CDT

Through Friday...

Lingering showers and thunderstorms have diminished in coverage and
exited to the east of the area this afternoon. Meanwhile, dew points
have surged into the low to mid 60s across the area. The extensive
cloud cover has limited surface warming somewhat, but current
satellite trends suggest the cirrus beginning to thin which should
help temperatures warm up into the low 70s across much of the area.

An east-west oriented boundary has set up across southern Iowa
into northern IL, combined with strengthening low level jet will
set the stage for potential severe weather and heavy rain this
evening and overnight.

The primary corridor for severe weather has shifted south mainly
along and south of I-80 where the better instability will be.
Conditions are not as favorable as once thought thanks to the
continued precip through the morning and early afternoon hours which
has prevented the surface boundary from lifting as far north. In
general, expecting things to begin in multi-cell clusters this
evening and transition to a more linear mode and track off to the
east. Primary severe weather threats will be strong winds on the
leading edge of this line. Some hail may be possible with the
stronger storms, especially at the onset and a QLCS tornado threat
cannot be entirely ruled out.

Of greater concern with this system is the potential for training
thunderstorms resulting in flash flooding. The upwind Corfidi
vectors for propagation into the inflow are light and even
oriented to the northeast which suggests that redevelopment
behind the leading edge of the line is possible resulting in heavy
rain falling over the same areas for multiple hours. Current
thinking and trends is that this will favor along and near the
I-80 corridor. Have fielded a Flash Flood Watch to account for the
potential for heavy rain. Wherever this ultimately sets up could
see locally heavy rain amounts of 3-4+ inches of rain when this is
all said and done. If the cold pool ends up being stronger it may
be able to help push this out of the area more quickly reducing
the flooding potential, but current thinking is this will be
limited in its influence.

Once overnight showers and thunderstorms move out Friday morning a
mostly quiet day between systems. Temperatures are expected in the
70s, (60s closest to the lake). A lake breeze is expected to develop
in the afternoon which will lead to quick cooling behind it.

Aside from the severe weather and flooding threat, dense marine fog
is expected across southern Lake Michigan through late morning
tomorrow.

Petr

&&

.LONG TERM...
154 PM CDT

Friday Night through Thursday...

Things look more tranquil to start of the the long term portion of
the forecast on Friday night as shortwave ridging briefly builds
in overhead northeasterly winds develop in the wake of our
departing storm system. Can't rule out a few showers developing
across our far southern counties through the overnight hours, but
think things look mainly dry. Persistent east to northeasterly
winds on Saturday look to hold lakeside temperatures down towards
60 degrees or so, but a notable temperature gradient may set up
inland and for areas south of I-80 should cloud cover hold off
through the afternoon with some mid and even upper 70s not out the
question.

Warm advection will really get cranking Saturday afternoon and
evening in advance of our next weather maker which looks to be a
rather stout shortwave. Will commensurately ramp PoPs up through
Saturday night and Sunday in advance of this system. With the
latest multi-model consensus supporting this disturbance arriving
during the overnight and early-morning hours, currently
anticipate the severe threat remaining on the lower end of the
spectrum with an overall dearth of surface-based instability
present although things do look impressive upstairs from a
kinematic standpoint. Should this system slow down, the severe
threat would increase owing to the potential for diurnal gains in
instability.

This disturbance peels east of the region through the day on
Sunday although  conditions may remain somewhat unsettled even
into Monday as the core of the cold upper-low looks like it may
take its time fully clearing the area to the east. We'll also have
to keep an eye on a period of potentially gustier onshore winds
which may develop on the backside of this departing low. Still
some uncertainty regarding the magnitude of the developing onshore
flow with things-- as they stand right now--looking like waves
may flirt with Lakeshore Advisory criteria into Monday and Monday
evening.

Guidance today continues to trend towards developing a rather
blocky upper air pattern into the middle of next week with the
potential for persistent east or northeasterly surface winds to
keep temperatures on the cooler side. Always some volatility with
these types of set ups, but have trended things cooler in line
with the most recent blended guidance into the middle and end of
the week as a result.

Carlaw

&&

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

Aviation concerns include:

* Period of thunderstorms expected to develop by mid-evening and
  persisting past midnight before diminishing.

* IFR/low MVFR ceilings likely with storms and lingering
  afterwards into early Friday morning.

* Winds generally southwest winds until shifting to northwest with
  cold frontal passage early Friday morning, with northeast lake
  breeze winds expected Friday afternoon/evening.

Outflow from convection earlier in the day had pushed the
effective warm frontal boundary well south of the terminals across
southern IL. However, decreasing cloudiness this afternoon has
allowed some warming and destabilization of the airmass north of
this boundary, allowing it to mix back northward late this
afternoon. A mid-level disturbance will track east along and just
north of this warm front from IA into northern IL and northwest
IN this evening/overnight, with an area of thunderstorms currently
over central/eastern expected to expand eastward across the
terminals during the mid-late evening hours. Thunderstorm coverage
should diminish after midnight (1-2 am) across the terminals,
though showers will likely linger for several hours into early
Friday morning before sliding off to the southeast with the
passage of a cold front.

Patchy MVFR ceilings have scattered late this afternoon as better
mixing has developed northward into the area, though a rather
moist boundary layer will likely support low MVFR and IFR ceilings
to develops as precipitation spreads across the area with the
storms. Guidance maintains IFR ceilings through most of the
overnight hours before a gradual improvement through MVFR occurs
behind the cold front Friday morning.

Surface winds are initially south-southwest ahead of the cluster
of storms this evening, and may back slightly as weak surface low
pressure approaches. Some variability in direction is possible as
the complex of showers/storms moves through. Cold frontal passage
looks to occur 12-13Z Friday morning for the Chicago terminals,
with a shift to northwest winds, and an expected lake breeze
should produce a shift to northeast winds for the afternoon and
evening hours.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flash Flood Watch...ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-
     ILZ020-ILZ021-ILZ023-ILZ032-ILZ103-ILZ104-ILZ105-ILZ106-
     ILZ107-ILZ108 until 7 AM Friday.

IN...Flash Flood Watch...INZ001-INZ002 until 7 AM Friday.

LM...Dense Fog Advisory...IL nearshore waters until 7 AM Friday.

&&

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
326 PM CDT Thu May 14 2020

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-152030-
Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-De Kalb-Kane-DuPage-
La Salle-Kendall-Grundy-Kankakee-Livingston-Iroquois-Ford-
Northern Cook-Central Cook-Southern Cook-Northern Will-
Southern Will-Eastern Will-Lake IN-Porter-Newton-Jasper-Benton-
326 PM CDT Thu May 14 2020 /426 PM EDT Thu May 14 2020/

This Hazardous Weather Outlook is for portions of North Central
Illinois...Northeast Illinois and Northwest Indiana.

.DAY ONE...Tonight.

Weather hazards expected...

  Significant Thunderstorm Risk...with an associated:
     Elevated hail risk...up to ping pong ball sized.
     Elevated damaging wind risk...up to 65 mph.
     Limited Tornado Risk
  Significant Flooding Risk.

DISCUSSION...

Thunderstorms are expected to redevelop later this afternoon and
evening, and some of these could be severe, capable of producing
some instances of damaging winds and large hail. The most likely
time of any potential severe thunderstorms is roughly between 6 pm
and 11 pm. The main severe threat appears to be near and south of
I-80.

In addition to the risk for severe weather, there is a threat for
several rounds of thunderstorms to produce locally very heavy
rainfall this evening and overnight with rainfall totals in
excess of three inches possible. A Flash Flood Watch is in effect
for locales near and south of I-88 and north of a Chatsworth to
DeMotte, Indiana line.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Saturday...
  Limited Flooding Risk.
  Limited Thunderstorm Risk.
Sunday...
  Limited Thunderstorm Risk.

.SPOTTER INFORMATION STATEMENT...

  Spotters may be needed tonight mainly south of I-80.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east northeast at 35 to 40 mph.

LMZ740>745-152030-
Winthrop Harbor to Wilmette Harbor IL-
Wilmette Harbor to Northerly Island IL-
Northerly Island to Calumet Harbor IL-
Calumet Harbor IL to Gary IN-Gary to Burns Harbor IN-
Burns Harbor to Michigan City IN-
326 PM CDT Thu May 14 2020

This Hazardous Weather Outlook is for portions of the Illinois
nearshore waters of Lake Michigan and the Indiana nearshore
waters of Lake Michigan.

.DAY ONE...Tonight.

Weather hazards expected...

  Elevated Thunderstorm Risk...with an associated:
     Elevated hail risk...up to ping pong ball size.
     Elevated thunderstorm wind gust risk...up to 60 kts.
  Elevated Fog Risk.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Friday...
  Limited Thunderstorm Risk.
Saturday and Sunday...
  Hazardous conditions for small craft are likely.
  Limited Thunderstorm Risk.
Monday...
  Hazardous conditions for small craft are likely.

Carlaw

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE CHICAGO IL
1056 PM CDT THU MAY 14 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1015 PM     TSTM WND DMG     3 SSE GRANT PARK        41.20N 87.63W
05/14/2020                   KANKAKEE           IL   TRAINED SPOTTER

            TRAINED SPOTTER REPORTS POWER POLES DAMAGED
            NORTHEAST OF MOMENCE. REPORT RELAYED VIA HAM
            RADIO OPERATOR. TIME ESTIMATED BY RADAR.

1010 PM     TSTM WND DMG     1 WSW MOMENCE           41.16N 87.69W
05/14/2020                   KANKAKEE           IL   TRAINED SPOTTER

            TREES AND POWER LINES DOWNED JUST WEST OF
            MOMENCE ALONG RIVER ROAD. REPORT RELAYED VIA
            HAM OPERATOR. TIME ESTIMATED VIA RADAR.

&&

WSL

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE CHICAGO IL
1155 PM CDT THU MAY 14 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1118 PM     FLASH FLOOD      3.3 E HERSCHER          41.05N 88.03W
05/14/2020                   KANKAKEE           IL   TRAINED SPOTTER

            3.3 INCHES OF RAIN. WATER ALL THE WAY ACROSS
            STATE HIGHWAY 115. WATER ALL THE WAY ACROSS
            MOST ROADS IN THE AREA. HORSE CREEK ABOUT
            90% FULL ON 9000 W RD AND WELL OVER ITS
            BANKS AT 10000W RD AND AT STATE HIGHWAY 15.
            PILOT ROAD FLOODED ALL TEH WAY ACROSS IN
            HERSCHER.

&&

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